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diff --git a/doc/Tech.Notes b/doc/Tech.Notes new file mode 100644 index 0000000..d485a4e --- /dev/null +++ b/doc/Tech.Notes @@ -0,0 +1,239 @@ +Technical Notes about PCRE +-------------------------- + +Many years ago I implemented some regular expression functions to an algorithm +suggested by Martin Richards. These were not Unix-like in form, and were quite +restricted in what they could do by comparison with Perl. The interesting part +about the algorithm was that the amount of space required to hold the compiled +form of an expression was known in advance. The code to apply an expression did +not operate by backtracking, as the Henry Spencer and Perl code does, but +instead checked all possibilities simultaneously by keeping a list of current +states and checking all of them as it advanced through the subject string. (In +the terminology of Jeffrey Friedl's book, it was a "DFA algorithm".) When the +pattern was all used up, all remaining states were possible matches, and the +one matching the longest subset of the subject string was chosen. This did not +necessarily maximize the individual wild portions of the pattern, as is +expected in Unix and Perl-style regular expressions. + +By contrast, the code originally written by Henry Spencer and subsequently +heavily modified for Perl actually compiles the expression twice: once in a +dummy mode in order to find out how much store will be needed, and then for +real. The execution function operates by backtracking and maximizing (or, +optionally, minimizing in Perl) the amount of the subject that matches +individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's +terminology. + +For this set of functions that forms PCRE, I tried at first to invent an +algorithm that used an amount of store bounded by a multiple of the number of +characters in the pattern, to save on compiling time. However, because of the +greater complexity in Perl regular expressions, I couldn't do this. In any +case, a first pass through the pattern is needed, in order to find internal +flag settings like (?i) at top level. So it works by running a very degenerate +first pass to calculate a maximum store size, and then a second pass to do the +real compile - which may use a bit less than the predicted amount of store. The +idea is that this is going to turn out faster because the first pass is +degenerate and the second can just store stuff straight into the vector. It +does make the compiling functions bigger, of course, but they have got quite +big anyway to handle all the Perl stuff. + +The compiled form of a pattern is a vector of bytes, containing items of +variable length. The first byte in an item is an opcode, and the length of the +item is either implicit in the opcode or contained in the data bytes which +follow it. A list of all the opcodes follows: + +Opcodes with no following data +------------------------------ + +These items are all just one byte long + + OP_END end of pattern + OP_ANY match any character + OP_SOD match start of data: \A + OP_CIRC ^ (start of data, or after \n in multiline) + OP_NOT_WORD_BOUNDARY \W + OP_WORD_BOUNDARY \w + OP_NOT_DIGIT \D + OP_DIGIT \d + OP_NOT_WHITESPACE \S + OP_WHITESPACE \s + OP_NOT_WORDCHAR \W + OP_WORDCHAR \w + OP_EODN match end of data or \n at end: \Z + OP_EOD match end of data: \z + OP_DOLL $ (end of data, or before \n in multiline) + + +Repeating single characters +--------------------------- + +The common repeats (*, +, ?) when applied to a single character appear as +two-byte items using the following opcodes: + + OP_STAR + OP_MINSTAR + OP_PLUS + OP_MINPLUS + OP_QUERY + OP_MINQUERY + +Those with "MIN" in their name are the minimizing versions. Each is followed by +the character that is to be repeated. Other repeats make use of + + OP_UPTO + OP_MINUPTO + OP_EXACT + +which are followed by a two-byte count (most significant first) and the +repeated character. OP_UPTO matches from 0 to the given number. A repeat with a +non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an +OP_UPTO (or OP_MINUPTO). + + +Repeating character types +------------------------- + +Repeats of things like \d are done exactly as for single characters, except +that instead of a character, the opcode for the type is stored in the data +byte. The opcodes are: + + OP_TYPESTAR + OP_TYPEMINSTAR + OP_TYPEPLUS + OP_TYPEMINPLUS + OP_TYPEQUERY + OP_TYPEMINQUERY + OP_TYPEUPTO + OP_TYPEMINUPTO + OP_TYPEEXACT + + +Matching a character string +--------------------------- + +The OP_CHARS opcode is followed by a one-byte count and then that number of +characters. If there are more than 255 characters in sequence, successive +instances of OP_CHARS are used. + + +Character classes +----------------- + +OP_CLASS is used for a character class, provided there are at least two +characters in the class. If there is only one character, OP_CHARS is used for a +positive class, and OP_NOT for a negative one (that is, for something like +[^a]). Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a +repeated, negated, single-character class. The normal ones (OP_STAR etc.) are +used for a repeated positive single-character class. + +OP_CLASS is followed by a 32-byte bit map containing a 1 +bit for every character that is acceptable. The bits are counted from the least +significant end of each byte. + + +Back references +--------------- + +OP_REF is followed by a single byte containing the reference number. + + +Repeating character classes and back references +----------------------------------------------- + +Single-character classes are handled specially (see above). This applies to +OP_CLASS and OP_REF. In both cases, the repeat information follows the base +item. The matching code looks at the following opcode to see if it is one of + + OP_CRSTAR + OP_CRMINSTAR + OP_CRPLUS + OP_CRMINPLUS + OP_CRQUERY + OP_CRMINQUERY + OP_CRRANGE + OP_CRMINRANGE + +All but the last two are just single-byte items. The others are followed by +four bytes of data, comprising the minimum and maximum repeat counts. + + +Brackets and alternation +------------------------ + +A pair of non-identifying (round) brackets is wrapped round each expression at +compile time, so alternation always happens in the context of brackets. +Non-identifying brackets use the opcode OP_BRA, while identifying brackets use +OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English +speakers, including myself, can be round, square, or curly. Hence this usage.] + +A bracket opcode is followed by two bytes which give the offset to the next +alternative OP_ALT or, if there aren't any branches, to the matching KET +opcode. Each OP_ALT is followed by two bytes giving the offset to the next one, +or to the KET opcode. + +OP_KET is used for subpatterns that do not repeat indefinitely, while +OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or +maximally respectively. All three are followed by two bytes giving (as a +positive number) the offset back to the matching BRA opcode. + +If a subpattern is quantified such that it is permitted to match zero times, it +is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte +opcodes which tell the matcher that skipping this subpattern entirely is a +valid branch. + +A subpattern with an indefinite maximum repetition is replicated in the +compiled data its minimum number of times (or once with a BRAZERO if the +minimum is zero), with the final copy terminating with a KETRMIN or KETRMAX as +appropriate. + +A subpattern with a bounded maximum repetition is replicated in a nested +fashion up to the maximum number of times, with BRAZERO or BRAMINZERO before +each replication after the minimum, so that, for example, (abc){2,5} is +compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 200-bracket limit does not +apply to these internally generated brackets. + + +Assertions +---------- + +Forward assertions are just like other subpatterns, but starting with one of +the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes +OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion +is OP_REVERSE, followed by a two byte count of the number of characters to move +back the pointer in the subject string. A separate count is present in each +alternative of a lookbehind assertion, allowing them to have different fixed +lengths. + + +Once-only subpatterns +--------------------- + +These are also just like other subpatterns, but they start with the opcode +OP_ONCE. + + +Conditional subpatterns +----------------------- + +These are like other subpatterns, but they start with the opcode OP_COND. If +the condition is a back reference, this is stored at the start of the +subpattern using the opcode OP_CREF followed by one byte containing the +reference number. Otherwise, a conditional subpattern will always start with +one of the assertions. + + +Changing options +---------------- + +If any of the /i, /m, or /s options are changed within a parenthesized group, +an OP_OPT opcode is compiled, followed by one byte containing the new settings +of these flags. If there are several alternatives in a group, there is an +occurrence of OP_OPT at the start of all those following the first options +change, to set appropriate options for the start of the alternative. +Immediately after the end of the group there is another such item to reset the +flags to their previous values. Other changes of flag within the pattern can be +handled entirely at compile time, and so do not cause anything to be put into +the compiled data. + + +Philip Hazel +January 1999 diff --git a/doc/pcre.3 b/doc/pcre.3 new file mode 100644 index 0000000..47971b9 --- /dev/null +++ b/doc/pcre.3 @@ -0,0 +1,1502 @@ +.TH PCRE 3 +.SH NAME +pcre - Perl-compatible regular expressions. +.SH SYNOPSIS +.B #include <pcre.h> +.PP +.SM +.br +.B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR, +.ti +5n +.B const char **\fIerrptr\fR, int *\fIerroffset\fR, +.ti +5n +.B const unsigned char *\fItableptr\fR); +.PP +.br +.B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR, +.ti +5n +.B const char **\fIerrptr\fR); +.PP +.br +.B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR," +.ti +5n +.B "const char *\fIsubject\fR," int \fIlength\fR, int \fIstartoffset\fR, +.ti +5n +.B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR); +.PP +.br +.B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR, +.ti +5n +.B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR, +.ti +5n +.B int \fIbuffersize\fR); +.PP +.br +.B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR, +.ti +5n +.B int \fIstringcount\fR, int \fIstringnumber\fR, +.ti +5n +.B const char **\fIstringptr\fR); +.PP +.br +.B int pcre_get_substring_list(const char *\fIsubject\fR, +.ti +5n +.B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);" +.PP +.br +.B const unsigned char *pcre_maketables(void); +.PP +.br +.B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int +.B *\fIfirstcharptr\fR); +.PP +.br +.B char *pcre_version(void); +.PP +.br +.B void *(*pcre_malloc)(size_t); +.PP +.br +.B void (*pcre_free)(void *); + + + +.SH DESCRIPTION +The PCRE library is a set of functions that implement regular expression +pattern matching using the same syntax and semantics as Perl 5, with just a few +differences (see below). The current implementation corresponds to Perl 5.005. + +PCRE has its own native API, which is described in this document. There is also +a set of wrapper functions that correspond to the POSIX API. These are +described in the \fBpcreposix\fR documentation. + +The native API function prototypes are defined in the header file \fBpcre.h\fR, +and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be +accessed by adding \fB-lpcre\fR to the command for linking an application which +calls it. + +The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR +are used for compiling and matching regular expressions, while +\fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and +\fBpcre_get_substring_list()\fR are convenience functions for extracting +captured substrings from a matched subject string. The function +\fBpcre_maketables()\fR is used (optionally) to build a set of character tables +in the current locale for passing to \fBpcre_compile()\fR. + +The function \fBpcre_info()\fR is used to find out information about a compiled +pattern, while the function \fBpcre_version()\fR returns a pointer to a string +containing the version of PCRE and its date of release. + +The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain +the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions +respectively. PCRE calls the memory management functions via these variables, +so a calling program can replace them if it wishes to intercept the calls. This +should be done before calling any PCRE functions. + + +.SH MULTI-THREADING +The PCRE functions can be used in multi-threading applications, with the +proviso that the memory management functions pointed to by \fBpcre_malloc\fR +and \fBpcre_free\fR are shared by all threads. + +The compiled form of a regular expression is not altered during matching, so +the same compiled pattern can safely be used by several threads at once. + + +.SH COMPILING A PATTERN +The function \fBpcre_compile()\fR is called to compile a pattern into an +internal form. The pattern is a C string terminated by a binary zero, and +is passed in the argument \fIpattern\fR. A pointer to a single block of memory +that is obtained via \fBpcre_malloc\fR is returned. This contains the +compiled code and related data. The \fBpcre\fR type is defined for this for +convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the +contents of the block are not externally defined. It is up to the caller to +free the memory when it is no longer required. +.PP +The size of a compiled pattern is roughly proportional to the length of the +pattern string, except that each character class (other than those containing +just a single character, negated or not) requires 33 bytes, and repeat +quantifiers with a minimum greater than one or a bounded maximum cause the +relevant portions of the compiled pattern to be replicated. +.PP +The \fIoptions\fR argument contains independent bits that affect the +compilation. It should be zero if no options are required. Some of the options, +in particular, those that are compatible with Perl, can also be set and unset +from within the pattern (see the detailed description of regular expressions +below). For these options, the contents of the \fIoptions\fR argument specifies +their initial settings at the start of compilation and execution. The +PCRE_ANCHORED option can be set at the time of matching as well as at compile +time. +.PP +If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately. +Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns +NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual +error message. The offset from the start of the pattern to the character where +the error was discovered is placed in the variable pointed to by +\fIerroffset\fR, which must not be NULL. If it is, an immediate error is given. +.PP +If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of +character tables which are built when it is compiled, using the default C +locale. Otherwise, \fItableptr\fR must be the result of a call to +\fBpcre_maketables()\fR. See the section on locale support below. +.PP +The following option bits are defined in the header file: + + PCRE_ANCHORED + +If this bit is set, the pattern is forced to be "anchored", that is, it is +constrained to match only at the start of the string which is being searched +(the "subject string"). This effect can also be achieved by appropriate +constructs in the pattern itself, which is the only way to do it in Perl. + + PCRE_CASELESS + +If this bit is set, letters in the pattern match both upper and lower case +letters. It is equivalent to Perl's /i option. + + PCRE_DOLLAR_ENDONLY + +If this bit is set, a dollar metacharacter in the pattern matches only at the +end of the subject string. Without this option, a dollar also matches +immediately before the final character if it is a newline (but not before any +other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is +set. There is no equivalent to this option in Perl. + + PCRE_DOTALL + +If this bit is set, a dot metacharater in the pattern matches all characters, +including newlines. Without it, newlines are excluded. This option is +equivalent to Perl's /s option. A negative class such as [^a] always matches a +newline character, independent of the setting of this option. + + PCRE_EXTENDED + +If this bit is set, whitespace data characters in the pattern are totally +ignored except when escaped or inside a character class, and characters between +an unescaped # outside a character class and the next newline character, +inclusive, are also ignored. This is equivalent to Perl's /x option, and makes +it possible to include comments inside complicated patterns. Note, however, +that this applies only to data characters. Whitespace characters may never +appear within special character sequences in a pattern, for example within the +sequence (?( which introduces a conditional subpattern. + + PCRE_EXTRA + +This option turns on additional functionality of PCRE that is incompatible with +Perl. Any backslash in a pattern that is followed by a letter that has no +special meaning causes an error, thus reserving these combinations for future +expansion. By default, as in Perl, a backslash followed by a letter with no +special meaning is treated as a literal. There are at present no other features +controlled by this option. + + PCRE_MULTILINE + +By default, PCRE treats the subject string as consisting of a single "line" of +characters (even if it actually contains several newlines). The "start of line" +metacharacter (^) matches only at the start of the string, while the "end of +line" metacharacter ($) matches only at the end of the string, or before a +terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as +Perl. + +When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs +match immediately following or immediately before any newline in the subject +string, respectively, as well as at the very start and end. This is equivalent +to Perl's /m option. If there are no "\\n" characters in a subject string, or +no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no +effect. + + PCRE_UNGREEDY + +This option inverts the "greediness" of the quantifiers so that they are not +greedy by default, but become greedy if followed by "?". It is not compatible +with Perl. It can also be set by a (?U) option setting within the pattern. + + +.SH STUDYING A PATTERN +When a pattern is going to be used several times, it is worth spending more +time analyzing it in order to speed up the time taken for matching. The +function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first +argument, and returns a pointer to a \fBpcre_extra\fR block (another \fBvoid\fR +typedef) containing additional information about the pattern; this can be +passed to \fBpcre_exec()\fR. If no additional information is available, NULL +is returned. + +The second argument contains option bits. At present, no options are defined +for \fBpcre_study()\fR, and this argument should always be zero. + +The third argument for \fBpcre_study()\fR is a pointer to an error message. If +studying succeeds (even if no data is returned), the variable it points to is +set to NULL. Otherwise it points to a textual error message. + +At present, studying a pattern is useful only for non-anchored patterns that do +not have a single fixed starting character. A bitmap of possible starting +characters is created. + + +.SH LOCALE SUPPORT +PCRE handles caseless matching, and determines whether characters are letters, +digits, or whatever, by reference to a set of tables. The library contains a +default set of tables which is created in the default C locale when PCRE is +compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL, +and is sufficient for many applications. + +An alternative set of tables can, however, be supplied. Such tables are built +by calling the \fBpcre_maketables()\fR function, which has no arguments, in the +relevant locale. The result can then be passed to \fBpcre_compile()\fR as often +as necessary. For example, to build and use tables that are appropriate for the +French locale (where accented characters with codes greater than 128 are +treated as letters), the following code could be used: + + setlocale(LC_CTYPE, "fr"); + tables = pcre_maketables(); + re = pcre_compile(..., tables); + +The tables are built in memory that is obtained via \fBpcre_malloc\fR. The +pointer that is passed to \fBpcre_compile\fR is saved with the compiled +pattern, and the same tables are used via this pointer by \fBpcre_study()\fR +and \fBpcre_exec()\fR. Thus for any single pattern, compilation, studying and +matching all happen in the same locale, but different patterns can be compiled +in different locales. It is the caller's responsibility to ensure that the +memory containing the tables remains available for as long as it is needed. + + +.SH INFORMATION ABOUT A PATTERN +The \fBpcre_info()\fR function returns information about a compiled pattern. +Its yield is the number of capturing subpatterns, or one of the following +negative numbers: + + PCRE_ERROR_NULL the argument \fIcode\fR was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + +If the \fIoptptr\fR argument is not NULL, a copy of the options with which the +pattern was compiled is placed in the integer it points to. These option bits +are those specified in the call to \fBpcre_compile()\fR, modified by any +top-level option settings within the pattern itself, and with the PCRE_ANCHORED +bit set if the form of the pattern implies that it can match only at the start +of a subject string. + +If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL, +it is used to pass back information about the first character of any matched +string. If there is a fixed first character, e.g. from a pattern such as +(cat|cow|coyote), then it is returned in the integer pointed to by +\fIfirstcharptr\fR. Otherwise, if either + +(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch +starts with "^", or + +(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set +(if it were set, the pattern would be anchored), + +then -1 is returned, indicating that the pattern matches only at the +start of a subject string or after any "\\n" within the string. Otherwise -2 is +returned. + + +.SH MATCHING A PATTERN +The function \fBpcre_exec()\fR is called to match a subject string against a +pre-compiled pattern, which is passed in the \fIcode\fR argument. If the +pattern has been studied, the result of the study should be passed in the +\fIextra\fR argument. Otherwise this must be NULL. + +The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose +unused bits must be zero. However, if a pattern was compiled with +PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it +cannot be made unachored at matching time. + +There are also three further options that can be set only at matching time: + + PCRE_NOTBOL + +The first character of the string is not the beginning of a line, so the +circumflex metacharacter should not match before it. Setting this without +PCRE_MULTILINE (at compile time) causes circumflex never to match. + + PCRE_NOTEOL + +The end of the string is not the end of a line, so the dollar metacharacter +should not match it nor (except in multiline mode) a newline immediately before +it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never +to match. + + PCRE_NOTEMPTY + +An empty string is not considered to be a valid match if this option is set. If +there are alternatives in the pattern, they are tried. If all the alternatives +match the empty string, the entire match fails. For example, if the pattern + + a?b? + +is applied to a string not beginning with "a" or "b", it matches the empty +string at the start of the subject. With PCRE_NOTEMPTY set, this match is not +valid, so PCRE searches further into the string for occurrences of "a" or "b". + +Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case +of a pattern match of the empty string within its \fBsplit()\fR function, and +when using the /g modifier. It is possible to emulate Perl's behaviour after +matching a null string by first trying the match again at the same offset with +PCRE_NOTEMPTY set, and then if that fails by advancing the starting offset (see +below) and trying an ordinary match again. + +The subject string is passed as a pointer in \fIsubject\fR, a length in +\fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern +string, it may contain binary zero characters. When the starting offset is +zero, the search for a match starts at the beginning of the subject, and this +is by far the most common case. + +A non-zero starting offset is useful when searching for another match in the +same subject by calling \fBpcre_exec()\fR again after a previous success. +Setting \fIstartoffset\fR differs from just passing over a shortened string and +setting PCRE_NOTBOL in the case of a pattern that begins with any kind of +lookbehind. For example, consider the pattern + + \\Biss\\B + +which finds occurrences of "iss" in the middle of words. (\\B matches only if +the current position in the subject is not a word boundary.) When applied to +the string "Mississipi" the first call to \fBpcre_exec()\fR finds the first +occurrence. If \fBpcre_exec()\fR is called again with just the remainder of the +subject, namely "issipi", it does not match, because \\B is always false at the +start of the subject, which is deemed to be a word boundary. However, if +\fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR +set to 4, it finds the second occurrence of "iss" because it is able to look +behind the starting point to discover that it is preceded by a letter. + +If a non-zero starting offset is passed when the pattern is anchored, one +attempt to match at the given offset is tried. This can only succeed if the +pattern does not require the match to be at the start of the subject. + +In general, a pattern matches a certain portion of the subject, and in +addition, further substrings from the subject may be picked out by parts of the +pattern. Following the usage in Jeffrey Friedl's book, this is called +"capturing" in what follows, and the phrase "capturing subpattern" is used for +a fragment of a pattern that picks out a substring. PCRE supports several other +kinds of parenthesized subpattern that do not cause substrings to be captured. + +Captured substrings are returned to the caller via a vector of integer offsets +whose address is passed in \fIovector\fR. The number of elements in the vector +is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass +back captured substrings, each substring using a pair of integers. The +remaining third of the vector is used as workspace by \fBpcre_exec()\fR while +matching capturing subpatterns, and is not available for passing back +information. The length passed in \fIovecsize\fR should always be a multiple of +three. If it is not, it is rounded down. + +When a match has been successful, information about captured substrings is +returned in pairs of integers, starting at the beginning of \fIovector\fR, and +continuing up to two-thirds of its length at the most. The first element of a +pair is set to the offset of the first character in a substring, and the second +is set to the offset of the first character after the end of a substring. The +first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the +subject string matched by the entire pattern. The next pair is used for the +first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR +is the number of pairs that have been set. If there are no capturing +subpatterns, the return value from a successful match is 1, indicating that +just the first pair of offsets has been set. + +Some convenience functions are provided for extracting the captured substrings +as separate strings. These are described in the following section. + +It is possible for an capturing subpattern number \fIn+1\fR to match some +part of the subject when subpattern \fIn\fR has not been used at all. For +example, if the string "abc" is matched against the pattern (a|(z))(bc) +subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset +values corresponding to the unused subpattern are set to -1. + +If a capturing subpattern is matched repeatedly, it is the last portion of the +string that it matched that gets returned. + +If the vector is too small to hold all the captured substrings, it is used as +far as possible (up to two-thirds of its length), and the function returns a +value of zero. In particular, if the substring offsets are not of interest, +\fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and +\fIovecsize\fR as zero. However, if the pattern contains back references and +the \fIovector\fR isn't big enough to remember the related substrings, PCRE has +to get additional memory for use during matching. Thus it is usually advisable +to supply an \fIovector\fR. + +Note that \fBpcre_info()\fR can be used to find out how many capturing +subpatterns there are in a compiled pattern. The smallest size for +\fIovector\fR that will allow for \fIn\fR captured substrings in addition to +the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3. + +If \fBpcre_exec()\fR fails, it returns a negative number. The following are +defined in the header file: + + PCRE_ERROR_NOMATCH (-1) + +The subject string did not match the pattern. + + PCRE_ERROR_NULL (-2) + +Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was +NULL and \fIovecsize\fR was not zero. + + PCRE_ERROR_BADOPTION (-3) + +An unrecognized bit was set in the \fIoptions\fR argument. + + PCRE_ERROR_BADMAGIC (-4) + +PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch +the case when it is passed a junk pointer. This is the error it gives when the +magic number isn't present. + + PCRE_ERROR_UNKNOWN_NODE (-5) + +While running the pattern match, an unknown item was encountered in the +compiled pattern. This error could be caused by a bug in PCRE or by overwriting +of the compiled pattern. + + PCRE_ERROR_NOMEMORY (-6) + +If a pattern contains back references, but the \fIovector\fR that is passed to +\fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE +gets a block of memory at the start of matching to use for this purpose. If the +call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at +the end of matching. + + +.SH EXTRACTING CAPTURED SUBSTRINGS +Captured substrings can be accessed directly by using the offsets returned by +\fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions +\fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and +\fBpcre_get_substring_list()\fR are provided for extracting captured substrings +as new, separate, zero-terminated strings. A substring that contains a binary +zero is correctly extracted and has a further zero added on the end, but the +result does not, of course, function as a C string. + +The first three arguments are the same for all three functions: \fIsubject\fR +is the subject string which has just been successfully matched, \fIovector\fR +is a pointer to the vector of integer offsets that was passed to +\fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that +were captured by the match, including the substring that matched the entire +regular expression. This is the value returned by \fBpcre_exec\fR if it +is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it +ran out of space in \fIovector\fR, then the value passed as +\fIstringcount\fR should be the size of the vector divided by three. + +The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR +extract a single substring, whose number is given as \fIstringnumber\fR. A +value of zero extracts the substring that matched the entire pattern, while +higher values extract the captured substrings. For \fBpcre_copy_substring()\fR, +the string is placed in \fIbuffer\fR, whose length is given by +\fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is +obtained via \fBpcre_malloc\fR, and its address is returned via +\fIstringptr\fR. The yield of the function is the length of the string, not +including the terminating zero, or one of + + PCRE_ERROR_NOMEMORY (-6) + +The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get +memory failed for \fBpcre_get_substring()\fR. + + PCRE_ERROR_NOSUBSTRING (-7) + +There is no substring whose number is \fIstringnumber\fR. + +The \fBpcre_get_substring_list()\fR function extracts all available substrings +and builds a list of pointers to them. All this is done in a single block of +memory which is obtained via \fBpcre_malloc\fR. The address of the memory block +is returned via \fIlistptr\fR, which is also the start of the list of string +pointers. The end of the list is marked by a NULL pointer. The yield of the +function is zero if all went well, or + + PCRE_ERROR_NOMEMORY (-6) + +if the attempt to get the memory block failed. + +When any of these functions encounter a substring that is unset, which can +happen when capturing subpattern number \fIn+1\fR matches some part of the +subject, but subpattern \fIn\fR has not been used at all, they return an empty +string. This can be distinguished from a genuine zero-length substring by +inspecting the appropriate offset in \fIovector\fR, which is negative for unset +substrings. + + + +.SH LIMITATIONS +There are some size limitations in PCRE but it is hoped that they will never in +practice be relevant. +The maximum length of a compiled pattern is 65539 (sic) bytes. +All values in repeating quantifiers must be less than 65536. +The maximum number of capturing subpatterns is 99. +The maximum number of all parenthesized subpatterns, including capturing +subpatterns, assertions, and other types of subpattern, is 200. + +The maximum length of a subject string is the largest positive number that an +integer variable can hold. However, PCRE uses recursion to handle subpatterns +and indefinite repetition. This means that the available stack space may limit +the size of a subject string that can be processed by certain patterns. + + +.SH DIFFERENCES FROM PERL +The differences described here are with respect to Perl 5.005. + +1. By default, a whitespace character is any character that the C library +function \fBisspace()\fR recognizes, though it is possible to compile PCRE with +alternative character type tables. Normally \fBisspace()\fR matches space, +formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 +no longer includes vertical tab in its set of whitespace characters. The \\v +escape that was in the Perl documentation for a long time was never in fact +recognized. However, the character itself was treated as whitespace at least +up to 5.002. In 5.004 and 5.005 it does not match \\s. + +2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits +them, but they do not mean what you might think. For example, (?!a){3} does +not assert that the next three characters are not "a". It just asserts that the +next character is not "a" three times. + +3. Capturing subpatterns that occur inside negative lookahead assertions are +counted, but their entries in the offsets vector are never set. Perl sets its +numerical variables from any such patterns that are matched before the +assertion fails to match something (thereby succeeding), but only if the +negative lookahead assertion contains just one branch. + +4. Though binary zero characters are supported in the subject string, they are +not allowed in a pattern string because it is passed as a normal C string, +terminated by zero. The escape sequence "\\0" can be used in the pattern to +represent a binary zero. + +5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U, +\\E, \\Q. In fact these are implemented by Perl's general string-handling and +are not part of its pattern matching engine. + +6. The Perl \\G assertion is not supported as it is not relevant to single +pattern matches. + +7. Fairly obviously, PCRE does not support the (?{code}) construction. + +8. There are at the time of writing some oddities in Perl 5.005_02 concerned +with the settings of captured strings when part of a pattern is repeated. For +example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value +"b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if +the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set. + +In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the +future Perl changes to a consistent state that is different, PCRE may change to +follow. + +9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern +/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. +However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset. + +10. PCRE provides some extensions to the Perl regular expression facilities: + +(a) Although lookbehind assertions must match fixed length strings, each +alternative branch of a lookbehind assertion can match a different length of +string. Perl 5.005 requires them all to have the same length. + +(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- +character matches only at the very end of the string. + +(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special +meaning is faulted. + +(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is +inverted, that is, by default they are not greedy, but if followed by a +question mark they are. + +(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start +of the subject. + +(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for +\fBpcre_exec()\fR have no Perl equivalents. + + +.SH REGULAR EXPRESSION DETAILS +The syntax and semantics of the regular expressions supported by PCRE are +described below. Regular expressions are also described in the Perl +documentation and in a number of other books, some of which have copious +examples. Jeffrey Friedl's "Mastering Regular Expressions", published by +O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description +here is intended as reference documentation. + +A regular expression is a pattern that is matched against a subject string from +left to right. Most characters stand for themselves in a pattern, and match the +corresponding characters in the subject. As a trivial example, the pattern + + The quick brown fox + +matches a portion of a subject string that is identical to itself. The power of +regular expressions comes from the ability to include alternatives and +repetitions in the pattern. These are encoded in the pattern by the use of +\fImeta-characters\fR, which do not stand for themselves but instead are +interpreted in some special way. + +There are two different sets of meta-characters: those that are recognized +anywhere in the pattern except within square brackets, and those that are +recognized in square brackets. Outside square brackets, the meta-characters are +as follows: + + \\ general escape character with several uses + ^ assert start of subject (or line, in multiline mode) + $ assert end of subject (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + { start min/max quantifier + +Part of a pattern that is in square brackets is called a "character class". In +a character class the only meta-characters are: + + \\ general escape character + ^ negate the class, but only if the first character + - indicates character range + ] terminates the character class + +The following sections describe the use of each of the meta-characters. + + +.SH BACKSLASH +The backslash character has several uses. Firstly, if it is followed by a +non-alphameric character, it takes away any special meaning that character may +have. This use of backslash as an escape character applies both inside and +outside character classes. + +For example, if you want to match a "*" character, you write "\\*" in the +pattern. This applies whether or not the following character would otherwise be +interpreted as a meta-character, so it is always safe to precede a +non-alphameric with "\\" to specify that it stands for itself. In particular, +if you want to match a backslash, you write "\\\\". + +If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the +pattern (other than in a character class) and characters between a "#" outside +a character class and the next newline character are ignored. An escaping +backslash can be used to include a whitespace or "#" character as part of the +pattern. + +A second use of backslash provides a way of encoding non-printing characters +in patterns in a visible manner. There is no restriction on the appearance of +non-printing characters, apart from the binary zero that terminates a pattern, +but when a pattern is being prepared by text editing, it is usually easier to +use one of the following escape sequences than the binary character it +represents: + + \\a alarm, that is, the BEL character (hex 07) + \\cx "control-x", where x is any character + \\e escape (hex 1B) + \\f formfeed (hex 0C) + \\n newline (hex 0A) + \\r carriage return (hex 0D) + \\t tab (hex 09) + \\xhh character with hex code hh + \\ddd character with octal code ddd, or backreference + +The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it +is converted to upper case. Then bit 6 of the character (hex 40) is inverted. +Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex +7B. + +After "\\x", up to two hexadecimal digits are read (letters can be in upper or +lower case). + +After "\\0" up to two further octal digits are read. In both cases, if there +are fewer than two digits, just those that are present are used. Thus the +sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character. +Make sure you supply two digits after the initial zero if the character that +follows is itself an octal digit. + +The handling of a backslash followed by a digit other than 0 is complicated. +Outside a character class, PCRE reads it and any following digits as a decimal +number. If the number is less than 10, or if there have been at least that many +previous capturing left parentheses in the expression, the entire sequence is +taken as a \fIback reference\fR. A description of how this works is given +later, following the discussion of parenthesized subpatterns. + +Inside a character class, or if the decimal number is greater than 9 and there +have not been that many capturing subpatterns, PCRE re-reads up to three octal +digits following the backslash, and generates a single byte from the least +significant 8 bits of the value. Any subsequent digits stand for themselves. +For example: + + \\040 is another way of writing a space + \\40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \\7 is always a back reference + \\11 might be a back reference, or another way of + writing a tab + \\011 is always a tab + \\0113 is a tab followed by the character "3" + \\113 is the character with octal code 113 (since there + can be no more than 99 back references) + \\377 is a byte consisting entirely of 1 bits + \\81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" + +Note that octal values of 100 or greater must not be introduced by a leading +zero, because no more than three octal digits are ever read. + +All the sequences that define a single byte value can be used both inside and +outside character classes. In addition, inside a character class, the sequence +"\\b" is interpreted as the backspace character (hex 08). Outside a character +class it has a different meaning (see below). + +The third use of backslash is for specifying generic character types: + + \\d any decimal digit + \\D any character that is not a decimal digit + \\s any whitespace character + \\S any character that is not a whitespace character + \\w any "word" character + \\W any "non-word" character + +Each pair of escape sequences partitions the complete set of characters into +two disjoint sets. Any given character matches one, and only one, of each pair. + +A "word" character is any letter or digit or the underscore character, that is, +any character which can be part of a Perl "word". The definition of letters and +digits is controlled by PCRE's character tables, and may vary if locale- +specific matching is taking place (see "Locale support" above). For example, in +the "fr" (French) locale, some character codes greater than 128 are used for +accented letters, and these are matched by \\w. + +These character type sequences can appear both inside and outside character +classes. They each match one character of the appropriate type. If the current +matching point is at the end of the subject string, all of them fail, since +there is no character to match. + +The fourth use of backslash is for certain simple assertions. An assertion +specifies a condition that has to be met at a particular point in a match, +without consuming any characters from the subject string. The use of +subpatterns for more complicated assertions is described below. The backslashed +assertions are + + \\b word boundary + \\B not a word boundary + \\A start of subject (independent of multiline mode) + \\Z end of subject or newline at end (independent of multiline mode) + \\z end of subject (independent of multiline mode) + +These assertions may not appear in character classes (but note that "\\b" has a +different meaning, namely the backspace character, inside a character class). + +A word boundary is a position in the subject string where the current character +and the previous character do not both match \\w or \\W (i.e. one matches +\\w and the other matches \\W), or the start or end of the string if the +first or last character matches \\w, respectively. + +The \\A, \\Z, and \\z assertions differ from the traditional circumflex and +dollar (described below) in that they only ever match at the very start and end +of the subject string, whatever options are set. They are not affected by the +PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of +\fBpcre_exec()\fR is non-zero, \\A can never match. The difference between \\Z +and \\z is that \\Z matches before a newline that is the last character of the +string as well as at the end of the string, whereas \\z matches only at the +end. + + +.SH CIRCUMFLEX AND DOLLAR +Outside a character class, in the default matching mode, the circumflex +character is an assertion which is true only if the current matching point is +at the start of the subject string. If the \fIstartoffset\fR argument of +\fBpcre_exec()\fR is non-zero, circumflex can never match. Inside a character +class, circumflex has an entirely different meaning (see below). + +Circumflex need not be the first character of the pattern if a number of +alternatives are involved, but it should be the first thing in each alternative +in which it appears if the pattern is ever to match that branch. If all +possible alternatives start with a circumflex, that is, if the pattern is +constrained to match only at the start of the subject, it is said to be an +"anchored" pattern. (There are also other constructs that can cause a pattern +to be anchored.) + +A dollar character is an assertion which is true only if the current matching +point is at the end of the subject string, or immediately before a newline +character that is the last character in the string (by default). Dollar need +not be the last character of the pattern if a number of alternatives are +involved, but it should be the last item in any branch in which it appears. +Dollar has no special meaning in a character class. + +The meaning of dollar can be changed so that it matches only at the very end of +the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching +time. This does not affect the \\Z assertion. + +The meanings of the circumflex and dollar characters are changed if the +PCRE_MULTILINE option is set. When this is the case, they match immediately +after and immediately before an internal "\\n" character, respectively, in +addition to matching at the start and end of the subject string. For example, +the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode, +but not otherwise. Consequently, patterns that are anchored in single line mode +because all branches start with "^" are not anchored in multiline mode, and a +match for circumflex is possible when the \fIstartoffset\fR argument of +\fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if +PCRE_MULTILINE is set. + +Note that the sequences \\A, \\Z, and \\z can be used to match the start and +end of the subject in both modes, and if all branches of a pattern start with +\\A is it always anchored, whether PCRE_MULTILINE is set or not. + + +.SH FULL STOP (PERIOD, DOT) +Outside a character class, a dot in the pattern matches any one character in +the subject, including a non-printing character, but not (by default) newline. +If the PCRE_DOTALL option is set, then dots match newlines as well. The +handling of dot is entirely independent of the handling of circumflex and +dollar, the only relationship being that they both involve newline characters. +Dot has no special meaning in a character class. + + +.SH SQUARE BRACKETS +An opening square bracket introduces a character class, terminated by a closing +square bracket. A closing square bracket on its own is not special. If a +closing square bracket is required as a member of the class, it should be the +first data character in the class (after an initial circumflex, if present) or +escaped with a backslash. + +A character class matches a single character in the subject; the character must +be in the set of characters defined by the class, unless the first character in +the class is a circumflex, in which case the subject character must not be in +the set defined by the class. If a circumflex is actually required as a member +of the class, ensure it is not the first character, or escape it with a +backslash. + +For example, the character class [aeiou] matches any lower case vowel, while +[^aeiou] matches any character that is not a lower case vowel. Note that a +circumflex is just a convenient notation for specifying the characters which +are in the class by enumerating those that are not. It is not an assertion: it +still consumes a character from the subject string, and fails if the current +pointer is at the end of the string. + +When caseless matching is set, any letters in a class represent both their +upper case and lower case versions, so for example, a caseless [aeiou] matches +"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a +caseful version would. + +The newline character is never treated in any special way in character classes, +whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class +such as [^a] will always match a newline. + +The minus (hyphen) character can be used to specify a range of characters in a +character class. For example, [d-m] matches any letter between d and m, +inclusive. If a minus character is required in a class, it must be escaped with +a backslash or appear in a position where it cannot be interpreted as +indicating a range, typically as the first or last character in the class. + +It is not possible to have the literal character "]" as the end character of a +range. A pattern such as [W-]46] is interpreted as a class of two characters +("W" and "-") followed by a literal string "46]", so it would match "W46]" or +"-46]". However, if the "]" is escaped with a backslash it is interpreted as +the end of range, so [W-\\]46] is interpreted as a single class containing a +range followed by two separate characters. The octal or hexadecimal +representation of "]" can also be used to end a range. + +Ranges operate in ASCII collating sequence. They can also be used for +characters specified numerically, for example [\\000-\\037]. If a range that +includes letters is used when caseless matching is set, it matches the letters +in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched +caselessly, and if character tables for the "fr" locale are in use, +[\\xc8-\\xcb] matches accented E characters in both cases. + +The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a +character class, and add the characters that they match to the class. For +example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can +conveniently be used with the upper case character types to specify a more +restricted set of characters than the matching lower case type. For example, +the class [^\\W_] matches any letter or digit, but not underscore. + +All non-alphameric characters other than \\, -, ^ (at the start) and the +terminating ] are non-special in character classes, but it does no harm if they +are escaped. + + +.SH VERTICAL BAR +Vertical bar characters are used to separate alternative patterns. For example, +the pattern + + gilbert|sullivan + +matches either "gilbert" or "sullivan". Any number of alternatives may appear, +and an empty alternative is permitted (matching the empty string). +The matching process tries each alternative in turn, from left to right, +and the first one that succeeds is used. If the alternatives are within a +subpattern (defined below), "succeeds" means matching the rest of the main +pattern as well as the alternative in the subpattern. + + +.SH INTERNAL OPTION SETTING +The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED +can be changed from within the pattern by a sequence of Perl option letters +enclosed between "(?" and ")". The option letters are + + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED + +For example, (?im) sets caseless, multiline matching. It is also possible to +unset these options by preceding the letter with a hyphen, and a combined +setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and +PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also +permitted. If a letter appears both before and after the hyphen, the option is +unset. + +The scope of these option changes depends on where in the pattern the setting +occurs. For settings that are outside any subpattern (defined below), the +effect is the same as if the options were set or unset at the start of +matching. The following patterns all behave in exactly the same way: + + (?i)abc + a(?i)bc + ab(?i)c + abc(?i) + +which in turn is the same as compiling the pattern abc with PCRE_CASELESS set. +In other words, such "top level" settings apply to the whole pattern (unless +there are other changes inside subpatterns). If there is more than one setting +of the same option at top level, the rightmost setting is used. + +If an option change occurs inside a subpattern, the effect is different. This +is a change of behaviour in Perl 5.005. An option change inside a subpattern +affects only that part of the subpattern that follows it, so + + (a(?i)b)c + +matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). +By this means, options can be made to have different settings in different +parts of the pattern. Any changes made in one alternative do carry on +into subsequent branches within the same subpattern. For example, + + (a(?i)b|c) + +matches "ab", "aB", "c", and "C", even though when matching "C" the first +branch is abandoned before the option setting. This is because the effects of +option settings happen at compile time. There would be some very weird +behaviour otherwise. + +The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the +same way as the Perl-compatible options by using the characters U and X +respectively. The (?X) flag setting is special in that it must always occur +earlier in the pattern than any of the additional features it turns on, even +when it is at top level. It is best put at the start. + + +.SH SUBPATTERNS +Subpatterns are delimited by parentheses (round brackets), which can be nested. +Marking part of a pattern as a subpattern does two things: + +1. It localizes a set of alternatives. For example, the pattern + + cat(aract|erpillar|) + +matches one of the words "cat", "cataract", or "caterpillar". Without the +parentheses, it would match "cataract", "erpillar" or the empty string. + +2. It sets up the subpattern as a capturing subpattern (as defined above). +When the whole pattern matches, that portion of the subject string that matched +the subpattern is passed back to the caller via the \fIovector\fR argument of +\fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting +from 1) to obtain the numbers of the capturing subpatterns. + +For example, if the string "the red king" is matched against the pattern + + the ((red|white) (king|queen)) + +the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3. + +The fact that plain parentheses fulfil two functions is not always helpful. +There are often times when a grouping subpattern is required without a +capturing requirement. If an opening parenthesis is followed by "?:", the +subpattern does not do any capturing, and is not counted when computing the +number of any subsequent capturing subpatterns. For example, if the string "the +white queen" is matched against the pattern + + the ((?:red|white) (king|queen)) + +the captured substrings are "white queen" and "queen", and are numbered 1 and +2. The maximum number of captured substrings is 99, and the maximum number of +all subpatterns, both capturing and non-capturing, is 200. + +As a convenient shorthand, if any option settings are required at the start of +a non-capturing subpattern, the option letters may appear between the "?" and +the ":". Thus the two patterns + + (?i:saturday|sunday) + (?:(?i)saturday|sunday) + +match exactly the same set of strings. Because alternative branches are tried +from left to right, and options are not reset until the end of the subpattern +is reached, an option setting in one branch does affect subsequent branches, so +the above patterns match "SUNDAY" as well as "Saturday". + + +.SH REPETITION +Repetition is specified by quantifiers, which can follow any of the following +items: + + a single character, possibly escaped + the . metacharacter + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion - see below) + +The general repetition quantifier specifies a minimum and maximum number of +permitted matches, by giving the two numbers in curly brackets (braces), +separated by a comma. The numbers must be less than 65536, and the first must +be less than or equal to the second. For example: + + z{2,4} + +matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special +character. If the second number is omitted, but the comma is present, there is +no upper limit; if the second number and the comma are both omitted, the +quantifier specifies an exact number of required matches. Thus + + [aeiou]{3,} + +matches at least 3 successive vowels, but may match many more, while + + \\d{8} + +matches exactly 8 digits. An opening curly bracket that appears in a position +where a quantifier is not allowed, or one that does not match the syntax of a +quantifier, is taken as a literal character. For example, {,6} is not a +quantifier, but a literal string of four characters. + +The quantifier {0} is permitted, causing the expression to behave as if the +previous item and the quantifier were not present. + +For convenience (and historical compatibility) the three most common +quantifiers have single-character abbreviations: + + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} + +It is possible to construct infinite loops by following a subpattern that can +match no characters with a quantifier that has no upper limit, for example: + + (a?)* + +Earlier versions of Perl and PCRE used to give an error at compile time for +such patterns. However, because there are cases where this can be useful, such +patterns are now accepted, but if any repetition of the subpattern does in fact +match no characters, the loop is forcibly broken. + +By default, the quantifiers are "greedy", that is, they match as much as +possible (up to the maximum number of permitted times), without causing the +rest of the pattern to fail. The classic example of where this gives problems +is in trying to match comments in C programs. These appear between the +sequences /* and */ and within the sequence, individual * and / characters may +appear. An attempt to match C comments by applying the pattern + + /\\*.*\\*/ + +to the string + + /* first command */ not comment /* second comment */ + +fails, because it matches the entire string due to the greediness of the .* +item. + +However, if a quantifier is followed by a question mark, then it ceases to be +greedy, and instead matches the minimum number of times possible, so the +pattern + + /\\*.*?\\*/ + +does the right thing with the C comments. The meaning of the various +quantifiers is not otherwise changed, just the preferred number of matches. +Do not confuse this use of question mark with its use as a quantifier in its +own right. Because it has two uses, it can sometimes appear doubled, as in + + \\d??\\d + +which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. + +If the PCRE_UNGREEDY option is set (an option which is not available in Perl) +then the quantifiers are not greedy by default, but individual ones can be made +greedy by following them with a question mark. In other words, it inverts the +default behaviour. + +When a parenthesized subpattern is quantified with a minimum repeat count that +is greater than 1 or with a limited maximum, more store is required for the +compiled pattern, in proportion to the size of the minimum or maximum. + +If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent +to Perl's /s) is set, thus allowing the . to match newlines, then the pattern +is implicitly anchored, because whatever follows will be tried against every +character position in the subject string, so there is no point in retrying the +overall match at any position after the first. PCRE treats such a pattern as +though it were preceded by \\A. In cases where it is known that the subject +string contains no newlines, it is worth setting PCRE_DOTALL when the pattern +begins with .* in order to obtain this optimization, or alternatively using ^ +to indicate anchoring explicitly. + +When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after + + (tweedle[dume]{3}\\s*)+ + +has matched "tweedledum tweedledee" the value of the captured substring is +"tweedledee". However, if there are nested capturing subpatterns, the +corresponding captured values may have been set in previous iterations. For +example, after + + /(a|(b))+/ + +matches "aba" the value of the second captured substring is "b". + + +.SH BACK REFERENCES +Outside a character class, a backslash followed by a digit greater than 0 (and +possibly further digits) is a back reference to a capturing subpattern earlier +(i.e. to its left) in the pattern, provided there have been that many previous +capturing left parentheses. + +However, if the decimal number following the backslash is less than 10, it is +always taken as a back reference, and causes an error only if there are not +that many capturing left parentheses in the entire pattern. In other words, the +parentheses that are referenced need not be to the left of the reference for +numbers less than 10. See the section entitled "Backslash" above for further +details of the handling of digits following a backslash. + +A back reference matches whatever actually matched the capturing subpattern in +the current subject string, rather than anything matching the subpattern +itself. So the pattern + + (sens|respons)e and \\1ibility + +matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If caseful matching is in force at the time of the +back reference, then the case of letters is relevant. For example, + + ((?i)rah)\\s+\\1 + +matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. + +There may be more than one back reference to the same subpattern. If a +subpattern has not actually been used in a particular match, then any back +references to it always fail. For example, the pattern + + (a|(bc))\\2 + +always fails if it starts to match "a" rather than "bc". Because there may be +up to 99 back references, all digits following the backslash are taken +as part of a potential back reference number. If the pattern continues with a +digit character, then some delimiter must be used to terminate the back +reference. If the PCRE_EXTENDED option is set, this can be whitespace. +Otherwise an empty comment can be used. + +A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\\1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern + + (a|b\\1)+ + +matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of +the subpattern, the back reference matches the character string corresponding +to the previous iteration. In order for this to work, the pattern must be such +that the first iteration does not need to match the back reference. This can be +done using alternation, as in the example above, or by a quantifier with a +minimum of zero. + + +.SH ASSERTIONS +An assertion is a test on the characters following or preceding the current +matching point that does not actually consume any characters. The simple +assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More +complicated assertions are coded as subpatterns. There are two kinds: those +that look ahead of the current position in the subject string, and those that +look behind it. + +An assertion subpattern is matched in the normal way, except that it does not +cause the current matching position to be changed. Lookahead assertions start +with (?= for positive assertions and (?! for negative assertions. For example, + + \\w+(?=;) + +matches a word followed by a semicolon, but does not include the semicolon in +the match, and + + foo(?!bar) + +matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern + + (?!foo)bar + +does not find an occurrence of "bar" that is preceded by something other than +"foo"; it finds any occurrence of "bar" whatsoever, because the assertion +(?!foo) is always true when the next three characters are "bar". A +lookbehind assertion is needed to achieve this effect. + +Lookbehind assertions start with (?<= for positive assertions and (?<! for +negative assertions. For example, + + (?<!foo)bar + +does find an occurrence of "bar" that is not preceded by "foo". The contents of +a lookbehind assertion are restricted such that all the strings it matches must +have a fixed length. However, if there are several alternatives, they do not +all have to have the same fixed length. Thus + + (?<=bullock|donkey) + +is permitted, but + + (?<!dogs?|cats?) + +causes an error at compile time. Branches that match different length strings +are permitted only at the top level of a lookbehind assertion. This is an +extension compared with Perl 5.005, which requires all branches to match the +same length of string. An assertion such as + + (?<=ab(c|de)) + +is not permitted, because its single top-level branch can match two different +lengths, but it is acceptable if rewritten to use two top-level branches: + + (?<=abc|abde) + +The implementation of lookbehind assertions is, for each alternative, to +temporarily move the current position back by the fixed width and then try to +match. If there are insufficient characters before the current position, the +match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns +can be particularly useful for matching at the ends of strings; an example is +given at the end of the section on once-only subpatterns. + +Several assertions (of any sort) may occur in succession. For example, + + (?<=\\d{3})(?<!999)foo + +matches "foo" preceded by three digits that are not "999". Notice that each of +the assertions is applied independently at the same point in the subject +string. First there is a check that the previous three characters are all +digits, then there is a check that the same three characters are not "999". +This pattern does \fInot\fR match "foo" preceded by six characters, the first +of which are digits and the last three of which are not "999". For example, it +doesn't match "123abcfoo". A pattern to do that is + + (?<=\\d{3}...)(?<!999)foo + +This time the first assertion looks at the preceding six characters, checking +that the first three are digits, and then the second assertion checks that the +preceding three characters are not "999". + +Assertions can be nested in any combination. For example, + + (?<=(?<!foo)bar)baz + +matches an occurrence of "baz" that is preceded by "bar" which in turn is not +preceded by "foo", while + + (?<=\\d{3}(?!999)...)foo + +is another pattern which matches "foo" preceded by three digits and any three +characters that are not "999". + +Assertion subpatterns are not capturing subpatterns, and may not be repeated, +because it makes no sense to assert the same thing several times. If any kind +of assertion contains capturing subpatterns within it, these are counted for +the purposes of numbering the capturing subpatterns in the whole pattern. +However, substring capturing is carried out only for positive assertions, +because it does not make sense for negative assertions. + +Assertions count towards the maximum of 200 parenthesized subpatterns. + + +.SH ONCE-ONLY SUBPATTERNS +With both maximizing and minimizing repetition, failure of what follows +normally causes the repeated item to be re-evaluated to see if a different +number of repeats allows the rest of the pattern to match. Sometimes it is +useful to prevent this, either to change the nature of the match, or to cause +it fail earlier than it otherwise might, when the author of the pattern knows +there is no point in carrying on. + +Consider, for example, the pattern \\d+foo when applied to the subject line + + 123456bar + +After matching all 6 digits and then failing to match "foo", the normal +action of the matcher is to try again with only 5 digits matching the \\d+ +item, and then with 4, and so on, before ultimately failing. Once-only +subpatterns provide the means for specifying that once a portion of the pattern +has matched, it is not to be re-evaluated in this way, so the matcher would +give up immediately on failing to match "foo" the first time. The notation is +another kind of special parenthesis, starting with (?> as in this example: + + (?>\\d+)bar + +This kind of parenthesis "locks up" the part of the pattern it contains once +it has matched, and a failure further into the pattern is prevented from +backtracking into it. Backtracking past it to previous items, however, works as +normal. + +An alternative description is that a subpattern of this type matches the string +of characters that an identical standalone pattern would match, if anchored at +the current point in the subject string. + +Once-only subpatterns are not capturing subpatterns. Simple cases such as the +above example can be thought of as a maximizing repeat that must swallow +everything it can. So, while both \\d+ and \\d+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\\d+) can only match an entire sequence of digits. + +This construction can of course contain arbitrarily complicated subpatterns, +and it can be nested. + +Once-only subpatterns can be used in conjunction with lookbehind assertions to +specify efficient matching at the end of the subject string. Consider a simple +pattern such as + + abcd$ + +when applied to a long string which does not match it. Because matching +proceeds from left to right, PCRE will look for each "a" in the subject and +then see if what follows matches the rest of the pattern. If the pattern is +specified as + + ^.*abcd$ + +then the initial .* matches the entire string at first, but when this fails, it +backtracks to match all but the last character, then all but the last two +characters, and so on. Once again the search for "a" covers the entire string, +from right to left, so we are no better off. However, if the pattern is written +as + + ^(?>.*)(?<=abcd) + +then there can be no backtracking for the .* item; it can match only the entire +string. The subsequent lookbehind assertion does a single test on the last four +characters. If it fails, the match fails immediately. For long strings, this +approach makes a significant difference to the processing time. + + +.SH CONDITIONAL SUBPATTERNS +It is possible to cause the matching process to obey a subpattern +conditionally or to choose between two alternative subpatterns, depending on +the result of an assertion, or whether a previous capturing subpattern matched +or not. The two possible forms of conditional subpattern are + + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) + +If the condition is satisfied, the yes-pattern is used; otherwise the +no-pattern (if present) is used. If there are more than two alternatives in the +subpattern, a compile-time error occurs. + +There are two kinds of condition. If the text between the parentheses consists +of a sequence of digits, then the condition is satisfied if the capturing +subpattern of that number has previously matched. Consider the following +pattern, which contains non-significant white space to make it more readable +(assume the PCRE_EXTENDED option) and to divide it into three parts for ease +of discussion: + + ( \\( )? [^()]+ (?(1) \\) ) + +The first part matches an optional opening parenthesis, and if that +character is present, sets it as the first captured substring. The second part +matches one or more characters that are not parentheses. The third part is a +conditional subpattern that tests whether the first set of parentheses matched +or not. If they did, that is, if subject started with an opening parenthesis, +the condition is true, and so the yes-pattern is executed and a closing +parenthesis is required. Otherwise, since no-pattern is not present, the +subpattern matches nothing. In other words, this pattern matches a sequence of +non-parentheses, optionally enclosed in parentheses. + +If the condition is not a sequence of digits, it must be an assertion. This may +be a positive or negative lookahead or lookbehind assertion. Consider this +pattern, again containing non-significant white space, and with the two +alternatives on the second line: + + (?(?=[^a-z]*[a-z]) + \\d{2}-[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} ) + +The condition is a positive lookahead assertion that matches an optional +sequence of non-letters followed by a letter. In other words, it tests for the +presence of at least one letter in the subject. If a letter is found, the +subject is matched against the first alternative; otherwise it is matched +against the second. This pattern matches strings in one of the two forms +dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. + + +.SH COMMENTS +The sequence (?# marks the start of a comment which continues up to the next +closing parenthesis. Nested parentheses are not permitted. The characters +that make up a comment play no part in the pattern matching at all. + +If the PCRE_EXTENDED option is set, an unescaped # character outside a +character class introduces a comment that continues up to the next newline +character in the pattern. + + +.SH PERFORMANCE +Certain items that may appear in patterns are more efficient than others. It is +more efficient to use a character class like [aeiou] than a set of alternatives +such as (a|e|i|o|u). In general, the simplest construction that provides the +required behaviour is usually the most efficient. Jeffrey Friedl's book +contains a lot of discussion about optimizing regular expressions for efficient +performance. + +When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is +implicitly anchored by PCRE, since it can match only at the start of a subject +string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization, +because the . metacharacter does not then match a newline, and if the subject +string contains newlines, the pattern may match from the character immediately +following one of them instead of from the very start. For example, the pattern + + (.*) second + +matches the subject "first\\nand second" (where \\n stands for a newline +character) with the first captured substring being "and". In order to do this, +PCRE has to retry the match starting after every newline in the subject. + +If you are using such a pattern with subject strings that do not contain +newlines, the best performance is obtained by setting PCRE_DOTALL, or starting +the pattern with ^.* to indicate explicit anchoring. That saves PCRE from +having to scan along the subject looking for a newline to restart at. + +Beware of patterns that contain nested indefinite repeats. These can take a +long time to run when applied to a string that does not match. Consider the +pattern fragment + + (a+)* + +This can match "aaaa" in 33 different ways, and this number increases very +rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 +times, and for each of those cases other than 0, the + repeats can match +different numbers of times.) When the remainder of the pattern is such that the +entire match is going to fail, PCRE has in principle to try every possible +variation, and this can take an extremely long time. + +An optimization catches some of the more simple cases such as + + (a+)*b + +where a literal character follows. Before embarking on the standard matching +procedure, PCRE checks that there is a "b" later in the subject string, and if +there is not, it fails the match immediately. However, when there is no +following literal this optimization cannot be used. You can see the difference +by comparing the behaviour of + + (a+)*\\d + +with the pattern above. The former gives a failure almost instantly when +applied to a whole line of "a" characters, whereas the latter takes an +appreciable time with strings longer than about 20 characters. + +.SH AUTHOR +Philip Hazel <ph10@cam.ac.uk> +.br +University Computing Service, +.br +New Museums Site, +.br +Cambridge CB2 3QG, England. +.br +Phone: +44 1223 334714 + +Last updated: 29 July 1999 +.br +Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pcre.html b/doc/pcre.html new file mode 100644 index 0000000..6d91a5c --- /dev/null +++ b/doc/pcre.html @@ -0,0 +1,1977 @@ +<HTML> +<HEAD> +<TITLE>pcre specification</TITLE> +</HEAD> +<body bgcolor="#FFFFFF" text="#00005A"> +<H1>pcre specification</H1> +This HTML document has been generated automatically from the original man page. +If there is any nonsense in it, please consult the man page in case the +conversion went wrong. +<UL> +<LI><A NAME="TOC1" HREF="#SEC1">NAME</A> +<LI><A NAME="TOC2" HREF="#SEC2">SYNOPSIS</A> +<LI><A NAME="TOC3" HREF="#SEC3">DESCRIPTION</A> +<LI><A NAME="TOC4" HREF="#SEC4">MULTI-THREADING</A> +<LI><A NAME="TOC5" HREF="#SEC5">COMPILING A PATTERN</A> +<LI><A NAME="TOC6" HREF="#SEC6">STUDYING A PATTERN</A> +<LI><A NAME="TOC7" HREF="#SEC7">LOCALE SUPPORT</A> +<LI><A NAME="TOC8" HREF="#SEC8">INFORMATION ABOUT A PATTERN</A> +<LI><A NAME="TOC9" HREF="#SEC9">MATCHING A PATTERN</A> +<LI><A NAME="TOC10" HREF="#SEC10">EXTRACTING CAPTURED SUBSTRINGS</A> +<LI><A NAME="TOC11" HREF="#SEC11">LIMITATIONS</A> +<LI><A NAME="TOC12" HREF="#SEC12">DIFFERENCES FROM PERL</A> +<LI><A NAME="TOC13" HREF="#SEC13">REGULAR EXPRESSION DETAILS</A> +<LI><A NAME="TOC14" HREF="#SEC14">BACKSLASH</A> +<LI><A NAME="TOC15" HREF="#SEC15">CIRCUMFLEX AND DOLLAR</A> +<LI><A NAME="TOC16" HREF="#SEC16">FULL STOP (PERIOD, DOT)</A> +<LI><A NAME="TOC17" HREF="#SEC17">SQUARE BRACKETS</A> +<LI><A NAME="TOC18" HREF="#SEC18">VERTICAL BAR</A> +<LI><A NAME="TOC19" HREF="#SEC19">INTERNAL OPTION SETTING</A> +<LI><A NAME="TOC20" HREF="#SEC20">SUBPATTERNS</A> +<LI><A NAME="TOC21" HREF="#SEC21">REPETITION</A> +<LI><A NAME="TOC22" HREF="#SEC22">BACK REFERENCES</A> +<LI><A NAME="TOC23" HREF="#SEC23">ASSERTIONS</A> +<LI><A NAME="TOC24" HREF="#SEC24">ONCE-ONLY SUBPATTERNS</A> +<LI><A NAME="TOC25" HREF="#SEC25">CONDITIONAL SUBPATTERNS</A> +<LI><A NAME="TOC26" HREF="#SEC26">COMMENTS</A> +<LI><A NAME="TOC27" HREF="#SEC27">PERFORMANCE</A> +<LI><A NAME="TOC28" HREF="#SEC28">AUTHOR</A> +</UL> +<LI><A NAME="SEC1" HREF="#TOC1">NAME</A> +<P> +pcre - Perl-compatible regular expressions. +</P> +<LI><A NAME="SEC2" HREF="#TOC1">SYNOPSIS</A> +<P> +<B>#include <pcre.h></B> +</P> +<P> +<B>pcre *pcre_compile(const char *<I>pattern</I>, int <I>options</I>,</B> +<B>const char **<I>errptr</I>, int *<I>erroffset</I>,</B> +<B>const unsigned char *<I>tableptr</I>);</B> +</P> +<P> +<B>pcre_extra *pcre_study(const pcre *<I>code</I>, int <I>options</I>,</B> +<B>const char **<I>errptr</I>);</B> +</P> +<P> +<B>int pcre_exec(const pcre *<I>code</I>, const pcre_extra *<I>extra</I>,</B> +<B>const char *<I>subject</I>, int <I>length</I>, int <I>startoffset</I>,</B> +<B>int <I>options</I>, int *<I>ovector</I>, int <I>ovecsize</I>);</B> +</P> +<P> +<B>int pcre_copy_substring(const char *<I>subject</I>, int *<I>ovector</I>,</B> +<B>int <I>stringcount</I>, int <I>stringnumber</I>, char *<I>buffer</I>,</B> +<B>int <I>buffersize</I>);</B> +</P> +<P> +<B>int pcre_get_substring(const char *<I>subject</I>, int *<I>ovector</I>,</B> +<B>int <I>stringcount</I>, int <I>stringnumber</I>,</B> +<B>const char **<I>stringptr</I>);</B> +</P> +<P> +<B>int pcre_get_substring_list(const char *<I>subject</I>,</B> +<B>int *<I>ovector</I>, int <I>stringcount</I>, const char ***<I>listptr</I>);</B> +</P> +<P> +<B>const unsigned char *pcre_maketables(void);</B> +</P> +<P> +<B>int pcre_info(const pcre *<I>code</I>, int *<I>optptr</I>, int</B> +<B>*<I>firstcharptr</I>);</B> +</P> +<P> +<B>char *pcre_version(void);</B> +</P> +<P> +<B>void *(*pcre_malloc)(size_t);</B> +</P> +<P> +<B>void (*pcre_free)(void *);</B> +</P> +<LI><A NAME="SEC3" HREF="#TOC1">DESCRIPTION</A> +<P> +The PCRE library is a set of functions that implement regular expression +pattern matching using the same syntax and semantics as Perl 5, with just a few +differences (see below). The current implementation corresponds to Perl 5.005. +</P> +<P> +PCRE has its own native API, which is described in this document. There is also +a set of wrapper functions that correspond to the POSIX API. These are +described in the <B>pcreposix</B> documentation. +</P> +<P> +The native API function prototypes are defined in the header file <B>pcre.h</B>, +and on Unix systems the library itself is called <B>libpcre.a</B>, so can be +accessed by adding <B>-lpcre</B> to the command for linking an application which +calls it. +</P> +<P> +The functions <B>pcre_compile()</B>, <B>pcre_study()</B>, and <B>pcre_exec()</B> +are used for compiling and matching regular expressions, while +<B>pcre_copy_substring()</B>, <B>pcre_get_substring()</B>, and +<B>pcre_get_substring_list()</B> are convenience functions for extracting +captured substrings from a matched subject string. The function +<B>pcre_maketables()</B> is used (optionally) to build a set of character tables +in the current locale for passing to <B>pcre_compile()</B>. +</P> +<P> +The function <B>pcre_info()</B> is used to find out information about a compiled +pattern, while the function <B>pcre_version()</B> returns a pointer to a string +containing the version of PCRE and its date of release. +</P> +<P> +The global variables <B>pcre_malloc</B> and <B>pcre_free</B> initially contain +the entry points of the standard <B>malloc()</B> and <B>free()</B> functions +respectively. PCRE calls the memory management functions via these variables, +so a calling program can replace them if it wishes to intercept the calls. This +should be done before calling any PCRE functions. +</P> +<LI><A NAME="SEC4" HREF="#TOC1">MULTI-THREADING</A> +<P> +The PCRE functions can be used in multi-threading applications, with the +proviso that the memory management functions pointed to by <B>pcre_malloc</B> +and <B>pcre_free</B> are shared by all threads. +</P> +<P> +The compiled form of a regular expression is not altered during matching, so +the same compiled pattern can safely be used by several threads at once. +</P> +<LI><A NAME="SEC5" HREF="#TOC1">COMPILING A PATTERN</A> +<P> +The function <B>pcre_compile()</B> is called to compile a pattern into an +internal form. The pattern is a C string terminated by a binary zero, and +is passed in the argument <I>pattern</I>. A pointer to a single block of memory +that is obtained via <B>pcre_malloc</B> is returned. This contains the +compiled code and related data. The <B>pcre</B> type is defined for this for +convenience, but in fact <B>pcre</B> is just a typedef for <B>void</B>, since the +contents of the block are not externally defined. It is up to the caller to +free the memory when it is no longer required. +</P> +<P> +The size of a compiled pattern is roughly proportional to the length of the +pattern string, except that each character class (other than those containing +just a single character, negated or not) requires 33 bytes, and repeat +quantifiers with a minimum greater than one or a bounded maximum cause the +relevant portions of the compiled pattern to be replicated. +</P> +<P> +The <I>options</I> argument contains independent bits that affect the +compilation. It should be zero if no options are required. Some of the options, +in particular, those that are compatible with Perl, can also be set and unset +from within the pattern (see the detailed description of regular expressions +below). For these options, the contents of the <I>options</I> argument specifies +their initial settings at the start of compilation and execution. The +PCRE_ANCHORED option can be set at the time of matching as well as at compile +time. +</P> +<P> +If <I>errptr</I> is NULL, <B>pcre_compile()</B> returns NULL immediately. +Otherwise, if compilation of a pattern fails, <B>pcre_compile()</B> returns +NULL, and sets the variable pointed to by <I>errptr</I> to point to a textual +error message. The offset from the start of the pattern to the character where +the error was discovered is placed in the variable pointed to by +<I>erroffset</I>, which must not be NULL. If it is, an immediate error is given. +</P> +<P> +If the final argument, <I>tableptr</I>, is NULL, PCRE uses a default set of +character tables which are built when it is compiled, using the default C +locale. Otherwise, <I>tableptr</I> must be the result of a call to +<B>pcre_maketables()</B>. See the section on locale support below. +</P> +<P> +The following option bits are defined in the header file: +</P> +<P> +<PRE> + PCRE_ANCHORED +</PRE> +</P> +<P> +If this bit is set, the pattern is forced to be "anchored", that is, it is +constrained to match only at the start of the string which is being searched +(the "subject string"). This effect can also be achieved by appropriate +constructs in the pattern itself, which is the only way to do it in Perl. +</P> +<P> +<PRE> + PCRE_CASELESS +</PRE> +</P> +<P> +If this bit is set, letters in the pattern match both upper and lower case +letters. It is equivalent to Perl's /i option. +</P> +<P> +<PRE> + PCRE_DOLLAR_ENDONLY +</PRE> +</P> +<P> +If this bit is set, a dollar metacharacter in the pattern matches only at the +end of the subject string. Without this option, a dollar also matches +immediately before the final character if it is a newline (but not before any +other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is +set. There is no equivalent to this option in Perl. +</P> +<P> +<PRE> + PCRE_DOTALL +</PRE> +</P> +<P> +If this bit is set, a dot metacharater in the pattern matches all characters, +including newlines. Without it, newlines are excluded. This option is +equivalent to Perl's /s option. A negative class such as [^a] always matches a +newline character, independent of the setting of this option. +</P> +<P> +<PRE> + PCRE_EXTENDED +</PRE> +</P> +<P> +If this bit is set, whitespace data characters in the pattern are totally +ignored except when escaped or inside a character class, and characters between +an unescaped # outside a character class and the next newline character, +inclusive, are also ignored. This is equivalent to Perl's /x option, and makes +it possible to include comments inside complicated patterns. Note, however, +that this applies only to data characters. Whitespace characters may never +appear within special character sequences in a pattern, for example within the +sequence (?( which introduces a conditional subpattern. +</P> +<P> +<PRE> + PCRE_EXTRA +</PRE> +</P> +<P> +This option turns on additional functionality of PCRE that is incompatible with +Perl. Any backslash in a pattern that is followed by a letter that has no +special meaning causes an error, thus reserving these combinations for future +expansion. By default, as in Perl, a backslash followed by a letter with no +special meaning is treated as a literal. There are at present no other features +controlled by this option. +</P> +<P> +<PRE> + PCRE_MULTILINE +</PRE> +</P> +<P> +By default, PCRE treats the subject string as consisting of a single "line" of +characters (even if it actually contains several newlines). The "start of line" +metacharacter (^) matches only at the start of the string, while the "end of +line" metacharacter ($) matches only at the end of the string, or before a +terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as +Perl. +</P> +<P> +When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs +match immediately following or immediately before any newline in the subject +string, respectively, as well as at the very start and end. This is equivalent +to Perl's /m option. If there are no "\n" characters in a subject string, or +no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no +effect. +</P> +<P> +<PRE> + PCRE_UNGREEDY +</PRE> +</P> +<P> +This option inverts the "greediness" of the quantifiers so that they are not +greedy by default, but become greedy if followed by "?". It is not compatible +with Perl. It can also be set by a (?U) option setting within the pattern. +</P> +<LI><A NAME="SEC6" HREF="#TOC1">STUDYING A PATTERN</A> +<P> +When a pattern is going to be used several times, it is worth spending more +time analyzing it in order to speed up the time taken for matching. The +function <B>pcre_study()</B> takes a pointer to a compiled pattern as its first +argument, and returns a pointer to a <B>pcre_extra</B> block (another <B>void</B> +typedef) containing additional information about the pattern; this can be +passed to <B>pcre_exec()</B>. If no additional information is available, NULL +is returned. +</P> +<P> +The second argument contains option bits. At present, no options are defined +for <B>pcre_study()</B>, and this argument should always be zero. +</P> +<P> +The third argument for <B>pcre_study()</B> is a pointer to an error message. If +studying succeeds (even if no data is returned), the variable it points to is +set to NULL. Otherwise it points to a textual error message. +</P> +<P> +At present, studying a pattern is useful only for non-anchored patterns that do +not have a single fixed starting character. A bitmap of possible starting +characters is created. +</P> +<LI><A NAME="SEC7" HREF="#TOC1">LOCALE SUPPORT</A> +<P> +PCRE handles caseless matching, and determines whether characters are letters, +digits, or whatever, by reference to a set of tables. The library contains a +default set of tables which is created in the default C locale when PCRE is +compiled. This is used when the final argument of <B>pcre_compile()</B> is NULL, +and is sufficient for many applications. +</P> +<P> +An alternative set of tables can, however, be supplied. Such tables are built +by calling the <B>pcre_maketables()</B> function, which has no arguments, in the +relevant locale. The result can then be passed to <B>pcre_compile()</B> as often +as necessary. For example, to build and use tables that are appropriate for the +French locale (where accented characters with codes greater than 128 are +treated as letters), the following code could be used: +</P> +<P> +<PRE> + setlocale(LC_CTYPE, "fr"); + tables = pcre_maketables(); + re = pcre_compile(..., tables); +</PRE> +</P> +<P> +The tables are built in memory that is obtained via <B>pcre_malloc</B>. The +pointer that is passed to <B>pcre_compile</B> is saved with the compiled +pattern, and the same tables are used via this pointer by <B>pcre_study()</B> +and <B>pcre_exec()</B>. Thus for any single pattern, compilation, studying and +matching all happen in the same locale, but different patterns can be compiled +in different locales. It is the caller's responsibility to ensure that the +memory containing the tables remains available for as long as it is needed. +</P> +<LI><A NAME="SEC8" HREF="#TOC1">INFORMATION ABOUT A PATTERN</A> +<P> +The <B>pcre_info()</B> function returns information about a compiled pattern. +Its yield is the number of capturing subpatterns, or one of the following +negative numbers: +</P> +<P> +<PRE> + PCRE_ERROR_NULL the argument <I>code</I> was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found +</PRE> +</P> +<P> +If the <I>optptr</I> argument is not NULL, a copy of the options with which the +pattern was compiled is placed in the integer it points to. These option bits +are those specified in the call to <B>pcre_compile()</B>, modified by any +top-level option settings within the pattern itself, and with the PCRE_ANCHORED +bit set if the form of the pattern implies that it can match only at the start +of a subject string. +</P> +<P> +If the pattern is not anchored and the <I>firstcharptr</I> argument is not NULL, +it is used to pass back information about the first character of any matched +string. If there is a fixed first character, e.g. from a pattern such as +(cat|cow|coyote), then it is returned in the integer pointed to by +<I>firstcharptr</I>. Otherwise, if either +</P> +<P> +(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch +starts with "^", or +</P> +<P> +(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set +(if it were set, the pattern would be anchored), +</P> +<P> +then -1 is returned, indicating that the pattern matches only at the +start of a subject string or after any "\n" within the string. Otherwise -2 is +returned. +</P> +<LI><A NAME="SEC9" HREF="#TOC1">MATCHING A PATTERN</A> +<P> +The function <B>pcre_exec()</B> is called to match a subject string against a +pre-compiled pattern, which is passed in the <I>code</I> argument. If the +pattern has been studied, the result of the study should be passed in the +<I>extra</I> argument. Otherwise this must be NULL. +</P> +<P> +The PCRE_ANCHORED option can be passed in the <I>options</I> argument, whose +unused bits must be zero. However, if a pattern was compiled with +PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it +cannot be made unachored at matching time. +</P> +<P> +There are also three further options that can be set only at matching time: +</P> +<P> +<PRE> + PCRE_NOTBOL +</PRE> +</P> +<P> +The first character of the string is not the beginning of a line, so the +circumflex metacharacter should not match before it. Setting this without +PCRE_MULTILINE (at compile time) causes circumflex never to match. +</P> +<P> +<PRE> + PCRE_NOTEOL +</PRE> +</P> +<P> +The end of the string is not the end of a line, so the dollar metacharacter +should not match it nor (except in multiline mode) a newline immediately before +it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never +to match. +</P> +<P> +<PRE> + PCRE_NOTEMPTY +</PRE> +</P> +<P> +An empty string is not considered to be a valid match if this option is set. If +there are alternatives in the pattern, they are tried. If all the alternatives +match the empty string, the entire match fails. For example, if the pattern +</P> +<P> +<PRE> + a?b? +</PRE> +</P> +<P> +is applied to a string not beginning with "a" or "b", it matches the empty +string at the start of the subject. With PCRE_NOTEMPTY set, this match is not +valid, so PCRE searches further into the string for occurrences of "a" or "b". +</P> +<P> +Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case +of a pattern match of the empty string within its <B>split()</B> function, and +when using the /g modifier. It is possible to emulate Perl's behaviour after +matching a null string by first trying the match again at the same offset with +PCRE_NOTEMPTY set, and then if that fails by advancing the starting offset (see +below) and trying an ordinary match again. +</P> +<P> +The subject string is passed as a pointer in <I>subject</I>, a length in +<I>length</I>, and a starting offset in <I>startoffset</I>. Unlike the pattern +string, it may contain binary zero characters. When the starting offset is +zero, the search for a match starts at the beginning of the subject, and this +is by far the most common case. +</P> +<P> +A non-zero starting offset is useful when searching for another match in the +same subject by calling <B>pcre_exec()</B> again after a previous success. +Setting <I>startoffset</I> differs from just passing over a shortened string and +setting PCRE_NOTBOL in the case of a pattern that begins with any kind of +lookbehind. For example, consider the pattern +</P> +<P> +<PRE> + \Biss\B +</PRE> +</P> +<P> +which finds occurrences of "iss" in the middle of words. (\B matches only if +the current position in the subject is not a word boundary.) When applied to +the string "Mississipi" the first call to <B>pcre_exec()</B> finds the first +occurrence. If <B>pcre_exec()</B> is called again with just the remainder of the +subject, namely "issipi", it does not match, because \B is always false at the +start of the subject, which is deemed to be a word boundary. However, if +<B>pcre_exec()</B> is passed the entire string again, but with <I>startoffset</I> +set to 4, it finds the second occurrence of "iss" because it is able to look +behind the starting point to discover that it is preceded by a letter. +</P> +<P> +If a non-zero starting offset is passed when the pattern is anchored, one +attempt to match at the given offset is tried. This can only succeed if the +pattern does not require the match to be at the start of the subject. +</P> +<P> +In general, a pattern matches a certain portion of the subject, and in +addition, further substrings from the subject may be picked out by parts of the +pattern. Following the usage in Jeffrey Friedl's book, this is called +"capturing" in what follows, and the phrase "capturing subpattern" is used for +a fragment of a pattern that picks out a substring. PCRE supports several other +kinds of parenthesized subpattern that do not cause substrings to be captured. +</P> +<P> +Captured substrings are returned to the caller via a vector of integer offsets +whose address is passed in <I>ovector</I>. The number of elements in the vector +is passed in <I>ovecsize</I>. The first two-thirds of the vector is used to pass +back captured substrings, each substring using a pair of integers. The +remaining third of the vector is used as workspace by <B>pcre_exec()</B> while +matching capturing subpatterns, and is not available for passing back +information. The length passed in <I>ovecsize</I> should always be a multiple of +three. If it is not, it is rounded down. +</P> +<P> +When a match has been successful, information about captured substrings is +returned in pairs of integers, starting at the beginning of <I>ovector</I>, and +continuing up to two-thirds of its length at the most. The first element of a +pair is set to the offset of the first character in a substring, and the second +is set to the offset of the first character after the end of a substring. The +first pair, <I>ovector[0]</I> and <I>ovector[1]</I>, identify the portion of the +subject string matched by the entire pattern. The next pair is used for the +first capturing subpattern, and so on. The value returned by <B>pcre_exec()</B> +is the number of pairs that have been set. If there are no capturing +subpatterns, the return value from a successful match is 1, indicating that +just the first pair of offsets has been set. +</P> +<P> +Some convenience functions are provided for extracting the captured substrings +as separate strings. These are described in the following section. +</P> +<P> +It is possible for an capturing subpattern number <I>n+1</I> to match some +part of the subject when subpattern <I>n</I> has not been used at all. For +example, if the string "abc" is matched against the pattern (a|(z))(bc) +subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset +values corresponding to the unused subpattern are set to -1. +</P> +<P> +If a capturing subpattern is matched repeatedly, it is the last portion of the +string that it matched that gets returned. +</P> +<P> +If the vector is too small to hold all the captured substrings, it is used as +far as possible (up to two-thirds of its length), and the function returns a +value of zero. In particular, if the substring offsets are not of interest, +<B>pcre_exec()</B> may be called with <I>ovector</I> passed as NULL and +<I>ovecsize</I> as zero. However, if the pattern contains back references and +the <I>ovector</I> isn't big enough to remember the related substrings, PCRE has +to get additional memory for use during matching. Thus it is usually advisable +to supply an <I>ovector</I>. +</P> +<P> +Note that <B>pcre_info()</B> can be used to find out how many capturing +subpatterns there are in a compiled pattern. The smallest size for +<I>ovector</I> that will allow for <I>n</I> captured substrings in addition to +the offsets of the substring matched by the whole pattern is (<I>n</I>+1)*3. +</P> +<P> +If <B>pcre_exec()</B> fails, it returns a negative number. The following are +defined in the header file: +</P> +<P> +<PRE> + PCRE_ERROR_NOMATCH (-1) +</PRE> +</P> +<P> +The subject string did not match the pattern. +</P> +<P> +<PRE> + PCRE_ERROR_NULL (-2) +</PRE> +</P> +<P> +Either <I>code</I> or <I>subject</I> was passed as NULL, or <I>ovector</I> was +NULL and <I>ovecsize</I> was not zero. +</P> +<P> +<PRE> + PCRE_ERROR_BADOPTION (-3) +</PRE> +</P> +<P> +An unrecognized bit was set in the <I>options</I> argument. +</P> +<P> +<PRE> + PCRE_ERROR_BADMAGIC (-4) +</PRE> +</P> +<P> +PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch +the case when it is passed a junk pointer. This is the error it gives when the +magic number isn't present. +</P> +<P> +<PRE> + PCRE_ERROR_UNKNOWN_NODE (-5) +</PRE> +</P> +<P> +While running the pattern match, an unknown item was encountered in the +compiled pattern. This error could be caused by a bug in PCRE or by overwriting +of the compiled pattern. +</P> +<P> +<PRE> + PCRE_ERROR_NOMEMORY (-6) +</PRE> +</P> +<P> +If a pattern contains back references, but the <I>ovector</I> that is passed to +<B>pcre_exec()</B> is not big enough to remember the referenced substrings, PCRE +gets a block of memory at the start of matching to use for this purpose. If the +call via <B>pcre_malloc()</B> fails, this error is given. The memory is freed at +the end of matching. +</P> +<LI><A NAME="SEC10" HREF="#TOC1">EXTRACTING CAPTURED SUBSTRINGS</A> +<P> +Captured substrings can be accessed directly by using the offsets returned by +<B>pcre_exec()</B> in <I>ovector</I>. For convenience, the functions +<B>pcre_copy_substring()</B>, <B>pcre_get_substring()</B>, and +<B>pcre_get_substring_list()</B> are provided for extracting captured substrings +as new, separate, zero-terminated strings. A substring that contains a binary +zero is correctly extracted and has a further zero added on the end, but the +result does not, of course, function as a C string. +</P> +<P> +The first three arguments are the same for all three functions: <I>subject</I> +is the subject string which has just been successfully matched, <I>ovector</I> +is a pointer to the vector of integer offsets that was passed to +<B>pcre_exec()</B>, and <I>stringcount</I> is the number of substrings that +were captured by the match, including the substring that matched the entire +regular expression. This is the value returned by <B>pcre_exec</B> if it +is greater than zero. If <B>pcre_exec()</B> returned zero, indicating that it +ran out of space in <I>ovector</I>, then the value passed as +<I>stringcount</I> should be the size of the vector divided by three. +</P> +<P> +The functions <B>pcre_copy_substring()</B> and <B>pcre_get_substring()</B> +extract a single substring, whose number is given as <I>stringnumber</I>. A +value of zero extracts the substring that matched the entire pattern, while +higher values extract the captured substrings. For <B>pcre_copy_substring()</B>, +the string is placed in <I>buffer</I>, whose length is given by +<I>buffersize</I>, while for <B>pcre_get_substring()</B> a new block of store is +obtained via <B>pcre_malloc</B>, and its address is returned via +<I>stringptr</I>. The yield of the function is the length of the string, not +including the terminating zero, or one of +</P> +<P> +<PRE> + PCRE_ERROR_NOMEMORY (-6) +</PRE> +</P> +<P> +The buffer was too small for <B>pcre_copy_substring()</B>, or the attempt to get +memory failed for <B>pcre_get_substring()</B>. +</P> +<P> +<PRE> + PCRE_ERROR_NOSUBSTRING (-7) +</PRE> +</P> +<P> +There is no substring whose number is <I>stringnumber</I>. +</P> +<P> +The <B>pcre_get_substring_list()</B> function extracts all available substrings +and builds a list of pointers to them. All this is done in a single block of +memory which is obtained via <B>pcre_malloc</B>. The address of the memory block +is returned via <I>listptr</I>, which is also the start of the list of string +pointers. The end of the list is marked by a NULL pointer. The yield of the +function is zero if all went well, or +</P> +<P> +<PRE> + PCRE_ERROR_NOMEMORY (-6) +</PRE> +</P> +<P> +if the attempt to get the memory block failed. +</P> +<P> +When any of these functions encounter a substring that is unset, which can +happen when capturing subpattern number <I>n+1</I> matches some part of the +subject, but subpattern <I>n</I> has not been used at all, they return an empty +string. This can be distinguished from a genuine zero-length substring by +inspecting the appropriate offset in <I>ovector</I>, which is negative for unset +substrings. +</P> +<LI><A NAME="SEC11" HREF="#TOC1">LIMITATIONS</A> +<P> +There are some size limitations in PCRE but it is hoped that they will never in +practice be relevant. +The maximum length of a compiled pattern is 65539 (sic) bytes. +All values in repeating quantifiers must be less than 65536. +The maximum number of capturing subpatterns is 99. +The maximum number of all parenthesized subpatterns, including capturing +subpatterns, assertions, and other types of subpattern, is 200. +</P> +<P> +The maximum length of a subject string is the largest positive number that an +integer variable can hold. However, PCRE uses recursion to handle subpatterns +and indefinite repetition. This means that the available stack space may limit +the size of a subject string that can be processed by certain patterns. +</P> +<LI><A NAME="SEC12" HREF="#TOC1">DIFFERENCES FROM PERL</A> +<P> +The differences described here are with respect to Perl 5.005. +</P> +<P> +1. By default, a whitespace character is any character that the C library +function <B>isspace()</B> recognizes, though it is possible to compile PCRE with +alternative character type tables. Normally <B>isspace()</B> matches space, +formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 +no longer includes vertical tab in its set of whitespace characters. The \v +escape that was in the Perl documentation for a long time was never in fact +recognized. However, the character itself was treated as whitespace at least +up to 5.002. In 5.004 and 5.005 it does not match \s. +</P> +<P> +2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits +them, but they do not mean what you might think. For example, (?!a){3} does +not assert that the next three characters are not "a". It just asserts that the +next character is not "a" three times. +</P> +<P> +3. Capturing subpatterns that occur inside negative lookahead assertions are +counted, but their entries in the offsets vector are never set. Perl sets its +numerical variables from any such patterns that are matched before the +assertion fails to match something (thereby succeeding), but only if the +negative lookahead assertion contains just one branch. +</P> +<P> +4. Though binary zero characters are supported in the subject string, they are +not allowed in a pattern string because it is passed as a normal C string, +terminated by zero. The escape sequence "\0" can be used in the pattern to +represent a binary zero. +</P> +<P> +5. The following Perl escape sequences are not supported: \l, \u, \L, \U, +\E, \Q. In fact these are implemented by Perl's general string-handling and +are not part of its pattern matching engine. +</P> +<P> +6. The Perl \G assertion is not supported as it is not relevant to single +pattern matches. +</P> +<P> +7. Fairly obviously, PCRE does not support the (?{code}) construction. +</P> +<P> +8. There are at the time of writing some oddities in Perl 5.005_02 concerned +with the settings of captured strings when part of a pattern is repeated. For +example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value +"b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if +the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set. +</P> +<P> +In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the +future Perl changes to a consistent state that is different, PCRE may change to +follow. +</P> +<P> +9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern +/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. +However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset. +</P> +<P> +10. PCRE provides some extensions to the Perl regular expression facilities: +</P> +<P> +(a) Although lookbehind assertions must match fixed length strings, each +alternative branch of a lookbehind assertion can match a different length of +string. Perl 5.005 requires them all to have the same length. +</P> +<P> +(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- +character matches only at the very end of the string. +</P> +<P> +(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special +meaning is faulted. +</P> +<P> +(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is +inverted, that is, by default they are not greedy, but if followed by a +question mark they are. +</P> +<P> +(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start +of the subject. +</P> +<P> +(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for +<B>pcre_exec()</B> have no Perl equivalents. +</P> +<LI><A NAME="SEC13" HREF="#TOC1">REGULAR EXPRESSION DETAILS</A> +<P> +The syntax and semantics of the regular expressions supported by PCRE are +described below. Regular expressions are also described in the Perl +documentation and in a number of other books, some of which have copious +examples. Jeffrey Friedl's "Mastering Regular Expressions", published by +O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description +here is intended as reference documentation. +</P> +<P> +A regular expression is a pattern that is matched against a subject string from +left to right. Most characters stand for themselves in a pattern, and match the +corresponding characters in the subject. As a trivial example, the pattern +</P> +<P> +<PRE> + The quick brown fox +</PRE> +</P> +<P> +matches a portion of a subject string that is identical to itself. The power of +regular expressions comes from the ability to include alternatives and +repetitions in the pattern. These are encoded in the pattern by the use of +<I>meta-characters</I>, which do not stand for themselves but instead are +interpreted in some special way. +</P> +<P> +There are two different sets of meta-characters: those that are recognized +anywhere in the pattern except within square brackets, and those that are +recognized in square brackets. Outside square brackets, the meta-characters are +as follows: +</P> +<P> +<PRE> + \ general escape character with several uses + ^ assert start of subject (or line, in multiline mode) + $ assert end of subject (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + { start min/max quantifier +</PRE> +</P> +<P> +Part of a pattern that is in square brackets is called a "character class". In +a character class the only meta-characters are: +</P> +<P> +<PRE> + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + ] terminates the character class +</PRE> +</P> +<P> +The following sections describe the use of each of the meta-characters. +</P> +<LI><A NAME="SEC14" HREF="#TOC1">BACKSLASH</A> +<P> +The backslash character has several uses. Firstly, if it is followed by a +non-alphameric character, it takes away any special meaning that character may +have. This use of backslash as an escape character applies both inside and +outside character classes. +</P> +<P> +For example, if you want to match a "*" character, you write "\*" in the +pattern. This applies whether or not the following character would otherwise be +interpreted as a meta-character, so it is always safe to precede a +non-alphameric with "\" to specify that it stands for itself. In particular, +if you want to match a backslash, you write "\\". +</P> +<P> +If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the +pattern (other than in a character class) and characters between a "#" outside +a character class and the next newline character are ignored. An escaping +backslash can be used to include a whitespace or "#" character as part of the +pattern. +</P> +<P> +A second use of backslash provides a way of encoding non-printing characters +in patterns in a visible manner. There is no restriction on the appearance of +non-printing characters, apart from the binary zero that terminates a pattern, +but when a pattern is being prepared by text editing, it is usually easier to +use one of the following escape sequences than the binary character it +represents: +</P> +<P> +<PRE> + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n newline (hex 0A) + \r carriage return (hex 0D) + \t tab (hex 09) + \xhh character with hex code hh + \ddd character with octal code ddd, or backreference +</PRE> +</P> +<P> +The precise effect of "\cx" is as follows: if "x" is a lower case letter, it +is converted to upper case. Then bit 6 of the character (hex 40) is inverted. +Thus "\cz" becomes hex 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex +7B. +</P> +<P> +After "\x", up to two hexadecimal digits are read (letters can be in upper or +lower case). +</P> +<P> +After "\0" up to two further octal digits are read. In both cases, if there +are fewer than two digits, just those that are present are used. Thus the +sequence "\0\x\07" specifies two binary zeros followed by a BEL character. +Make sure you supply two digits after the initial zero if the character that +follows is itself an octal digit. +</P> +<P> +The handling of a backslash followed by a digit other than 0 is complicated. +Outside a character class, PCRE reads it and any following digits as a decimal +number. If the number is less than 10, or if there have been at least that many +previous capturing left parentheses in the expression, the entire sequence is +taken as a <I>back reference</I>. A description of how this works is given +later, following the discussion of parenthesized subpatterns. +</P> +<P> +Inside a character class, or if the decimal number is greater than 9 and there +have not been that many capturing subpatterns, PCRE re-reads up to three octal +digits following the backslash, and generates a single byte from the least +significant 8 bits of the value. Any subsequent digits stand for themselves. +For example: +</P> +<P> +<PRE> + \040 is another way of writing a space + \40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of + writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 is the character with octal code 113 (since there + can be no more than 99 back references) + \377 is a byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" +</PRE> +</P> +<P> +Note that octal values of 100 or greater must not be introduced by a leading +zero, because no more than three octal digits are ever read. +</P> +<P> +All the sequences that define a single byte value can be used both inside and +outside character classes. In addition, inside a character class, the sequence +"\b" is interpreted as the backspace character (hex 08). Outside a character +class it has a different meaning (see below). +</P> +<P> +The third use of backslash is for specifying generic character types: +</P> +<P> +<PRE> + \d any decimal digit + \D any character that is not a decimal digit + \s any whitespace character + \S any character that is not a whitespace character + \w any "word" character + \W any "non-word" character +</PRE> +</P> +<P> +Each pair of escape sequences partitions the complete set of characters into +two disjoint sets. Any given character matches one, and only one, of each pair. +</P> +<P> +A "word" character is any letter or digit or the underscore character, that is, +any character which can be part of a Perl "word". The definition of letters and +digits is controlled by PCRE's character tables, and may vary if locale- +specific matching is taking place (see "Locale support" above). For example, in +the "fr" (French) locale, some character codes greater than 128 are used for +accented letters, and these are matched by \w. +</P> +<P> +These character type sequences can appear both inside and outside character +classes. They each match one character of the appropriate type. If the current +matching point is at the end of the subject string, all of them fail, since +there is no character to match. +</P> +<P> +The fourth use of backslash is for certain simple assertions. An assertion +specifies a condition that has to be met at a particular point in a match, +without consuming any characters from the subject string. The use of +subpatterns for more complicated assertions is described below. The backslashed +assertions are +</P> +<P> +<PRE> + \b word boundary + \B not a word boundary + \A start of subject (independent of multiline mode) + \Z end of subject or newline at end (independent of multiline mode) + \z end of subject (independent of multiline mode) +</PRE> +</P> +<P> +These assertions may not appear in character classes (but note that "\b" has a +different meaning, namely the backspace character, inside a character class). +</P> +<P> +A word boundary is a position in the subject string where the current character +and the previous character do not both match \w or \W (i.e. one matches +\w and the other matches \W), or the start or end of the string if the +first or last character matches \w, respectively. +</P> +<P> +The \A, \Z, and \z assertions differ from the traditional circumflex and +dollar (described below) in that they only ever match at the very start and end +of the subject string, whatever options are set. They are not affected by the +PCRE_NOTBOL or PCRE_NOTEOL options. If the <I>startoffset</I> argument of +<B>pcre_exec()</B> is non-zero, \A can never match. The difference between \Z +and \z is that \Z matches before a newline that is the last character of the +string as well as at the end of the string, whereas \z matches only at the +end. +</P> +<LI><A NAME="SEC15" HREF="#TOC1">CIRCUMFLEX AND DOLLAR</A> +<P> +Outside a character class, in the default matching mode, the circumflex +character is an assertion which is true only if the current matching point is +at the start of the subject string. If the <I>startoffset</I> argument of +<B>pcre_exec()</B> is non-zero, circumflex can never match. Inside a character +class, circumflex has an entirely different meaning (see below). +</P> +<P> +Circumflex need not be the first character of the pattern if a number of +alternatives are involved, but it should be the first thing in each alternative +in which it appears if the pattern is ever to match that branch. If all +possible alternatives start with a circumflex, that is, if the pattern is +constrained to match only at the start of the subject, it is said to be an +"anchored" pattern. (There are also other constructs that can cause a pattern +to be anchored.) +</P> +<P> +A dollar character is an assertion which is true only if the current matching +point is at the end of the subject string, or immediately before a newline +character that is the last character in the string (by default). Dollar need +not be the last character of the pattern if a number of alternatives are +involved, but it should be the last item in any branch in which it appears. +Dollar has no special meaning in a character class. +</P> +<P> +The meaning of dollar can be changed so that it matches only at the very end of +the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching +time. This does not affect the \Z assertion. +</P> +<P> +The meanings of the circumflex and dollar characters are changed if the +PCRE_MULTILINE option is set. When this is the case, they match immediately +after and immediately before an internal "\n" character, respectively, in +addition to matching at the start and end of the subject string. For example, +the pattern /^abc$/ matches the subject string "def\nabc" in multiline mode, +but not otherwise. Consequently, patterns that are anchored in single line mode +because all branches start with "^" are not anchored in multiline mode, and a +match for circumflex is possible when the <I>startoffset</I> argument of +<B>pcre_exec()</B> is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if +PCRE_MULTILINE is set. +</P> +<P> +Note that the sequences \A, \Z, and \z can be used to match the start and +end of the subject in both modes, and if all branches of a pattern start with +\A is it always anchored, whether PCRE_MULTILINE is set or not. +</P> +<LI><A NAME="SEC16" HREF="#TOC1">FULL STOP (PERIOD, DOT)</A> +<P> +Outside a character class, a dot in the pattern matches any one character in +the subject, including a non-printing character, but not (by default) newline. +If the PCRE_DOTALL option is set, then dots match newlines as well. The +handling of dot is entirely independent of the handling of circumflex and +dollar, the only relationship being that they both involve newline characters. +Dot has no special meaning in a character class. +</P> +<LI><A NAME="SEC17" HREF="#TOC1">SQUARE BRACKETS</A> +<P> +An opening square bracket introduces a character class, terminated by a closing +square bracket. A closing square bracket on its own is not special. If a +closing square bracket is required as a member of the class, it should be the +first data character in the class (after an initial circumflex, if present) or +escaped with a backslash. +</P> +<P> +A character class matches a single character in the subject; the character must +be in the set of characters defined by the class, unless the first character in +the class is a circumflex, in which case the subject character must not be in +the set defined by the class. If a circumflex is actually required as a member +of the class, ensure it is not the first character, or escape it with a +backslash. +</P> +<P> +For example, the character class [aeiou] matches any lower case vowel, while +[^aeiou] matches any character that is not a lower case vowel. Note that a +circumflex is just a convenient notation for specifying the characters which +are in the class by enumerating those that are not. It is not an assertion: it +still consumes a character from the subject string, and fails if the current +pointer is at the end of the string. +</P> +<P> +When caseless matching is set, any letters in a class represent both their +upper case and lower case versions, so for example, a caseless [aeiou] matches +"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a +caseful version would. +</P> +<P> +The newline character is never treated in any special way in character classes, +whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class +such as [^a] will always match a newline. +</P> +<P> +The minus (hyphen) character can be used to specify a range of characters in a +character class. For example, [d-m] matches any letter between d and m, +inclusive. If a minus character is required in a class, it must be escaped with +a backslash or appear in a position where it cannot be interpreted as +indicating a range, typically as the first or last character in the class. +</P> +<P> +It is not possible to have the literal character "]" as the end character of a +range. A pattern such as [W-]46] is interpreted as a class of two characters +("W" and "-") followed by a literal string "46]", so it would match "W46]" or +"-46]". However, if the "]" is escaped with a backslash it is interpreted as +the end of range, so [W-\]46] is interpreted as a single class containing a +range followed by two separate characters. The octal or hexadecimal +representation of "]" can also be used to end a range. +</P> +<P> +Ranges operate in ASCII collating sequence. They can also be used for +characters specified numerically, for example [\000-\037]. If a range that +includes letters is used when caseless matching is set, it matches the letters +in either case. For example, [W-c] is equivalent to [][\^_`wxyzabc], matched +caselessly, and if character tables for the "fr" locale are in use, +[\xc8-\xcb] matches accented E characters in both cases. +</P> +<P> +The character types \d, \D, \s, \S, \w, and \W may also appear in a +character class, and add the characters that they match to the class. For +example, [\dABCDEF] matches any hexadecimal digit. A circumflex can +conveniently be used with the upper case character types to specify a more +restricted set of characters than the matching lower case type. For example, +the class [^\W_] matches any letter or digit, but not underscore. +</P> +<P> +All non-alphameric characters other than \, -, ^ (at the start) and the +terminating ] are non-special in character classes, but it does no harm if they +are escaped. +</P> +<LI><A NAME="SEC18" HREF="#TOC1">VERTICAL BAR</A> +<P> +Vertical bar characters are used to separate alternative patterns. For example, +the pattern +</P> +<P> +<PRE> + gilbert|sullivan +</PRE> +</P> +<P> +matches either "gilbert" or "sullivan". Any number of alternatives may appear, +and an empty alternative is permitted (matching the empty string). +The matching process tries each alternative in turn, from left to right, +and the first one that succeeds is used. If the alternatives are within a +subpattern (defined below), "succeeds" means matching the rest of the main +pattern as well as the alternative in the subpattern. +</P> +<LI><A NAME="SEC19" HREF="#TOC1">INTERNAL OPTION SETTING</A> +<P> +The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED +can be changed from within the pattern by a sequence of Perl option letters +enclosed between "(?" and ")". The option letters are +</P> +<P> +<PRE> + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED +</PRE> +</P> +<P> +For example, (?im) sets caseless, multiline matching. It is also possible to +unset these options by preceding the letter with a hyphen, and a combined +setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and +PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also +permitted. If a letter appears both before and after the hyphen, the option is +unset. +</P> +<P> +The scope of these option changes depends on where in the pattern the setting +occurs. For settings that are outside any subpattern (defined below), the +effect is the same as if the options were set or unset at the start of +matching. The following patterns all behave in exactly the same way: +</P> +<P> +<PRE> + (?i)abc + a(?i)bc + ab(?i)c + abc(?i) +</PRE> +</P> +<P> +which in turn is the same as compiling the pattern abc with PCRE_CASELESS set. +In other words, such "top level" settings apply to the whole pattern (unless +there are other changes inside subpatterns). If there is more than one setting +of the same option at top level, the rightmost setting is used. +</P> +<P> +If an option change occurs inside a subpattern, the effect is different. This +is a change of behaviour in Perl 5.005. An option change inside a subpattern +affects only that part of the subpattern that follows it, so +</P> +<P> +<PRE> + (a(?i)b)c +</PRE> +</P> +<P> +matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). +By this means, options can be made to have different settings in different +parts of the pattern. Any changes made in one alternative do carry on +into subsequent branches within the same subpattern. For example, +</P> +<P> +<PRE> + (a(?i)b|c) +</PRE> +</P> +<P> +matches "ab", "aB", "c", and "C", even though when matching "C" the first +branch is abandoned before the option setting. This is because the effects of +option settings happen at compile time. There would be some very weird +behaviour otherwise. +</P> +<P> +The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the +same way as the Perl-compatible options by using the characters U and X +respectively. The (?X) flag setting is special in that it must always occur +earlier in the pattern than any of the additional features it turns on, even +when it is at top level. It is best put at the start. +</P> +<LI><A NAME="SEC20" HREF="#TOC1">SUBPATTERNS</A> +<P> +Subpatterns are delimited by parentheses (round brackets), which can be nested. +Marking part of a pattern as a subpattern does two things: +</P> +<P> +1. It localizes a set of alternatives. For example, the pattern +</P> +<P> +<PRE> + cat(aract|erpillar|) +</PRE> +</P> +<P> +matches one of the words "cat", "cataract", or "caterpillar". Without the +parentheses, it would match "cataract", "erpillar" or the empty string. +</P> +<P> +2. It sets up the subpattern as a capturing subpattern (as defined above). +When the whole pattern matches, that portion of the subject string that matched +the subpattern is passed back to the caller via the <I>ovector</I> argument of +<B>pcre_exec()</B>. Opening parentheses are counted from left to right (starting +from 1) to obtain the numbers of the capturing subpatterns. +</P> +<P> +For example, if the string "the red king" is matched against the pattern +</P> +<P> +<PRE> + the ((red|white) (king|queen)) +</PRE> +</P> +<P> +the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3. +</P> +<P> +The fact that plain parentheses fulfil two functions is not always helpful. +There are often times when a grouping subpattern is required without a +capturing requirement. If an opening parenthesis is followed by "?:", the +subpattern does not do any capturing, and is not counted when computing the +number of any subsequent capturing subpatterns. For example, if the string "the +white queen" is matched against the pattern +</P> +<P> +<PRE> + the ((?:red|white) (king|queen)) +</PRE> +</P> +<P> +the captured substrings are "white queen" and "queen", and are numbered 1 and +2. The maximum number of captured substrings is 99, and the maximum number of +all subpatterns, both capturing and non-capturing, is 200. +</P> +<P> +As a convenient shorthand, if any option settings are required at the start of +a non-capturing subpattern, the option letters may appear between the "?" and +the ":". Thus the two patterns +</P> +<P> +<PRE> + (?i:saturday|sunday) + (?:(?i)saturday|sunday) +</PRE> +</P> +<P> +match exactly the same set of strings. Because alternative branches are tried +from left to right, and options are not reset until the end of the subpattern +is reached, an option setting in one branch does affect subsequent branches, so +the above patterns match "SUNDAY" as well as "Saturday". +</P> +<LI><A NAME="SEC21" HREF="#TOC1">REPETITION</A> +<P> +Repetition is specified by quantifiers, which can follow any of the following +items: +</P> +<P> +<PRE> + a single character, possibly escaped + the . metacharacter + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion - see below) +</PRE> +</P> +<P> +The general repetition quantifier specifies a minimum and maximum number of +permitted matches, by giving the two numbers in curly brackets (braces), +separated by a comma. The numbers must be less than 65536, and the first must +be less than or equal to the second. For example: +</P> +<P> +<PRE> + z{2,4} +</PRE> +</P> +<P> +matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special +character. If the second number is omitted, but the comma is present, there is +no upper limit; if the second number and the comma are both omitted, the +quantifier specifies an exact number of required matches. Thus +</P> +<P> +<PRE> + [aeiou]{3,} +</PRE> +</P> +<P> +matches at least 3 successive vowels, but may match many more, while +</P> +<P> +<PRE> + \d{8} +</PRE> +</P> +<P> +matches exactly 8 digits. An opening curly bracket that appears in a position +where a quantifier is not allowed, or one that does not match the syntax of a +quantifier, is taken as a literal character. For example, {,6} is not a +quantifier, but a literal string of four characters. +</P> +<P> +The quantifier {0} is permitted, causing the expression to behave as if the +previous item and the quantifier were not present. +</P> +<P> +For convenience (and historical compatibility) the three most common +quantifiers have single-character abbreviations: +</P> +<P> +<PRE> + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} +</PRE> +</P> +<P> +It is possible to construct infinite loops by following a subpattern that can +match no characters with a quantifier that has no upper limit, for example: +</P> +<P> +<PRE> + (a?)* +</PRE> +</P> +<P> +Earlier versions of Perl and PCRE used to give an error at compile time for +such patterns. However, because there are cases where this can be useful, such +patterns are now accepted, but if any repetition of the subpattern does in fact +match no characters, the loop is forcibly broken. +</P> +<P> +By default, the quantifiers are "greedy", that is, they match as much as +possible (up to the maximum number of permitted times), without causing the +rest of the pattern to fail. The classic example of where this gives problems +is in trying to match comments in C programs. These appear between the +sequences /* and */ and within the sequence, individual * and / characters may +appear. An attempt to match C comments by applying the pattern +</P> +<P> +<PRE> + /\*.*\*/ +</PRE> +</P> +<P> +to the string +</P> +<P> +<PRE> + /* first command */ not comment /* second comment */ +</PRE> +</P> +<P> +fails, because it matches the entire string due to the greediness of the .* +item. +</P> +<P> +However, if a quantifier is followed by a question mark, then it ceases to be +greedy, and instead matches the minimum number of times possible, so the +pattern +</P> +<P> +<PRE> + /\*.*?\*/ +</PRE> +</P> +<P> +does the right thing with the C comments. The meaning of the various +quantifiers is not otherwise changed, just the preferred number of matches. +Do not confuse this use of question mark with its use as a quantifier in its +own right. Because it has two uses, it can sometimes appear doubled, as in +</P> +<P> +<PRE> + \d??\d +</PRE> +</P> +<P> +which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. +</P> +<P> +If the PCRE_UNGREEDY option is set (an option which is not available in Perl) +then the quantifiers are not greedy by default, but individual ones can be made +greedy by following them with a question mark. In other words, it inverts the +default behaviour. +</P> +<P> +When a parenthesized subpattern is quantified with a minimum repeat count that +is greater than 1 or with a limited maximum, more store is required for the +compiled pattern, in proportion to the size of the minimum or maximum. +</P> +<P> +If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent +to Perl's /s) is set, thus allowing the . to match newlines, then the pattern +is implicitly anchored, because whatever follows will be tried against every +character position in the subject string, so there is no point in retrying the +overall match at any position after the first. PCRE treats such a pattern as +though it were preceded by \A. In cases where it is known that the subject +string contains no newlines, it is worth setting PCRE_DOTALL when the pattern +begins with .* in order to obtain this optimization, or alternatively using ^ +to indicate anchoring explicitly. +</P> +<P> +When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after +</P> +<P> +<PRE> + (tweedle[dume]{3}\s*)+ +</PRE> +</P> +<P> +has matched "tweedledum tweedledee" the value of the captured substring is +"tweedledee". However, if there are nested capturing subpatterns, the +corresponding captured values may have been set in previous iterations. For +example, after +</P> +<P> +<PRE> + /(a|(b))+/ +</PRE> +</P> +<P> +matches "aba" the value of the second captured substring is "b". +</P> +<LI><A NAME="SEC22" HREF="#TOC1">BACK REFERENCES</A> +<P> +Outside a character class, a backslash followed by a digit greater than 0 (and +possibly further digits) is a back reference to a capturing subpattern earlier +(i.e. to its left) in the pattern, provided there have been that many previous +capturing left parentheses. +</P> +<P> +However, if the decimal number following the backslash is less than 10, it is +always taken as a back reference, and causes an error only if there are not +that many capturing left parentheses in the entire pattern. In other words, the +parentheses that are referenced need not be to the left of the reference for +numbers less than 10. See the section entitled "Backslash" above for further +details of the handling of digits following a backslash. +</P> +<P> +A back reference matches whatever actually matched the capturing subpattern in +the current subject string, rather than anything matching the subpattern +itself. So the pattern +</P> +<P> +<PRE> + (sens|respons)e and \1ibility +</PRE> +</P> +<P> +matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If caseful matching is in force at the time of the +back reference, then the case of letters is relevant. For example, +</P> +<P> +<PRE> + ((?i)rah)\s+\1 +</PRE> +</P> +<P> +matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. +</P> +<P> +There may be more than one back reference to the same subpattern. If a +subpattern has not actually been used in a particular match, then any back +references to it always fail. For example, the pattern +</P> +<P> +<PRE> + (a|(bc))\2 +</PRE> +</P> +<P> +always fails if it starts to match "a" rather than "bc". Because there may be +up to 99 back references, all digits following the backslash are taken +as part of a potential back reference number. If the pattern continues with a +digit character, then some delimiter must be used to terminate the back +reference. If the PCRE_EXTENDED option is set, this can be whitespace. +Otherwise an empty comment can be used. +</P> +<P> +A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern +</P> +<P> +<PRE> + (a|b\1)+ +</PRE> +</P> +<P> +matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of +the subpattern, the back reference matches the character string corresponding +to the previous iteration. In order for this to work, the pattern must be such +that the first iteration does not need to match the back reference. This can be +done using alternation, as in the example above, or by a quantifier with a +minimum of zero. +</P> +<LI><A NAME="SEC23" HREF="#TOC1">ASSERTIONS</A> +<P> +An assertion is a test on the characters following or preceding the current +matching point that does not actually consume any characters. The simple +assertions coded as \b, \B, \A, \Z, \z, ^ and $ are described above. More +complicated assertions are coded as subpatterns. There are two kinds: those +that look ahead of the current position in the subject string, and those that +look behind it. +</P> +<P> +An assertion subpattern is matched in the normal way, except that it does not +cause the current matching position to be changed. Lookahead assertions start +with (?= for positive assertions and (?! for negative assertions. For example, +</P> +<P> +<PRE> + \w+(?=;) +</PRE> +</P> +<P> +matches a word followed by a semicolon, but does not include the semicolon in +the match, and +</P> +<P> +<PRE> + foo(?!bar) +</PRE> +</P> +<P> +matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern +</P> +<P> +<PRE> + (?!foo)bar +</PRE> +</P> +<P> +does not find an occurrence of "bar" that is preceded by something other than +"foo"; it finds any occurrence of "bar" whatsoever, because the assertion +(?!foo) is always true when the next three characters are "bar". A +lookbehind assertion is needed to achieve this effect. +</P> +<P> +Lookbehind assertions start with (?<= for positive assertions and (?<! for +negative assertions. For example, +</P> +<P> +<PRE> + (?<!foo)bar +</PRE> +</P> +<P> +does find an occurrence of "bar" that is not preceded by "foo". The contents of +a lookbehind assertion are restricted such that all the strings it matches must +have a fixed length. However, if there are several alternatives, they do not +all have to have the same fixed length. Thus +</P> +<P> +<PRE> + (?<=bullock|donkey) +</PRE> +</P> +<P> +is permitted, but +</P> +<P> +<PRE> + (?<!dogs?|cats?) +</PRE> +</P> +<P> +causes an error at compile time. Branches that match different length strings +are permitted only at the top level of a lookbehind assertion. This is an +extension compared with Perl 5.005, which requires all branches to match the +same length of string. An assertion such as +</P> +<P> +<PRE> + (?<=ab(c|de)) +</PRE> +</P> +<P> +is not permitted, because its single top-level branch can match two different +lengths, but it is acceptable if rewritten to use two top-level branches: +</P> +<P> +<PRE> + (?<=abc|abde) +</PRE> +</P> +<P> +The implementation of lookbehind assertions is, for each alternative, to +temporarily move the current position back by the fixed width and then try to +match. If there are insufficient characters before the current position, the +match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns +can be particularly useful for matching at the ends of strings; an example is +given at the end of the section on once-only subpatterns. +</P> +<P> +Several assertions (of any sort) may occur in succession. For example, +</P> +<P> +<PRE> + (?<=\d{3})(?<!999)foo +</PRE> +</P> +<P> +matches "foo" preceded by three digits that are not "999". Notice that each of +the assertions is applied independently at the same point in the subject +string. First there is a check that the previous three characters are all +digits, then there is a check that the same three characters are not "999". +This pattern does <I>not</I> match "foo" preceded by six characters, the first +of which are digits and the last three of which are not "999". For example, it +doesn't match "123abcfoo". A pattern to do that is +</P> +<P> +<PRE> + (?<=\d{3}...)(?<!999)foo +</PRE> +</P> +<P> +This time the first assertion looks at the preceding six characters, checking +that the first three are digits, and then the second assertion checks that the +preceding three characters are not "999". +</P> +<P> +Assertions can be nested in any combination. For example, +</P> +<P> +<PRE> + (?<=(?<!foo)bar)baz +</PRE> +</P> +<P> +matches an occurrence of "baz" that is preceded by "bar" which in turn is not +preceded by "foo", while +</P> +<P> +<PRE> + (?<=\d{3}(?!999)...)foo +</PRE> +</P> +<P> +is another pattern which matches "foo" preceded by three digits and any three +characters that are not "999". +</P> +<P> +Assertion subpatterns are not capturing subpatterns, and may not be repeated, +because it makes no sense to assert the same thing several times. If any kind +of assertion contains capturing subpatterns within it, these are counted for +the purposes of numbering the capturing subpatterns in the whole pattern. +However, substring capturing is carried out only for positive assertions, +because it does not make sense for negative assertions. +</P> +<P> +Assertions count towards the maximum of 200 parenthesized subpatterns. +</P> +<LI><A NAME="SEC24" HREF="#TOC1">ONCE-ONLY SUBPATTERNS</A> +<P> +With both maximizing and minimizing repetition, failure of what follows +normally causes the repeated item to be re-evaluated to see if a different +number of repeats allows the rest of the pattern to match. Sometimes it is +useful to prevent this, either to change the nature of the match, or to cause +it fail earlier than it otherwise might, when the author of the pattern knows +there is no point in carrying on. +</P> +<P> +Consider, for example, the pattern \d+foo when applied to the subject line +</P> +<P> +<PRE> + 123456bar +</PRE> +</P> +<P> +After matching all 6 digits and then failing to match "foo", the normal +action of the matcher is to try again with only 5 digits matching the \d+ +item, and then with 4, and so on, before ultimately failing. Once-only +subpatterns provide the means for specifying that once a portion of the pattern +has matched, it is not to be re-evaluated in this way, so the matcher would +give up immediately on failing to match "foo" the first time. The notation is +another kind of special parenthesis, starting with (?> as in this example: +</P> +<P> +<PRE> + (?>\d+)bar +</PRE> +</P> +<P> +This kind of parenthesis "locks up" the part of the pattern it contains once +it has matched, and a failure further into the pattern is prevented from +backtracking into it. Backtracking past it to previous items, however, works as +normal. +</P> +<P> +An alternative description is that a subpattern of this type matches the string +of characters that an identical standalone pattern would match, if anchored at +the current point in the subject string. +</P> +<P> +Once-only subpatterns are not capturing subpatterns. Simple cases such as the +above example can be thought of as a maximizing repeat that must swallow +everything it can. So, while both \d+ and \d+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\d+) can only match an entire sequence of digits. +</P> +<P> +This construction can of course contain arbitrarily complicated subpatterns, +and it can be nested. +</P> +<P> +Once-only subpatterns can be used in conjunction with lookbehind assertions to +specify efficient matching at the end of the subject string. Consider a simple +pattern such as +</P> +<P> +<PRE> + abcd$ +</PRE> +</P> +<P> +when applied to a long string which does not match it. Because matching +proceeds from left to right, PCRE will look for each "a" in the subject and +then see if what follows matches the rest of the pattern. If the pattern is +specified as +</P> +<P> +<PRE> + ^.*abcd$ +</PRE> +</P> +<P> +then the initial .* matches the entire string at first, but when this fails, it +backtracks to match all but the last character, then all but the last two +characters, and so on. Once again the search for "a" covers the entire string, +from right to left, so we are no better off. However, if the pattern is written +as +</P> +<P> +<PRE> + ^(?>.*)(?<=abcd) +</PRE> +</P> +<P> +then there can be no backtracking for the .* item; it can match only the entire +string. The subsequent lookbehind assertion does a single test on the last four +characters. If it fails, the match fails immediately. For long strings, this +approach makes a significant difference to the processing time. +</P> +<LI><A NAME="SEC25" HREF="#TOC1">CONDITIONAL SUBPATTERNS</A> +<P> +It is possible to cause the matching process to obey a subpattern +conditionally or to choose between two alternative subpatterns, depending on +the result of an assertion, or whether a previous capturing subpattern matched +or not. The two possible forms of conditional subpattern are +</P> +<P> +<PRE> + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) +</PRE> +</P> +<P> +If the condition is satisfied, the yes-pattern is used; otherwise the +no-pattern (if present) is used. If there are more than two alternatives in the +subpattern, a compile-time error occurs. +</P> +<P> +There are two kinds of condition. If the text between the parentheses consists +of a sequence of digits, then the condition is satisfied if the capturing +subpattern of that number has previously matched. Consider the following +pattern, which contains non-significant white space to make it more readable +(assume the PCRE_EXTENDED option) and to divide it into three parts for ease +of discussion: +</P> +<P> +<PRE> + ( \( )? [^()]+ (?(1) \) ) +</PRE> +</P> +<P> +The first part matches an optional opening parenthesis, and if that +character is present, sets it as the first captured substring. The second part +matches one or more characters that are not parentheses. The third part is a +conditional subpattern that tests whether the first set of parentheses matched +or not. If they did, that is, if subject started with an opening parenthesis, +the condition is true, and so the yes-pattern is executed and a closing +parenthesis is required. Otherwise, since no-pattern is not present, the +subpattern matches nothing. In other words, this pattern matches a sequence of +non-parentheses, optionally enclosed in parentheses. +</P> +<P> +If the condition is not a sequence of digits, it must be an assertion. This may +be a positive or negative lookahead or lookbehind assertion. Consider this +pattern, again containing non-significant white space, and with the two +alternatives on the second line: +</P> +<P> +<PRE> + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) +</PRE> +</P> +<P> +The condition is a positive lookahead assertion that matches an optional +sequence of non-letters followed by a letter. In other words, it tests for the +presence of at least one letter in the subject. If a letter is found, the +subject is matched against the first alternative; otherwise it is matched +against the second. This pattern matches strings in one of the two forms +dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. +</P> +<LI><A NAME="SEC26" HREF="#TOC1">COMMENTS</A> +<P> +The sequence (?# marks the start of a comment which continues up to the next +closing parenthesis. Nested parentheses are not permitted. The characters +that make up a comment play no part in the pattern matching at all. +</P> +<P> +If the PCRE_EXTENDED option is set, an unescaped # character outside a +character class introduces a comment that continues up to the next newline +character in the pattern. +</P> +<LI><A NAME="SEC27" HREF="#TOC1">PERFORMANCE</A> +<P> +Certain items that may appear in patterns are more efficient than others. It is +more efficient to use a character class like [aeiou] than a set of alternatives +such as (a|e|i|o|u). In general, the simplest construction that provides the +required behaviour is usually the most efficient. Jeffrey Friedl's book +contains a lot of discussion about optimizing regular expressions for efficient +performance. +</P> +<P> +When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is +implicitly anchored by PCRE, since it can match only at the start of a subject +string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization, +because the . metacharacter does not then match a newline, and if the subject +string contains newlines, the pattern may match from the character immediately +following one of them instead of from the very start. For example, the pattern +</P> +<P> +<PRE> + (.*) second +</PRE> +</P> +<P> +matches the subject "first\nand second" (where \n stands for a newline +character) with the first captured substring being "and". In order to do this, +PCRE has to retry the match starting after every newline in the subject. +</P> +<P> +If you are using such a pattern with subject strings that do not contain +newlines, the best performance is obtained by setting PCRE_DOTALL, or starting +the pattern with ^.* to indicate explicit anchoring. That saves PCRE from +having to scan along the subject looking for a newline to restart at. +</P> +<P> +Beware of patterns that contain nested indefinite repeats. These can take a +long time to run when applied to a string that does not match. Consider the +pattern fragment +</P> +<P> +<PRE> + (a+)* +</PRE> +</P> +<P> +This can match "aaaa" in 33 different ways, and this number increases very +rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 +times, and for each of those cases other than 0, the + repeats can match +different numbers of times.) When the remainder of the pattern is such that the +entire match is going to fail, PCRE has in principle to try every possible +variation, and this can take an extremely long time. +</P> +<P> +An optimization catches some of the more simple cases such as +</P> +<P> +<PRE> + (a+)*b +</PRE> +</P> +<P> +where a literal character follows. Before embarking on the standard matching +procedure, PCRE checks that there is a "b" later in the subject string, and if +there is not, it fails the match immediately. However, when there is no +following literal this optimization cannot be used. You can see the difference +by comparing the behaviour of +</P> +<P> +<PRE> + (a+)*\d +</PRE> +</P> +<P> +with the pattern above. The former gives a failure almost instantly when +applied to a whole line of "a" characters, whereas the latter takes an +appreciable time with strings longer than about 20 characters. +</P> +<LI><A NAME="SEC28" HREF="#TOC1">AUTHOR</A> +<P> +Philip Hazel <ph10@cam.ac.uk> +<BR> +University Computing Service, +<BR> +New Museums Site, +<BR> +Cambridge CB2 3QG, England. +<BR> +Phone: +44 1223 334714 +</P> +<P> +Last updated: 29 July 1999 +<BR> +Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pcre.txt b/doc/pcre.txt new file mode 100644 index 0000000..2374f7c --- /dev/null +++ b/doc/pcre.txt @@ -0,0 +1,1746 @@ +NAME + pcre - Perl-compatible regular expressions. + + + +SYNOPSIS + #include <pcre.h> + + pcre *pcre_compile(const char *pattern, int options, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + pcre_extra *pcre_study(const pcre *code, int options, + const char **errptr); + + int pcre_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize); + + int pcre_copy_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, char *buffer, + int buffersize); + + int pcre_get_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, + const char **stringptr); + + int pcre_get_substring_list(const char *subject, + int *ovector, int stringcount, const char ***listptr); + + const unsigned char *pcre_maketables(void); + + int pcre_info(const pcre *code, int *optptr, *firstcharptr); + + char *pcre_version(void); + + void *(*pcre_malloc)(size_t); + + void (*pcre_free)(void *); + + + + +DESCRIPTION + The PCRE library is a set of functions that implement regu- + lar expression pattern matching using the same syntax and + semantics as Perl 5, with just a few differences (see + below). The current implementation corresponds to Perl + 5.005. + + PCRE has its own native API, which is described in this + document. There is also a set of wrapper functions that + correspond to the POSIX API. These are described in the + pcreposix documentation. + The native API function prototypes are defined in the header + file pcre.h, and on Unix systems the library itself is + called libpcre.a, so can be accessed by adding -lpcre to the + command for linking an application which calls it. + + The functions pcre_compile(), pcre_study(), and pcre_exec() + are used for compiling and matching regular expressions, + while pcre_copy_substring(), pcre_get_substring(), and + pcre_get_substring_list() are convenience functions for + extracting captured substrings from a matched subject + string. The function pcre_maketables() is used (optionally) + to build a set of character tables in the current locale for + passing to pcre_compile(). + + The function pcre_info() is used to find out information + about a compiled pattern, while the function pcre_version() + returns a pointer to a string containing the version of PCRE + and its date of release. + + The global variables pcre_malloc and pcre_free initially + contain the entry points of the standard malloc() and free() + functions respectively. PCRE calls the memory management + functions via these variables, so a calling program can + replace them if it wishes to intercept the calls. This + should be done before calling any PCRE functions. + + + +MULTI-THREADING + The PCRE functions can be used in multi-threading applica- + tions, with the proviso that the memory management functions + pointed to by pcre_malloc and pcre_free are shared by all + threads. + + The compiled form of a regular expression is not altered + during matching, so the same compiled pattern can safely be + used by several threads at once. + + + +COMPILING A PATTERN + The function pcre_compile() is called to compile a pattern + into an internal form. The pattern is a C string terminated + by a binary zero, and is passed in the argument pattern. A + pointer to a single block of memory that is obtained via + pcre_malloc is returned. This contains the compiled code and + related data. The pcre type is defined for this for conveni- + ence, but in fact pcre is just a typedef for void, since the + contents of the block are not externally defined. It is up + to the caller to free the memory when it is no longer + required. + + The size of a compiled pattern is roughly proportional to + the length of the pattern string, except that each character + class (other than those containing just a single character, + negated or not) requires 33 bytes, and repeat quantifiers + with a minimum greater than one or a bounded maximum cause + the relevant portions of the compiled pattern to be repli- + cated. + + The options argument contains independent bits that affect + the compilation. It should be zero if no options are + required. Some of the options, in particular, those that are + compatible with Perl, can also be set and unset from within + the pattern (see the detailed description of regular expres- + sions below). For these options, the contents of the options + argument specifies their initial settings at the start of + compilation and execution. The PCRE_ANCHORED option can be + set at the time of matching as well as at compile time. + + If errptr is NULL, pcre_compile() returns NULL immediately. + Otherwise, if compilation of a pattern fails, pcre_compile() + returns NULL, and sets the variable pointed to by errptr to + point to a textual error message. The offset from the start + of the pattern to the character where the error was + discovered is placed in the variable pointed to by + erroffset, which must not be NULL. If it is, an immediate + error is given. + + If the final argument, tableptr, is NULL, PCRE uses a + default set of character tables which are built when it is + compiled, using the default C locale. Otherwise, tableptr + must be the result of a call to pcre_maketables(). See the + section on locale support below. + + The following option bits are defined in the header file: + + PCRE_ANCHORED + + If this bit is set, the pattern is forced to be "anchored", + that is, it is constrained to match only at the start of the + string which is being searched (the "subject string"). This + effect can also be achieved by appropriate constructs in the + pattern itself, which is the only way to do it in Perl. + + PCRE_CASELESS + + If this bit is set, letters in the pattern match both upper + and lower case letters. It is equivalent to Perl's /i + option. + + PCRE_DOLLAR_ENDONLY + + If this bit is set, a dollar metacharacter in the pattern + matches only at the end of the subject string. Without this + option, a dollar also matches immediately before the final + character if it is a newline (but not before any other new- + lines). The PCRE_DOLLAR_ENDONLY option is ignored if + PCRE_MULTILINE is set. There is no equivalent to this option + in Perl. + + PCRE_DOTALL + + If this bit is set, a dot metacharater in the pattern + matches all characters, including newlines. Without it, new- + lines are excluded. This option is equivalent to Perl's /s + option. A negative class such as [^a] always matches a new- + line character, independent of the setting of this option. + + PCRE_EXTENDED + + If this bit is set, whitespace data characters in the pat- + tern are totally ignored except when escaped or inside a + character class, and characters between an unescaped # out- + side a character class and the next newline character, + inclusive, are also ignored. This is equivalent to Perl's /x + option, and makes it possible to include comments inside + complicated patterns. Note, however, that this applies only + to data characters. Whitespace characters may never appear + within special character sequences in a pattern, for example + within the sequence (?( which introduces a conditional sub- + pattern. + + PCRE_EXTRA + + This option turns on additional functionality of PCRE that + is incompatible with Perl. Any backslash in a pattern that + is followed by a letter that has no special meaning causes + an error, thus reserving these combinations for future + expansion. By default, as in Perl, a backslash followed by a + letter with no special meaning is treated as a literal. + There are at present no other features controlled by this + option. + + PCRE_MULTILINE + + By default, PCRE treats the subject string as consisting of + a single "line" of characters (even if it actually contains + several newlines). The "start of line" metacharacter (^) + matches only at the start of the string, while the "end of + line" metacharacter ($) matches only at the end of the + string, or before a terminating newline (unless + PCRE_DOLLAR_ENDONLY is set). This is the same as Perl. + + When PCRE_MULTILINE it is set, the "start of line" and "end + of line" constructs match immediately following or + immediately before any newline in the subject string, + respectively, as well as at the very start and end. This is + equivalent to Perl's /m option. If there are no "\n" charac- + ters in a subject string, or no occurrences of ^ or $ in a + pattern, setting PCRE_MULTILINE has no effect. + + PCRE_UNGREEDY + + This option inverts the "greediness" of the quantifiers so + that they are not greedy by default, but become greedy if + followed by "?". It is not compatible with Perl. It can also + be set by a (?U) option setting within the pattern. + + + +STUDYING A PATTERN + When a pattern is going to be used several times, it is + worth spending more time analyzing it in order to speed up + the time taken for matching. The function pcre_study() takes + a pointer to a compiled pattern as its first argument, and + returns a pointer to a pcre_extra block (another void + typedef) containing additional information about the pat- + tern; this can be passed to pcre_exec(). If no additional + information is available, NULL is returned. + + The second argument contains option bits. At present, no + options are defined for pcre_study(), and this argument + should always be zero. + + The third argument for pcre_study() is a pointer to an error + message. If studying succeeds (even if no data is returned), + the variable it points to is set to NULL. Otherwise it + points to a textual error message. + + At present, studying a pattern is useful only for non- + anchored patterns that do not have a single fixed starting + character. A bitmap of possible starting characters is + created. + + + +LOCALE SUPPORT + PCRE handles caseless matching, and determines whether char- + acters are letters, digits, or whatever, by reference to a + set of tables. The library contains a default set of tables + which is created in the default C locale when PCRE is com- + piled. This is used when the final argument of + pcre_compile() is NULL, and is sufficient for many applica- + tions. + + An alternative set of tables can, however, be supplied. Such + tables are built by calling the pcre_maketables() function, + which has no arguments, in the relevant locale. The result + can then be passed to pcre_compile() as often as necessary. + For example, to build and use tables that are appropriate + for the French locale (where accented characters with codes + greater than 128 are treated as letters), the following code + could be used: + + setlocale(LC_CTYPE, "fr"); + tables = pcre_maketables(); + re = pcre_compile(..., tables); + + The tables are built in memory that is obtained via + pcre_malloc. The pointer that is passed to pcre_compile is + saved with the compiled pattern, and the same tables are + used via this pointer by pcre_study() and pcre_exec(). Thus + for any single pattern, compilation, studying and matching + all happen in the same locale, but different patterns can be + compiled in different locales. It is the caller's responsi- + bility to ensure that the memory containing the tables + remains available for as long as it is needed. + + + +INFORMATION ABOUT A PATTERN + The pcre_info() function returns information about a com- + piled pattern. Its yield is the number of capturing subpat- + terns, or one of the following negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + + If the optptr argument is not NULL, a copy of the options + with which the pattern was compiled is placed in the integer + it points to. These option bits are those specified in the + call to pcre_compile(), modified by any top-level option + settings within the pattern itself, and with the + PCRE_ANCHORED bit set if the form of the pattern implies + that it can match only at the start of a subject string. + + If the pattern is not anchored and the firstcharptr argument + is not NULL, it is used to pass back information about the + first character of any matched string. If there is a fixed + first character, e.g. from a pattern such as + (cat|cow|coyote), then it is returned in the integer pointed + to by firstcharptr. Otherwise, if either + + (a) the pattern was compiled with the PCRE_MULTILINE option, + and every branch starts with "^", or + + (b) every branch of the pattern starts with ".*" and + PCRE_DOTALL is not set (if it were set, the pattern would be + anchored), + then -1 is returned, indicating that the pattern matches + only at the start of a subject string or after any "\n" + within the string. Otherwise -2 is returned. + + + +MATCHING A PATTERN + The function pcre_exec() is called to match a subject string + against a pre-compiled pattern, which is passed in the code + argument. If the pattern has been studied, the result of the + study should be passed in the extra argument. Otherwise this + must be NULL. + + The PCRE_ANCHORED option can be passed in the options argu- + ment, whose unused bits must be zero. However, if a pattern + was compiled with PCRE_ANCHORED, or turned out to be + anchored by virtue of its contents, it cannot be made + unachored at matching time. + + There are also three further options that can be set only at + matching time: + + PCRE_NOTBOL + + The first character of the string is not the beginning of a + line, so the circumflex metacharacter should not match + before it. Setting this without PCRE_MULTILINE (at compile + time) causes circumflex never to match. + + PCRE_NOTEOL + + The end of the string is not the end of a line, so the dol- + lar metacharacter should not match it nor (except in multi- + line mode) a newline immediately before it. Setting this + without PCRE_MULTILINE (at compile time) causes dollar never + to match. + + PCRE_NOTEMPTY + + An empty string is not considered to be a valid match if + this option is set. If there are alternatives in the pat- + tern, they are tried. If all the alternatives match the + empty string, the entire match fails. For example, if the + pattern + + a?b? + + is applied to a string not beginning with "a" or "b", it + matches the empty string at the start of the subject. With + PCRE_NOTEMPTY set, this match is not valid, so PCRE searches + further into the string for occurrences of "a" or "b". + + Perl has no direct equivalent of PCRE_NOTEMPTY, but it does + make a special case of a pattern match of the empty string + within its split() function, and when using the /g modifier. + It is possible to emulate Perl's behaviour after matching a + null string by first trying the match again at the same + offset with PCRE_NOTEMPTY set, and then if that fails by + advancing the starting offset (see below) and trying an + ordinary match again. + + The subject string is passed as a pointer in subject, a + length in length, and a starting offset in startoffset. + Unlike the pattern string, it may contain binary zero char- + acters. When the starting offset is zero, the search for a + match starts at the beginning of the subject, and this is by + far the most common case. + + A non-zero starting offset is useful when searching for + another match in the same subject by calling pcre_exec() + again after a previous success. Setting startoffset differs + from just passing over a shortened string and setting + PCRE_NOTBOL in the case of a pattern that begins with any + kind of lookbehind. For example, consider the pattern + + \Biss\B + + which finds occurrences of "iss" in the middle of words. (\B + matches only if the current position in the subject is not a + word boundary.) When applied to the string "Mississipi" the + first call to pcre_exec() finds the first occurrence. If + pcre_exec() is called again with just the remainder of the + subject, namely "issipi", it does not match, because \B is + always false at the start of the subject, which is deemed to + be a word boundary. However, if pcre_exec() is passed the + entire string again, but with startoffset set to 4, it finds + the second occurrence of "iss" because it is able to look + behind the starting point to discover that it is preceded by + a letter. + + If a non-zero starting offset is passed when the pattern is + anchored, one attempt to match at the given offset is tried. + This can only succeed if the pattern does not require the + match to be at the start of the subject. + + In general, a pattern matches a certain portion of the sub- + ject, and in addition, further substrings from the subject + may be picked out by parts of the pattern. Following the + usage in Jeffrey Friedl's book, this is called "capturing" + in what follows, and the phrase "capturing subpattern" is + used for a fragment of a pattern that picks out a substring. + PCRE supports several other kinds of parenthesized subpat- + tern that do not cause substrings to be captured. + + Captured substrings are returned to the caller via a vector + of integer offsets whose address is passed in ovector. The + number of elements in the vector is passed in ovecsize. The + first two-thirds of the vector is used to pass back captured + substrings, each substring using a pair of integers. The + remaining third of the vector is used as workspace by + pcre_exec() while matching capturing subpatterns, and is not + available for passing back information. The length passed in + ovecsize should always be a multiple of three. If it is not, + it is rounded down. + + When a match has been successful, information about captured + substrings is returned in pairs of integers, starting at the + beginning of ovector, and continuing up to two-thirds of its + length at the most. The first element of a pair is set to + the offset of the first character in a substring, and the + second is set to the offset of the first character after the + end of a substring. The first pair, ovector[0] and ovec- + tor[1], identify the portion of the subject string matched + by the entire pattern. The next pair is used for the first + capturing subpattern, and so on. The value returned by + pcre_exec() is the number of pairs that have been set. If + there are no capturing subpatterns, the return value from a + successful match is 1, indicating that just the first pair + of offsets has been set. + + Some convenience functions are provided for extracting the + captured substrings as separate strings. These are described + in the following section. + + It is possible for an capturing subpattern number n+1 to + match some part of the subject when subpattern n has not + been used at all. For example, if the string "abc" is + matched against the pattern (a|(z))(bc) subpatterns 1 and 3 + are matched, but 2 is not. When this happens, both offset + values corresponding to the unused subpattern are set to -1. + + If a capturing subpattern is matched repeatedly, it is the + last portion of the string that it matched that gets + returned. + + If the vector is too small to hold all the captured sub- + strings, it is used as far as possible (up to two-thirds of + its length), and the function returns a value of zero. In + particular, if the substring offsets are not of interest, + pcre_exec() may be called with ovector passed as NULL and + ovecsize as zero. However, if the pattern contains back + references and the ovector isn't big enough to remember the + related substrings, PCRE has to get additional memory for + use during matching. Thus it is usually advisable to supply + an ovector. + + Note that pcre_info() can be used to find out how many cap- + turing subpatterns there are in a compiled pattern. The + smallest size for ovector that will allow for n captured + substrings in addition to the offsets of the substring + matched by the whole pattern is (n+1)*3. + + If pcre_exec() fails, it returns a negative number. The fol- + lowing are defined in the header file: + + PCRE_ERROR_NOMATCH (-1) + + The subject string did not match the pattern. + + PCRE_ERROR_NULL (-2) + + Either code or subject was passed as NULL, or ovector was + NULL and ovecsize was not zero. + + PCRE_ERROR_BADOPTION (-3) + + An unrecognized bit was set in the options argument. + + PCRE_ERROR_BADMAGIC (-4) + + PCRE stores a 4-byte "magic number" at the start of the com- + piled code, to catch the case when it is passed a junk + pointer. This is the error it gives when the magic number + isn't present. + + PCRE_ERROR_UNKNOWN_NODE (-5) + + While running the pattern match, an unknown item was encoun- + tered in the compiled pattern. This error could be caused by + a bug in PCRE or by overwriting of the compiled pattern. + + PCRE_ERROR_NOMEMORY (-6) + + If a pattern contains back references, but the ovector that + is passed to pcre_exec() is not big enough to remember the + referenced substrings, PCRE gets a block of memory at the + start of matching to use for this purpose. If the call via + pcre_malloc() fails, this error is given. The memory is + freed at the end of matching. + + + +EXTRACTING CAPTURED SUBSTRINGS + Captured substrings can be accessed directly by using the + offsets returned by pcre_exec() in ovector. For convenience, + the functions pcre_copy_substring(), pcre_get_substring(), + and pcre_get_substring_list() are provided for extracting + captured substrings as new, separate, zero-terminated + strings. A substring that contains a binary zero is + correctly extracted and has a further zero added on the end, + but the result does not, of course, function as a C string. + + The first three arguments are the same for all three func- + tions: subject is the subject string which has just been + successfully matched, ovector is a pointer to the vector of + integer offsets that was passed to pcre_exec(), and + stringcount is the number of substrings that were captured + by the match, including the substring that matched the + entire regular expression. This is the value returned by + pcre_exec if it is greater than zero. If pcre_exec() + returned zero, indicating that it ran out of space in ovec- + tor, then the value passed as stringcount should be the size + of the vector divided by three. + + The functions pcre_copy_substring() and pcre_get_substring() + extract a single substring, whose number is given as string- + number. A value of zero extracts the substring that matched + the entire pattern, while higher values extract the captured + substrings. For pcre_copy_substring(), the string is placed + in buffer, whose length is given by buffersize, while for + pcre_get_substring() a new block of store is obtained via + pcre_malloc, and its address is returned via stringptr. The + yield of the function is the length of the string, not + including the terminating zero, or one of + + PCRE_ERROR_NOMEMORY (-6) + + The buffer was too small for pcre_copy_substring(), or the + attempt to get memory failed for pcre_get_substring(). + + PCRE_ERROR_NOSUBSTRING (-7) + + There is no substring whose number is stringnumber. + + The pcre_get_substring_list() function extracts all avail- + able substrings and builds a list of pointers to them. All + this is done in a single block of memory which is obtained + via pcre_malloc. The address of the memory block is returned + via listptr, which is also the start of the list of string + pointers. The end of the list is marked by a NULL pointer. + The yield of the function is zero if all went well, or + + PCRE_ERROR_NOMEMORY (-6) + + if the attempt to get the memory block failed. + + When any of these functions encounter a substring that is + unset, which can happen when capturing subpattern number n+1 + matches some part of the subject, but subpattern n has not + been used at all, they return an empty string. This can be + distinguished from a genuine zero-length substring by + inspecting the appropriate offset in ovector, which is nega- + tive for unset substrings. + + + + +LIMITATIONS + There are some size limitations in PCRE but it is hoped that + they will never in practice be relevant. The maximum length + of a compiled pattern is 65539 (sic) bytes. All values in + repeating quantifiers must be less than 65536. The maximum + number of capturing subpatterns is 99. The maximum number + of all parenthesized subpatterns, including capturing sub- + patterns, assertions, and other types of subpattern, is 200. + + The maximum length of a subject string is the largest posi- + tive number that an integer variable can hold. However, PCRE + uses recursion to handle subpatterns and indefinite repeti- + tion. This means that the available stack space may limit + the size of a subject string that can be processed by cer- + tain patterns. + + + +DIFFERENCES FROM PERL + The differences described here are with respect to Perl + 5.005. + + 1. By default, a whitespace character is any character that + the C library function isspace() recognizes, though it is + possible to compile PCRE with alternative character type + tables. Normally isspace() matches space, formfeed, newline, + carriage return, horizontal tab, and vertical tab. Perl 5 no + longer includes vertical tab in its set of whitespace char- + acters. The \v escape that was in the Perl documentation for + a long time was never in fact recognized. However, the char- + acter itself was treated as whitespace at least up to 5.002. + In 5.004 and 5.005 it does not match \s. + + 2. PCRE does not allow repeat quantifiers on lookahead + assertions. Perl permits them, but they do not mean what you + might think. For example, (?!a){3} does not assert that the + next three characters are not "a". It just asserts that the + next character is not "a" three times. + + 3. Capturing subpatterns that occur inside negative looka- + head assertions are counted, but their entries in the + offsets vector are never set. Perl sets its numerical vari- + ables from any such patterns that are matched before the + assertion fails to match something (thereby succeeding), but + only if the negative lookahead assertion contains just one + branch. + + 4. Though binary zero characters are supported in the sub- + ject string, they are not allowed in a pattern string + because it is passed as a normal C string, terminated by + zero. The escape sequence "\0" can be used in the pattern to + represent a binary zero. + + 5. The following Perl escape sequences are not supported: + \l, \u, \L, \U, \E, \Q. In fact these are implemented by + Perl's general string-handling and are not part of its pat- + tern matching engine. + + 6. The Perl \G assertion is not supported as it is not + relevant to single pattern matches. + + 7. Fairly obviously, PCRE does not support the (?{code}) + construction. + + 8. There are at the time of writing some oddities in Perl + 5.005_02 concerned with the settings of captured strings + when part of a pattern is repeated. For example, matching + "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value + "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 + unset. However, if the pattern is changed to + /^(aa(b(b))?)+$/ then $2 (and $3) get set. + + In Perl 5.004 $2 is set in both cases, and that is also true + of PCRE. If in the future Perl changes to a consistent state + that is different, PCRE may change to follow. + + 9. Another as yet unresolved discrepancy is that in Perl + 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string + "a", whereas in PCRE it does not. However, in both Perl and + PCRE /^(a)?a/ matched against "a" leaves $1 unset. + + 10. PCRE provides some extensions to the Perl regular + expression facilities: + + (a) Although lookbehind assertions must match fixed length + strings, each alternative branch of a lookbehind assertion + can match a different length of string. Perl 5.005 requires + them all to have the same length. + + (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not + set, the $ meta- character matches only at the very end of + the string. + + (c) If PCRE_EXTRA is set, a backslash followed by a letter + with no special meaning is faulted. + + (d) If PCRE_UNGREEDY is set, the greediness of the + repetition quantifiers is inverted, that is, by default they + are not greedy, but if followed by a question mark they are. + + (e) PCRE_ANCHORED can be used to force a pattern to be tried + only at the start of the subject. + + (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options + for pcre_exec() have no Perl equivalents. + + + +REGULAR EXPRESSION DETAILS + The syntax and semantics of the regular expressions sup- + ported by PCRE are described below. Regular expressions are + also described in the Perl documentation and in a number of + other books, some of which have copious examples. Jeffrey + Friedl's "Mastering Regular Expressions", published by + O'Reilly (ISBN 1-56592-257-3), covers them in great detail. + The description here is intended as reference documentation. + + A regular expression is a pattern that is matched against a + subject string from left to right. Most characters stand for + themselves in a pattern, and match the corresponding charac- + ters in the subject. As a trivial example, the pattern + + The quick brown fox + + matches a portion of a subject string that is identical to + itself. The power of regular expressions comes from the + ability to include alternatives and repetitions in the pat- + tern. These are encoded in the pattern by the use of meta- + characters, which do not stand for themselves but instead + are interpreted in some special way. + + There are two different sets of meta-characters: those that + are recognized anywhere in the pattern except within square + brackets, and those that are recognized in square brackets. + Outside square brackets, the meta-characters are as follows: + + \ general escape character with several uses + ^ assert start of subject (or line, in multiline + mode) + $ assert end of subject (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + { start min/max quantifier + + Part of a pattern that is in square brackets is called a + "character class". In a character class the only meta- + characters are: + + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + ] terminates the character class + + The following sections describe the use of each of the + meta-characters. + + + +BACKSLASH + The backslash character has several uses. Firstly, if it is + followed by a non-alphameric character, it takes away any + special meaning that character may have. This use of + backslash as an escape character applies both inside and + outside character classes. + + For example, if you want to match a "*" character, you write + "\*" in the pattern. This applies whether or not the follow- + ing character would otherwise be interpreted as a meta- + character, so it is always safe to precede a non-alphameric + with "\" to specify that it stands for itself. In particu- + lar, if you want to match a backslash, you write "\\". + + If a pattern is compiled with the PCRE_EXTENDED option, whi- + tespace in the pattern (other than in a character class) and + characters between a "#" outside a character class and the + next newline character are ignored. An escaping backslash + can be used to include a whitespace or "#" character as part + of the pattern. + + A second use of backslash provides a way of encoding non- + printing characters in patterns in a visible manner. There + is no restriction on the appearance of non-printing charac- + ters, apart from the binary zero that terminates a pattern, + but when a pattern is being prepared by text editing, it is + usually easier to use one of the following escape sequences + than the binary character it represents: + + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n newline (hex 0A) + \r carriage return (hex 0D) + + tab (hex 09) + \xhh character with hex code hh + \ddd character with octal code ddd, or backreference + + The precise effect of "\cx" is as follows: if "x" is a lower + case letter, it is converted to upper case. Then bit 6 of + the character (hex 40) is inverted. Thus "\cz" becomes hex + 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B. + + After "\x", up to two hexadecimal digits are read (letters + can be in upper or lower case). + + After "\0" up to two further octal digits are read. In both + cases, if there are fewer than two digits, just those that + are present are used. Thus the sequence "\0\x\07" specifies + two binary zeros followed by a BEL character. Make sure you + supply two digits after the initial zero if the character + that follows is itself an octal digit. + + The handling of a backslash followed by a digit other than 0 + is complicated. Outside a character class, PCRE reads it + and any following digits as a decimal number. If the number + is less than 10, or if there have been at least that many + previous capturing left parentheses in the expression, the + entire sequence is taken as a back reference. A description + of how this works is given later, following the discussion + of parenthesized subpatterns. + + Inside a character class, or if the decimal number is + greater than 9 and there have not been that many capturing + subpatterns, PCRE re-reads up to three octal digits follow- + ing the backslash, and generates a single byte from the + least significant 8 bits of the value. Any subsequent digits + stand for themselves. For example: + + \040 is another way of writing a space + \40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of + writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 is the character with octal code 113 (since there + can be no more than 99 back references) + \377 is a byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" + + Note that octal values of 100 or greater must not be intro- + duced by a leading zero, because no more than three octal + digits are ever read. + All the sequences that define a single byte value can be + used both inside and outside character classes. In addition, + inside a character class, the sequence "\b" is interpreted + as the backspace character (hex 08). Outside a character + class it has a different meaning (see below). + + The third use of backslash is for specifying generic charac- + ter types: + + \d any decimal digit + \D any character that is not a decimal digit + \s any whitespace character + \S any character that is not a whitespace character + \w any "word" character + \W any "non-word" character + + Each pair of escape sequences partitions the complete set of + characters into two disjoint sets. Any given character + matches one, and only one, of each pair. + + A "word" character is any letter or digit or the underscore + character, that is, any character which can be part of a + Perl "word". The definition of letters and digits is con- + trolled by PCRE's character tables, and may vary if locale- + specific matching is taking place (see "Locale support" + above). For example, in the "fr" (French) locale, some char- + acter codes greater than 128 are used for accented letters, + and these are matched by \w. + + These character type sequences can appear both inside and + outside character classes. They each match one character of + the appropriate type. If the current matching point is at + the end of the subject string, all of them fail, since there + is no character to match. + + The fourth use of backslash is for certain simple asser- + tions. An assertion specifies a condition that has to be met + at a particular point in a match, without consuming any + characters from the subject string. The use of subpatterns + for more complicated assertions is described below. The + backslashed assertions are + + \b word boundary + \B not a word boundary + \A start of subject (independent of multiline mode) + \Z end of subject or newline at end (independent of + multiline mode) + \z end of subject (independent of multiline mode) + + These assertions may not appear in character classes (but + note that "\b" has a different meaning, namely the backspace + character, inside a character class). + A word boundary is a position in the subject string where + the current character and the previous character do not both + match \w or \W (i.e. one matches \w and the other matches + \W), or the start or end of the string if the first or last + character matches \w, respectively. + + The \A, \Z, and \z assertions differ from the traditional + circumflex and dollar (described below) in that they only + ever match at the very start and end of the subject string, + whatever options are set. They are not affected by the + PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu- + ment of pcre_exec() is non-zero, \A can never match. The + difference between \Z and \z is that \Z matches before a + newline that is the last character of the string as well as + at the end of the string, whereas \z matches only at the + end. + + + +CIRCUMFLEX AND DOLLAR + Outside a character class, in the default matching mode, the + circumflex character is an assertion which is true only if + the current matching point is at the start of the subject + string. If the startoffset argument of pcre_exec() is non- + zero, circumflex can never match. Inside a character class, + circumflex has an entirely different meaning (see below). + + Circumflex need not be the first character of the pattern if + a number of alternatives are involved, but it should be the + first thing in each alternative in which it appears if the + pattern is ever to match that branch. If all possible alter- + natives start with a circumflex, that is, if the pattern is + constrained to match only at the start of the subject, it is + said to be an "anchored" pattern. (There are also other con- + structs that can cause a pattern to be anchored.) + + A dollar character is an assertion which is true only if the + current matching point is at the end of the subject string, + or immediately before a newline character that is the last + character in the string (by default). Dollar need not be the + last character of the pattern if a number of alternatives + are involved, but it should be the last item in any branch + in which it appears. Dollar has no special meaning in a + character class. + + The meaning of dollar can be changed so that it matches only + at the very end of the string, by setting the + PCRE_DOLLAR_ENDONLY option at compile or matching time. This + does not affect the \Z assertion. + + The meanings of the circumflex and dollar characters are + changed if the PCRE_MULTILINE option is set. When this is + the case, they match immediately after and immediately + before an internal "\n" character, respectively, in addition + to matching at the start and end of the subject string. For + example, the pattern /^abc$/ matches the subject string + "def\nabc" in multiline mode, but not otherwise. Conse- + quently, patterns that are anchored in single line mode + because all branches start with "^" are not anchored in mul- + tiline mode, and a match for circumflex is possible when the + startoffset argument of pcre_exec() is non-zero. The + PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is + set. + + Note that the sequences \A, \Z, and \z can be used to match + the start and end of the subject in both modes, and if all + branches of a pattern start with \A is it always anchored, + whether PCRE_MULTILINE is set or not. + + + +FULL STOP (PERIOD, DOT) + Outside a character class, a dot in the pattern matches any + one character in the subject, including a non-printing char- + acter, but not (by default) newline. If the PCRE_DOTALL + option is set, then dots match newlines as well. The han- + dling of dot is entirely independent of the handling of cir- + cumflex and dollar, the only relationship being that they + both involve newline characters. Dot has no special meaning + in a character class. + + + +SQUARE BRACKETS + An opening square bracket introduces a character class, ter- + minated by a closing square bracket. A closing square + bracket on its own is not special. If a closing square + bracket is required as a member of the class, it should be + the first data character in the class (after an initial cir- + cumflex, if present) or escaped with a backslash. + + A character class matches a single character in the subject; + the character must be in the set of characters defined by + the class, unless the first character in the class is a cir- + cumflex, in which case the subject character must not be in + the set defined by the class. If a circumflex is actually + required as a member of the class, ensure it is not the + first character, or escape it with a backslash. + + For example, the character class [aeiou] matches any lower + case vowel, while [^aeiou] matches any character that is not + a lower case vowel. Note that a circumflex is just a con- + venient notation for specifying the characters which are in + the class by enumerating those that are not. It is not an + assertion: it still consumes a character from the subject + string, and fails if the current pointer is at the end of + the string. + + When caseless matching is set, any letters in a class + represent both their upper case and lower case versions, so + for example, a caseless [aeiou] matches "A" as well as "a", + and a caseless [^aeiou] does not match "A", whereas a case- + ful version would. + + The newline character is never treated in any special way in + character classes, whatever the setting of the PCRE_DOTALL + or PCRE_MULTILINE options is. A class such as [^a] will + always match a newline. + + The minus (hyphen) character can be used to specify a range + of characters in a character class. For example, [d-m] + matches any letter between d and m, inclusive. If a minus + character is required in a class, it must be escaped with a + backslash or appear in a position where it cannot be inter- + preted as indicating a range, typically as the first or last + character in the class. + + It is not possible to have the literal character "]" as the + end character of a range. A pattern such as [W-]46] is + interpreted as a class of two characters ("W" and "-") fol- + lowed by a literal string "46]", so it would match "W46]" or + "-46]". However, if the "]" is escaped with a backslash it + is interpreted as the end of range, so [W-\]46] is inter- + preted as a single class containing a range followed by two + separate characters. The octal or hexadecimal representation + of "]" can also be used to end a range. + + Ranges operate in ASCII collating sequence. They can also be + used for characters specified numerically, for example + [\000-\037]. If a range that includes letters is used when + caseless matching is set, it matches the letters in either + case. For example, [W-c] is equivalent to [][\^_`wxyzabc], + matched caselessly, and if character tables for the "fr" + locale are in use, [\xc8-\xcb] matches accented E characters + in both cases. + + The character types \d, \D, \s, \S, \w, and \W may also + appear in a character class, and add the characters that + they match to the class. For example, [\dABCDEF] matches any + hexadecimal digit. A circumflex can conveniently be used + with the upper case character types to specify a more res- + tricted set of characters than the matching lower case type. + For example, the class [^\W_] matches any letter or digit, + but not underscore. + + All non-alphameric characters other than \, -, ^ (at the + start) and the terminating ] are non-special in character + classes, but it does no harm if they are escaped. + + + +VERTICAL BAR + Vertical bar characters are used to separate alternative + patterns. For example, the pattern + + gilbert|sullivan + + matches either "gilbert" or "sullivan". Any number of alter- + natives may appear, and an empty alternative is permitted + (matching the empty string). The matching process tries + each alternative in turn, from left to right, and the first + one that succeeds is used. If the alternatives are within a + subpattern (defined below), "succeeds" means matching the + rest of the main pattern as well as the alternative in the + subpattern. + + + +INTERNAL OPTION SETTING + The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, + and PCRE_EXTENDED can be changed from within the pattern by + a sequence of Perl option letters enclosed between "(?" and + ")". The option letters are + + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED + + For example, (?im) sets caseless, multiline matching. It is + also possible to unset these options by preceding the letter + with a hyphen, and a combined setting and unsetting such as + (?im-sx), which sets PCRE_CASELESS and PCRE_MULTILINE while + unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. + If a letter appears both before and after the hyphen, the + option is unset. + + The scope of these option changes depends on where in the + pattern the setting occurs. For settings that are outside + any subpattern (defined below), the effect is the same as if + the options were set or unset at the start of matching. The + following patterns all behave in exactly the same way: + + (?i)abc + a(?i)bc + ab(?i)c + abc(?i) + + which in turn is the same as compiling the pattern abc with + PCRE_CASELESS set. In other words, such "top level" set- + tings apply to the whole pattern (unless there are other + changes inside subpatterns). If there is more than one set- + ting of the same option at top level, the rightmost setting + is used. + + If an option change occurs inside a subpattern, the effect + is different. This is a change of behaviour in Perl 5.005. + An option change inside a subpattern affects only that part + of the subpattern that follows it, so + + (a(?i)b)c + + matches abc and aBc and no other strings (assuming + PCRE_CASELESS is not used). By this means, options can be + made to have different settings in different parts of the + pattern. Any changes made in one alternative do carry on + into subsequent branches within the same subpattern. For + example, + + (a(?i)b|c) + + matches "ab", "aB", "c", and "C", even though when matching + "C" the first branch is abandoned before the option setting. + This is because the effects of option settings happen at + compile time. There would be some very weird behaviour oth- + erwise. + + The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can + be changed in the same way as the Perl-compatible options by + using the characters U and X respectively. The (?X) flag + setting is special in that it must always occur earlier in + the pattern than any of the additional features it turns on, + even when it is at top level. It is best put at the start. + + + +SUBPATTERNS + Subpatterns are delimited by parentheses (round brackets), + which can be nested. Marking part of a pattern as a subpat- + tern does two things: + + 1. It localizes a set of alternatives. For example, the pat- + tern + + cat(aract|erpillar|) + + matches one of the words "cat", "cataract", or "caterpil- + lar". Without the parentheses, it would match "cataract", + "erpillar" or the empty string. + + 2. It sets up the subpattern as a capturing subpattern (as + defined above). When the whole pattern matches, that por- + tion of the subject string that matched the subpattern is + passed back to the caller via the ovector argument of + pcre_exec(). Opening parentheses are counted from left to + right (starting from 1) to obtain the numbers of the captur- + ing subpatterns. + + For example, if the string "the red king" is matched against + the pattern + + the ((red|white) (king|queen)) + + the captured substrings are "red king", "red", and "king", + and are numbered 1, 2, and 3. + + The fact that plain parentheses fulfil two functions is not + always helpful. There are often times when a grouping sub- + pattern is required without a capturing requirement. If an + opening parenthesis is followed by "?:", the subpattern does + not do any capturing, and is not counted when computing the + number of any subsequent capturing subpatterns. For example, + if the string "the white queen" is matched against the pat- + tern + + the ((?:red|white) (king|queen)) + + the captured substrings are "white queen" and "queen", and + are numbered 1 and 2. The maximum number of captured sub- + strings is 99, and the maximum number of all subpatterns, + both capturing and non-capturing, is 200. + + As a convenient shorthand, if any option settings are + required at the start of a non-capturing subpattern, the + option letters may appear between the "?" and the ":". Thus + the two patterns + + (?i:saturday|sunday) + (?:(?i)saturday|sunday) + + match exactly the same set of strings. Because alternative + branches are tried from left to right, and options are not + reset until the end of the subpattern is reached, an option + setting in one branch does affect subsequent branches, so + the above patterns match "SUNDAY" as well as "Saturday". + + + +REPETITION + Repetition is specified by quantifiers, which can follow any + of the following items: + + + a single character, possibly escaped + the . metacharacter + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion - + see below) + + The general repetition quantifier specifies a minimum and + maximum number of permitted matches, by giving the two + numbers in curly brackets (braces), separated by a comma. + The numbers must be less than 65536, and the first must be + less than or equal to the second. For example: + + z{2,4} + + matches "zz", "zzz", or "zzzz". A closing brace on its own + is not a special character. If the second number is omitted, + but the comma is present, there is no upper limit; if the + second number and the comma are both omitted, the quantifier + specifies an exact number of required matches. Thus + + [aeiou]{3,} + + matches at least 3 successive vowels, but may match many + more, while + + \d{8} + + matches exactly 8 digits. An opening curly bracket that + appears in a position where a quantifier is not allowed, or + one that does not match the syntax of a quantifier, is taken + as a literal character. For example, {,6} is not a quantif- + ier, but a literal string of four characters. + + The quantifier {0} is permitted, causing the expression to + behave as if the previous item and the quantifier were not + present. + + For convenience (and historical compatibility) the three + most common quantifiers have single-character abbreviations: + + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} + + It is possible to construct infinite loops by following a + subpattern that can match no characters with a quantifier + that has no upper limit, for example: + + (a?)* + + Earlier versions of Perl and PCRE used to give an error at + compile time for such patterns. However, because there are + cases where this can be useful, such patterns are now + accepted, but if any repetition of the subpattern does in + fact match no characters, the loop is forcibly broken. + + By default, the quantifiers are "greedy", that is, they + match as much as possible (up to the maximum number of per- + mitted times), without causing the rest of the pattern to + fail. The classic example of where this gives problems is in + trying to match comments in C programs. These appear between + the sequences /* and */ and within the sequence, individual + * and / characters may appear. An attempt to match C com- + ments by applying the pattern + + /\*.*\*/ + + to the string + + /* first command */ not comment /* second comment */ + + fails, because it matches the entire string due to the + greediness of the .* item. + + However, if a quantifier is followed by a question mark, + then it ceases to be greedy, and instead matches the minimum + number of times possible, so the pattern + + /\*.*?\*/ + + does the right thing with the C comments. The meaning of the + various quantifiers is not otherwise changed, just the pre- + ferred number of matches. Do not confuse this use of ques- + tion mark with its use as a quantifier in its own right. + Because it has two uses, it can sometimes appear doubled, as + in + + \d??\d + + which matches one digit by preference, but can match two if + that is the only way the rest of the pattern matches. + + If the PCRE_UNGREEDY option is set (an option which is not + available in Perl) then the quantifiers are not greedy by + default, but individual ones can be made greedy by following + them with a question mark. In other words, it inverts the + default behaviour. + + When a parenthesized subpattern is quantified with a minimum + repeat count that is greater than 1 or with a limited max- + imum, more store is required for the compiled pattern, in + proportion to the size of the minimum or maximum. + + If a pattern starts with .* or .{0,} and the PCRE_DOTALL + option (equivalent to Perl's /s) is set, thus allowing the . + to match newlines, then the pattern is implicitly anchored, + because whatever follows will be tried against every charac- + ter position in the subject string, so there is no point in + retrying the overall match at any position after the first. + PCRE treats such a pattern as though it were preceded by \A. + In cases where it is known that the subject string contains + no newlines, it is worth setting PCRE_DOTALL when the pat- + tern begins with .* in order to obtain this optimization, or + alternatively using ^ to indicate anchoring explicitly. + + When a capturing subpattern is repeated, the value captured + is the substring that matched the final iteration. For exam- + ple, after + + (tweedle[dume]{3}\s*)+ + + has matched "tweedledum tweedledee" the value of the cap- + tured substring is "tweedledee". However, if there are + nested capturing subpatterns, the corresponding captured + values may have been set in previous iterations. For exam- + ple, after + + /(a|(b))+/ + + matches "aba" the value of the second captured substring is + "b". + + + +BACK REFERENCES + Outside a character class, a backslash followed by a digit + greater than 0 (and possibly further digits) is a back + reference to a capturing subpattern earlier (i.e. to its + left) in the pattern, provided there have been that many + previous capturing left parentheses. + + However, if the decimal number following the backslash is + less than 10, it is always taken as a back reference, and + causes an error only if there are not that many capturing + left parentheses in the entire pattern. In other words, the + parentheses that are referenced need not be to the left of + the reference for numbers less than 10. See the section + entitled "Backslash" above for further details of the han- + dling of digits following a backslash. + + A back reference matches whatever actually matched the cap- + turing subpattern in the current subject string, rather than + anything matching the subpattern itself. So the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsi- + bility", but not "sense and responsibility". If caseful + matching is in force at the time of the back reference, then + the case of letters is relevant. For example, + + ((?i)rah)\s+\1 + + matches "rah rah" and "RAH RAH", but not "RAH rah", even + though the original capturing subpattern is matched case- + lessly. + + There may be more than one back reference to the same sub- + pattern. If a subpattern has not actually been used in a + particular match, then any back references to it always + fail. For example, the pattern + + (a|(bc))\2 + + always fails if it starts to match "a" rather than "bc". + Because there may be up to 99 back references, all digits + following the backslash are taken as part of a potential + back reference number. If the pattern continues with a digit + character, then some delimiter must be used to terminate the + back reference. If the PCRE_EXTENDED option is set, this can + be whitespace. Otherwise an empty comment can be used. + + A back reference that occurs inside the parentheses to which + it refers fails when the subpattern is first used, so, for + example, (a\1) never matches. However, such references can + be useful inside repeated subpatterns. For example, the pat- + tern + + (a|b\1)+ + + matches any number of "a"s and also "aba", "ababaa" etc. At + each iteration of the subpattern, the back reference matches + the character string corresponding to the previous itera- + tion. In order for this to work, the pattern must be such + that the first iteration does not need to match the back + reference. This can be done using alternation, as in the + example above, or by a quantifier with a minimum of zero. + + + +ASSERTIONS + An assertion is a test on the characters following or + preceding the current matching point that does not actually + consume any characters. The simple assertions coded as \b, + \B, \A, \Z, \z, ^ and $ are described above. More compli- + cated assertions are coded as subpatterns. There are two + kinds: those that look ahead of the current position in the + subject string, and those that look behind it. + An assertion subpattern is matched in the normal way, except + that it does not cause the current matching position to be + changed. Lookahead assertions start with (?= for positive + assertions and (?! for negative assertions. For example, + + \w+(?=;) + + matches a word followed by a semicolon, but does not include + the semicolon in the match, and + + foo(?!bar) + + matches any occurrence of "foo" that is not followed by + "bar". Note that the apparently similar pattern + + (?!foo)bar + + does not find an occurrence of "bar" that is preceded by + something other than "foo"; it finds any occurrence of "bar" + whatsoever, because the assertion (?!foo) is always true + when the next three characters are "bar". A lookbehind + assertion is needed to achieve this effect. + + Lookbehind assertions start with (?<= for positive asser- + tions and (?<! for negative assertions. For example, + + (?<!foo)bar + + does find an occurrence of "bar" that is not preceded by + "foo". The contents of a lookbehind assertion are restricted + such that all the strings it matches must have a fixed + length. However, if there are several alternatives, they do + not all have to have the same fixed length. Thus + + (?<=bullock|donkey) + + is permitted, but + + (?<!dogs?|cats?) + + causes an error at compile time. Branches that match dif- + ferent length strings are permitted only at the top level of + a lookbehind assertion. This is an extension compared with + Perl 5.005, which requires all branches to match the same + length of string. An assertion such as + + (?<=ab(c|de)) + + is not permitted, because its single top-level branch can + match two different lengths, but it is acceptable if rewrit- + ten to use two top-level branches: + + (?<=abc|abde) + + The implementation of lookbehind assertions is, for each + alternative, to temporarily move the current position back + by the fixed width and then try to match. If there are + insufficient characters before the current position, the + match is deemed to fail. Lookbehinds in conjunction with + once-only subpatterns can be particularly useful for match- + ing at the ends of strings; an example is given at the end + of the section on once-only subpatterns. + + Several assertions (of any sort) may occur in succession. + For example, + + (?<=\d{3})(?<!999)foo + + matches "foo" preceded by three digits that are not "999". + Notice that each of the assertions is applied independently + at the same point in the subject string. First there is a + check that the previous three characters are all digits, + then there is a check that the same three characters are not + "999". This pattern does not match "foo" preceded by six + characters, the first of which are digits and the last three + of which are not "999". For example, it doesn't match + "123abcfoo". A pattern to do that is + + (?<=\d{3}...)(?<!999)foo + + This time the first assertion looks at the preceding six + characters, checking that the first three are digits, and + then the second assertion checks that the preceding three + characters are not "999". + + Assertions can be nested in any combination. For example, + + (?<=(?<!foo)bar)baz + + matches an occurrence of "baz" that is preceded by "bar" + which in turn is not preceded by "foo", while + + (?<=\d{3}(?!999)...)foo + + is another pattern which matches "foo" preceded by three + digits and any three characters that are not "999". + + Assertion subpatterns are not capturing subpatterns, and may + not be repeated, because it makes no sense to assert the + same thing several times. If any kind of assertion contains + capturing subpatterns within it, these are counted for the + purposes of numbering the capturing subpatterns in the whole + pattern. However, substring capturing is carried out only + for positive assertions, because it does not make sense for + negative assertions. + + Assertions count towards the maximum of 200 parenthesized + subpatterns. + + + +ONCE-ONLY SUBPATTERNS + With both maximizing and minimizing repetition, failure of + what follows normally causes the repeated item to be re- + evaluated to see if a different number of repeats allows the + rest of the pattern to match. Sometimes it is useful to + prevent this, either to change the nature of the match, or + to cause it fail earlier than it otherwise might, when the + author of the pattern knows there is no point in carrying + on. + + Consider, for example, the pattern \d+foo when applied to + the subject line + + 123456bar + + After matching all 6 digits and then failing to match "foo", + the normal action of the matcher is to try again with only 5 + digits matching the \d+ item, and then with 4, and so on, + before ultimately failing. Once-only subpatterns provide the + means for specifying that once a portion of the pattern has + matched, it is not to be re-evaluated in this way, so the + matcher would give up immediately on failing to match "foo" + the first time. The notation is another kind of special + parenthesis, starting with (?> as in this example: + + (?>\d+)bar + + This kind of parenthesis "locks up" the part of the pattern + it contains once it has matched, and a failure further into + the pattern is prevented from backtracking into it. Back- + tracking past it to previous items, however, works as nor- + mal. + + An alternative description is that a subpattern of this type + matches the string of characters that an identical stan- + dalone pattern would match, if anchored at the current point + in the subject string. + + Once-only subpatterns are not capturing subpatterns. Simple + cases such as the above example can be thought of as a max- + imizing repeat that must swallow everything it can. So, + while both \d+ and \d+? are prepared to adjust the number of + digits they match in order to make the rest of the pattern + match, (?>\d+) can only match an entire sequence of digits. + + This construction can of course contain arbitrarily compli- + cated subpatterns, and it can be nested. + + Once-only subpatterns can be used in conjunction with look- + behind assertions to specify efficient matching at the end + of the subject string. Consider a simple pattern such as + + abcd$ + + when applied to a long string which does not match it. + Because matching proceeds from left to right, PCRE will look + for each "a" in the subject and then see if what follows + matches the rest of the pattern. If the pattern is specified + as + + ^.*abcd$ + + then the initial .* matches the entire string at first, but + when this fails, it backtracks to match all but the last + character, then all but the last two characters, and so on. + Once again the search for "a" covers the entire string, from + right to left, so we are no better off. However, if the pat- + tern is written as + + ^(?>.*)(?<=abcd) + + then there can be no backtracking for the .* item; it can + match only the entire string. The subsequent lookbehind + assertion does a single test on the last four characters. If + it fails, the match fails immediately. For long strings, + this approach makes a significant difference to the process- + ing time. + + + +CONDITIONAL SUBPATTERNS + It is possible to cause the matching process to obey a sub- + pattern conditionally or to choose between two alternative + subpatterns, depending on the result of an assertion, or + whether a previous capturing subpattern matched or not. The + two possible forms of conditional subpattern are + + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) + + If the condition is satisfied, the yes-pattern is used; oth- + erwise the no-pattern (if present) is used. If there are + more than two alternatives in the subpattern, a compile-time + error occurs. + + There are two kinds of condition. If the text between the + parentheses consists of a sequence of digits, then the + condition is satisfied if the capturing subpattern of that + number has previously matched. Consider the following pat- + tern, which contains non-significant white space to make it + more readable (assume the PCRE_EXTENDED option) and to + divide it into three parts for ease of discussion: + + ( \( )? [^()]+ (?(1) \) ) + + The first part matches an optional opening parenthesis, and + if that character is present, sets it as the first captured + substring. The second part matches one or more characters + that are not parentheses. The third part is a conditional + subpattern that tests whether the first set of parentheses + matched or not. If they did, that is, if subject started + with an opening parenthesis, the condition is true, and so + the yes-pattern is executed and a closing parenthesis is + required. Otherwise, since no-pattern is not present, the + subpattern matches nothing. In other words, this pattern + matches a sequence of non-parentheses, optionally enclosed + in parentheses. + + If the condition is not a sequence of digits, it must be an + assertion. This may be a positive or negative lookahead or + lookbehind assertion. Consider this pattern, again contain- + ing non-significant white space, and with the two alterna- + tives on the second line: + + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) + + The condition is a positive lookahead assertion that matches + an optional sequence of non-letters followed by a letter. In + other words, it tests for the presence of at least one + letter in the subject. If a letter is found, the subject is + matched against the first alternative; otherwise it is + matched against the second. This pattern matches strings in + one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are + letters and dd are digits. + + + +COMMENTS + The sequence (?# marks the start of a comment which contin- + ues up to the next closing parenthesis. Nested parentheses + are not permitted. The characters that make up a comment + play no part in the pattern matching at all. + + If the PCRE_EXTENDED option is set, an unescaped # character + outside a character class introduces a comment that contin- + ues up to the next newline character in the pattern. + + + +PERFORMANCE + Certain items that may appear in patterns are more efficient + than others. It is more efficient to use a character class + like [aeiou] than a set of alternatives such as (a|e|i|o|u). + In general, the simplest construction that provides the + required behaviour is usually the most efficient. Jeffrey + Friedl's book contains a lot of discussion about optimizing + regular expressions for efficient performance. + + When a pattern begins with .* and the PCRE_DOTALL option is + set, the pattern is implicitly anchored by PCRE, since it + can match only at the start of a subject string. However, if + PCRE_DOTALL is not set, PCRE cannot make this optimization, + because the . metacharacter does not then match a newline, + and if the subject string contains newlines, the pattern may + match from the character immediately following one of them + instead of from the very start. For example, the pattern + + (.*) second + + matches the subject "first\nand second" (where \n stands for + a newline character) with the first captured substring being + "and". In order to do this, PCRE has to retry the match + starting after every newline in the subject. + + If you are using such a pattern with subject strings that do + not contain newlines, the best performance is obtained by + setting PCRE_DOTALL, or starting the pattern with ^.* to + indicate explicit anchoring. That saves PCRE from having to + scan along the subject looking for a newline to restart at. + + Beware of patterns that contain nested indefinite repeats. + These can take a long time to run when applied to a string + that does not match. Consider the pattern fragment + + (a+)* + + This can match "aaaa" in 33 different ways, and this number + increases very rapidly as the string gets longer. (The * + repeat can match 0, 1, 2, 3, or 4 times, and for each of + those cases other than 0, the + repeats can match different + numbers of times.) When the remainder of the pattern is such + that the entire match is going to fail, PCRE has in princi- + ple to try every possible variation, and this can take an + extremely long time. + + An optimization catches some of the more simple cases such + as + + (a+)*b + + where a literal character follows. Before embarking on the + standard matching procedure, PCRE checks that there is a "b" + later in the subject string, and if there is not, it fails + the match immediately. However, when there is no following + literal this optimization cannot be used. You can see the + difference by comparing the behaviour of + + (a+)*\d + + with the pattern above. The former gives a failure almost + instantly when applied to a whole line of "a" characters, + whereas the latter takes an appreciable time with strings + longer than about 20 characters. + + + +AUTHOR + Philip Hazel <ph10@cam.ac.uk> + University Computing Service, + New Museums Site, + Cambridge CB2 3QG, England. + Phone: +44 1223 334714 + + Last updated: 29 July 1999 + Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pcreposix.3 b/doc/pcreposix.3 new file mode 100644 index 0000000..0a40369 --- /dev/null +++ b/doc/pcreposix.3 @@ -0,0 +1,141 @@ +.TH PCRE 3 +.SH NAME +pcreposix - POSIX API for Perl-compatible regular expressions. +.SH SYNOPSIS +.B #include <pcreposix.h> +.PP +.SM +.br +.B int regcomp(regex_t *\fIpreg\fR, const char *\fIpattern\fR, +.ti +5n +.B int \fIcflags\fR); +.PP +.br +.B int regexec(regex_t *\fIpreg\fR, const char *\fIstring\fR, +.ti +5n +.B size_t \fInmatch\fR, regmatch_t \fIpmatch\fR[], int \fIeflags\fR); +.PP +.br +.B size_t regerror(int \fIerrcode\fR, const regex_t *\fIpreg\fR, +.ti +5n +.B char *\fIerrbuf\fR, size_t \fIerrbuf_size\fR); +.PP +.br +.B void regfree(regex_t *\fIpreg\fR); + + +.SH DESCRIPTION +This set of functions provides a POSIX-style API to the PCRE regular expression +package. See the \fBpcre\fR documentation for a description of the native API, +which contains additional functionality. + +The functions described here are just wrapper functions that ultimately call +the native API. Their prototypes are defined in the \fBpcreposix.h\fR header +file, and on Unix systems the library itself is called \fBpcreposix.a\fR, so +can be accessed by adding \fB-lpcreposix\fR to the command for linking an +application which uses them. Because the POSIX functions call the native ones, +it is also necessary to add \fR-lpcre\fR. + +As I am pretty ignorant about POSIX, these functions must be considered as +experimental. I have implemented only those option bits that can be reasonably +mapped to PCRE native options. Other POSIX options are not even defined. It may +be that it is useful to define, but ignore, other options. Feedback from more +knowledgeable folk may cause this kind of detail to change. + +When PCRE is called via these functions, it is only the API that is POSIX-like +in style. The syntax and semantics of the regular expressions themselves are +still those of Perl, subject to the setting of various PCRE options, as +described below. + +The header for these functions is supplied as \fBpcreposix.h\fR to avoid any +potential clash with other POSIX libraries. It can, of course, be renamed or +aliased as \fBregex.h\fR, which is the "correct" name. It provides two +structure types, \fIregex_t\fR for compiled internal forms, and +\fIregmatch_t\fR for returning captured substrings. It also defines some +constants whose names start with "REG_"; these are used for setting options and +identifying error codes. + + +.SH COMPILING A PATTERN + +The function \fBregcomp()\fR is called to compile a pattern into an +internal form. The pattern is a C string terminated by a binary zero, and +is passed in the argument \fIpattern\fR. The \fIpreg\fR argument is a pointer +to a regex_t structure which is used as a base for storing information about +the compiled expression. + +The argument \fIcflags\fR is either zero, or contains one or more of the bits +defined by the following macros: + + REG_ICASE + +The PCRE_CASELESS option is set when the expression is passed for compilation +to the native function. + + REG_NEWLINE + +The PCRE_MULTILINE option is set when the expression is passed for compilation +to the native function. + +The yield of \fBregcomp()\fR is zero on success, and non-zero otherwise. The +\fIpreg\fR structure is filled in on success, and one member of the structure +is publicized: \fIre_nsub\fR contains the number of capturing subpatterns in +the regular expression. Various error codes are defined in the header file. + + +.SH MATCHING A PATTERN +The function \fBregexec()\fR is called to match a pre-compiled pattern +\fIpreg\fR against a given \fIstring\fR, which is terminated by a zero byte, +subject to the options in \fIeflags\fR. These can be: + + REG_NOTBOL + +The PCRE_NOTBOL option is set when calling the underlying PCRE matching +function. + + REG_NOTEOL + +The PCRE_NOTEOL option is set when calling the underlying PCRE matching +function. + +The portion of the string that was matched, and also any captured substrings, +are returned via the \fIpmatch\fR argument, which points to an array of +\fInmatch\fR structures of type \fIregmatch_t\fR, containing the members +\fIrm_so\fR and \fIrm_eo\fR. These contain the offset to the first character of +each substring and the offset to the first character after the end of each +substring, respectively. The 0th element of the vector relates to the entire +portion of \fIstring\fR that was matched; subsequent elements relate to the +capturing subpatterns of the regular expression. Unused entries in the array +have both structure members set to -1. + +A successful match yields a zero return; various error codes are defined in the +header file, of which REG_NOMATCH is the "expected" failure code. + + +.SH ERROR MESSAGES +The \fBregerror()\fR function maps a non-zero errorcode from either +\fBregcomp\fR or \fBregexec\fR to a printable message. If \fIpreg\fR is not +NULL, the error should have arisen from the use of that structure. A message +terminated by a binary zero is placed in \fIerrbuf\fR. The length of the +message, including the zero, is limited to \fIerrbuf_size\fR. The yield of the +function is the size of buffer needed to hold the whole message. + + +.SH STORAGE +Compiling a regular expression causes memory to be allocated and associated +with the \fIpreg\fR structure. The function \fBregfree()\fR frees all such +memory, after which \fIpreg\fR may no longer be used as a compiled expression. + + +.SH AUTHOR +Philip Hazel <ph10@cam.ac.uk> +.br +University Computing Service, +.br +New Museums Site, +.br +Cambridge CB2 3QG, England. +.br +Phone: +44 1223 334714 + +Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pcreposix.html b/doc/pcreposix.html new file mode 100644 index 0000000..2c764b6 --- /dev/null +++ b/doc/pcreposix.html @@ -0,0 +1,182 @@ +<HTML> +<HEAD> +<TITLE>pcreposix specification</TITLE> +</HEAD> +<body bgcolor="#FFFFFF" text="#00005A"> +<H1>pcreposix specification</H1> +This HTML document has been generated automatically from the original man page. +If there is any nonsense in it, please consult the man page in case the +conversion went wrong. +<UL> +<LI><A NAME="TOC1" HREF="#SEC1">NAME</A> +<LI><A NAME="TOC2" HREF="#SEC2">SYNOPSIS</A> +<LI><A NAME="TOC3" HREF="#SEC3">DESCRIPTION</A> +<LI><A NAME="TOC4" HREF="#SEC4">COMPILING A PATTERN</A> +<LI><A NAME="TOC5" HREF="#SEC5">MATCHING A PATTERN</A> +<LI><A NAME="TOC6" HREF="#SEC6">ERROR MESSAGES</A> +<LI><A NAME="TOC7" HREF="#SEC7">STORAGE</A> +<LI><A NAME="TOC8" HREF="#SEC8">AUTHOR</A> +</UL> +<LI><A NAME="SEC1" HREF="#TOC1">NAME</A> +<P> +pcreposix - POSIX API for Perl-compatible regular expressions. +</P> +<LI><A NAME="SEC2" HREF="#TOC1">SYNOPSIS</A> +<P> +<B>#include <pcreposix.h></B> +</P> +<P> +<B>int regcomp(regex_t *<I>preg</I>, const char *<I>pattern</I>,</B> +<B>int <I>cflags</I>);</B> +</P> +<P> +<B>int regexec(regex_t *<I>preg</I>, const char *<I>string</I>,</B> +<B>size_t <I>nmatch</I>, regmatch_t <I>pmatch</I>[], int <I>eflags</I>);</B> +</P> +<P> +<B>size_t regerror(int <I>errcode</I>, const regex_t *<I>preg</I>,</B> +<B>char *<I>errbuf</I>, size_t <I>errbuf_size</I>);</B> +</P> +<P> +<B>void regfree(regex_t *<I>preg</I>);</B> +</P> +<LI><A NAME="SEC3" HREF="#TOC1">DESCRIPTION</A> +<P> +This set of functions provides a POSIX-style API to the PCRE regular expression +package. See the <B>pcre</B> documentation for a description of the native API, +which contains additional functionality. +</P> +<P> +The functions described here are just wrapper functions that ultimately call +the native API. Their prototypes are defined in the <B>pcreposix.h</B> header +file, and on Unix systems the library itself is called <B>pcreposix.a</B>, so +can be accessed by adding <B>-lpcreposix</B> to the command for linking an +application which uses them. Because the POSIX functions call the native ones, +it is also necessary to add \fR-lpcre\fR. +</P> +<P> +As I am pretty ignorant about POSIX, these functions must be considered as +experimental. I have implemented only those option bits that can be reasonably +mapped to PCRE native options. Other POSIX options are not even defined. It may +be that it is useful to define, but ignore, other options. Feedback from more +knowledgeable folk may cause this kind of detail to change. +</P> +<P> +When PCRE is called via these functions, it is only the API that is POSIX-like +in style. The syntax and semantics of the regular expressions themselves are +still those of Perl, subject to the setting of various PCRE options, as +described below. +</P> +<P> +The header for these functions is supplied as <B>pcreposix.h</B> to avoid any +potential clash with other POSIX libraries. It can, of course, be renamed or +aliased as <B>regex.h</B>, which is the "correct" name. It provides two +structure types, <I>regex_t</I> for compiled internal forms, and +<I>regmatch_t</I> for returning captured substrings. It also defines some +constants whose names start with "REG_"; these are used for setting options and +identifying error codes. +</P> +<LI><A NAME="SEC4" HREF="#TOC1">COMPILING A PATTERN</A> +<P> +The function <B>regcomp()</B> is called to compile a pattern into an +internal form. The pattern is a C string terminated by a binary zero, and +is passed in the argument <I>pattern</I>. The <I>preg</I> argument is a pointer +to a regex_t structure which is used as a base for storing information about +the compiled expression. +</P> +<P> +The argument <I>cflags</I> is either zero, or contains one or more of the bits +defined by the following macros: +</P> +<P> +<PRE> + REG_ICASE +</PRE> +</P> +<P> +The PCRE_CASELESS option is set when the expression is passed for compilation +to the native function. +</P> +<P> +<PRE> + REG_NEWLINE +</PRE> +</P> +<P> +The PCRE_MULTILINE option is set when the expression is passed for compilation +to the native function. +</P> +<P> +The yield of <B>regcomp()</B> is zero on success, and non-zero otherwise. The +<I>preg</I> structure is filled in on success, and one member of the structure +is publicized: <I>re_nsub</I> contains the number of capturing subpatterns in +the regular expression. Various error codes are defined in the header file. +</P> +<LI><A NAME="SEC5" HREF="#TOC1">MATCHING A PATTERN</A> +<P> +The function <B>regexec()</B> is called to match a pre-compiled pattern +<I>preg</I> against a given <I>string</I>, which is terminated by a zero byte, +subject to the options in <I>eflags</I>. These can be: +</P> +<P> +<PRE> + REG_NOTBOL +</PRE> +</P> +<P> +The PCRE_NOTBOL option is set when calling the underlying PCRE matching +function. +</P> +<P> +<PRE> + REG_NOTEOL +</PRE> +</P> +<P> +The PCRE_NOTEOL option is set when calling the underlying PCRE matching +function. +</P> +<P> +The portion of the string that was matched, and also any captured substrings, +are returned via the <I>pmatch</I> argument, which points to an array of +<I>nmatch</I> structures of type <I>regmatch_t</I>, containing the members +<I>rm_so</I> and <I>rm_eo</I>. These contain the offset to the first character of +each substring and the offset to the first character after the end of each +substring, respectively. The 0th element of the vector relates to the entire +portion of <I>string</I> that was matched; subsequent elements relate to the +capturing subpatterns of the regular expression. Unused entries in the array +have both structure members set to -1. +</P> +<P> +A successful match yields a zero return; various error codes are defined in the +header file, of which REG_NOMATCH is the "expected" failure code. +</P> +<LI><A NAME="SEC6" HREF="#TOC1">ERROR MESSAGES</A> +<P> +The <B>regerror()</B> function maps a non-zero errorcode from either +<B>regcomp</B> or <B>regexec</B> to a printable message. If <I>preg</I> is not +NULL, the error should have arisen from the use of that structure. A message +terminated by a binary zero is placed in <I>errbuf</I>. The length of the +message, including the zero, is limited to <I>errbuf_size</I>. The yield of the +function is the size of buffer needed to hold the whole message. +</P> +<LI><A NAME="SEC7" HREF="#TOC1">STORAGE</A> +<P> +Compiling a regular expression causes memory to be allocated and associated +with the <I>preg</I> structure. The function <B>regfree()</B> frees all such +memory, after which <I>preg</I> may no longer be used as a compiled expression. +</P> +<LI><A NAME="SEC8" HREF="#TOC1">AUTHOR</A> +<P> +Philip Hazel <ph10@cam.ac.uk> +<BR> +University Computing Service, +<BR> +New Museums Site, +<BR> +Cambridge CB2 3QG, England. +<BR> +Phone: +44 1223 334714 +</P> +<P> +Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pcreposix.txt b/doc/pcreposix.txt new file mode 100644 index 0000000..c85fb84 --- /dev/null +++ b/doc/pcreposix.txt @@ -0,0 +1,150 @@ +NAME + pcreposix - POSIX API for Perl-compatible regular expres- + sions. + + + +SYNOPSIS + #include <pcreposix.h> + + int regcomp(regex_t *preg, const char *pattern, + int cflags); + + int regexec(regex_t *preg, const char *string, + size_t nmatch, regmatch_t pmatch[], int eflags); + + size_t regerror(int errcode, const regex_t *preg, + char *errbuf, size_t errbuf_size); + + void regfree(regex_t *preg); + + + +DESCRIPTION + This set of functions provides a POSIX-style API to the PCRE + regular expression package. See the pcre documentation for a + description of the native API, which contains additional + functionality. + + The functions described here are just wrapper functions that + ultimately call the native API. Their prototypes are defined + in the pcreposix.h header file, and on Unix systems the + library itself is called pcreposix.a, so can be accessed by + adding -lpcreposix to the command for linking an application + which uses them. Because the POSIX functions call the native + ones, it is also necessary to add -lpcre. + + As I am pretty ignorant about POSIX, these functions must be + considered as experimental. I have implemented only those + option bits that can be reasonably mapped to PCRE native + options. Other POSIX options are not even defined. It may be + that it is useful to define, but ignore, other options. + Feedback from more knowledgeable folk may cause this kind of + detail to change. + + When PCRE is called via these functions, it is only the API + that is POSIX-like in style. The syntax and semantics of the + regular expressions themselves are still those of Perl, sub- + ject to the setting of various PCRE options, as described + below. + + The header for these functions is supplied as pcreposix.h to + avoid any potential clash with other POSIX libraries. It + can, of course, be renamed or aliased as regex.h, which is + the "correct" name. It provides two structure types, regex_t + for compiled internal forms, and regmatch_t for returning + captured substrings. It also defines some constants whose + names start with "REG_"; these are used for setting options + and identifying error codes. + + + +COMPILING A PATTERN + The function regcomp() is called to compile a pattern into + an internal form. The pattern is a C string terminated by a + binary zero, and is passed in the argument pattern. The preg + argument is a pointer to a regex_t structure which is used + as a base for storing information about the compiled expres- + sion. + + The argument cflags is either zero, or contains one or more + of the bits defined by the following macros: + + REG_ICASE + + The PCRE_CASELESS option is set when the expression is + passed for compilation to the native function. + + REG_NEWLINE + + The PCRE_MULTILINE option is set when the expression is + passed for compilation to the native function. + + The yield of regcomp() is zero on success, and non-zero oth- + erwise. The preg structure is filled in on success, and one + member of the structure is publicized: re_nsub contains the + number of capturing subpatterns in the regular expression. + Various error codes are defined in the header file. + + + +MATCHING A PATTERN + The function regexec() is called to match a pre-compiled + pattern preg against a given string, which is terminated by + a zero byte, subject to the options in eflags. These can be: + + REG_NOTBOL + + The PCRE_NOTBOL option is set when calling the underlying + PCRE matching function. + + REG_NOTEOL + + The PCRE_NOTEOL option is set when calling the underlying + PCRE matching function. + + The portion of the string that was matched, and also any + captured substrings, are returned via the pmatch argument, + which points to an array of nmatch structures of type + regmatch_t, containing the members rm_so and rm_eo. These + contain the offset to the first character of each substring + and the offset to the first character after the end of each + substring, respectively. The 0th element of the vector + relates to the entire portion of string that was matched; + subsequent elements relate to the capturing subpatterns of + the regular expression. Unused entries in the array have + both structure members set to -1. + + A successful match yields a zero return; various error codes + are defined in the header file, of which REG_NOMATCH is the + "expected" failure code. + + + +ERROR MESSAGES + The regerror() function maps a non-zero errorcode from + either regcomp or regexec to a printable message. If preg is + not NULL, the error should have arisen from the use of that + structure. A message terminated by a binary zero is placed + in errbuf. The length of the message, including the zero, is + limited to errbuf_size. The yield of the function is the + size of buffer needed to hold the whole message. + + + +STORAGE + Compiling a regular expression causes memory to be allocated + and associated with the preg structure. The function reg- + free() frees all such memory, after which preg may no longer + be used as a compiled expression. + + + +AUTHOR + Philip Hazel <ph10@cam.ac.uk> + University Computing Service, + New Museums Site, + Cambridge CB2 3QG, England. + Phone: +44 1223 334714 + + Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pcretest.txt b/doc/pcretest.txt new file mode 100644 index 0000000..29e2f5c --- /dev/null +++ b/doc/pcretest.txt @@ -0,0 +1,213 @@ +The pcretest program +-------------------- + +This program is intended for testing PCRE, but it can also be used for +experimenting with regular expressions. + +If it is given two filename arguments, it reads from the first and writes to +the second. If it is given only one filename argument, it reads from that file +and writes to stdout. Otherwise, it reads from stdin and writes to stdout, and +prompts for each line of input. + +The program handles any number of sets of input on a single input file. Each +set starts with a regular expression, and continues with any number of data +lines to be matched against the pattern. An empty line signals the end of the +set. The regular expressions are given enclosed in any non-alphameric +delimiters other than backslash, for example + + /(a|bc)x+yz/ + +White space before the initial delimiter is ignored. A regular expression may +be continued over several input lines, in which case the newline characters are +included within it. See the testinput files for many examples. It is possible +to include the delimiter within the pattern by escaping it, for example + + /abc\/def/ + +If you do so, the escape and the delimiter form part of the pattern, but since +delimiters are always non-alphameric, this does not affect its interpretation. +If the terminating delimiter is immediately followed by a backslash, for +example, + + /abc/\ + +then a backslash is added to the end of the pattern. This is done to provide a +way of testing the error condition that arises if a pattern finishes with a +backslash, because + + /abc\/ + +is interpreted as the first line of a pattern that starts with "abc/", causing +pcretest to read the next line as a continuation of the regular expression. + +The pattern may be followed by i, m, s, or x to set the PCRE_CASELESS, +PCRE_MULTILINE, PCRE_DOTALL, or PCRE_EXTENDED options, respectively. For +example: + + /caseless/i + +These modifier letters have the same effect as they do in Perl. There are +others which set PCRE options that do not correspond to anything in Perl: /A, +/E, and /X set PCRE_ANCHORED, PCRE_DOLLAR_ENDONLY, and PCRE_EXTRA respectively. + +Searching for all possible matches within each subject string can be requested +by the /g or /G modifier. After finding a match, PCRE is called again to search +the remainder of the subject string. The difference between /g and /G is that +the former uses the startoffset argument to pcre_exec() to start searching at +a new point within the entire string (which is in effect what Perl does), +whereas the latter passes over a shortened substring. This makes a difference +to the matching process if the pattern begins with a lookbehind assertion +(including \b or \B). + +If any call to pcre_exec() in a /g or /G sequence matches an empty string, the +next call is done with the PCRE_NOTEMPTY flag set so that it cannot match an +empty string again at the same point. If however, this second match fails, the +start offset is advanced by one, and the match is retried. This imitates the +way Perl handles such cases when using the /g modifier or the split() function. + +There are a number of other modifiers for controlling the way pcretest +operates. + +The /+ modifier requests that as well as outputting the substring that matched +the entire pattern, pcretest should in addition output the remainder of the +subject string. This is useful for tests where the subject contains multiple +copies of the same substring. + +The /L modifier must be followed directly by the name of a locale, for example, + + /pattern/Lfr + +For this reason, it must be the last modifier letter. The given locale is set, +pcre_maketables() is called to build a set of character tables for the locale, +and this is then passed to pcre_compile() when compiling the regular +expression. Without an /L modifier, NULL is passed as the tables pointer; that +is, /L applies only to the expression on which it appears. + +The /I modifier requests that pcretest output information about the compiled +expression (whether it is anchored, has a fixed first character, and so on). It +does this by calling pcre_info() after compiling an expression, and outputting +the information it gets back. If the pattern is studied, the results of that +are also output. + +The /D modifier is a PCRE debugging feature, which also assumes /I. It causes +the internal form of compiled regular expressions to be output after +compilation. + +The /S modifier causes pcre_study() to be called after the expression has been +compiled, and the results used when the expression is matched. + +The /M modifier causes the size of memory block used to hold the compiled +pattern to be output. + +Finally, the /P modifier causes pcretest to call PCRE via the POSIX wrapper API +rather than its native API. When this is done, all other modifiers except /i, +/m, and /+ are ignored. REG_ICASE is set if /i is present, and REG_NEWLINE is +set if /m is present. The wrapper functions force PCRE_DOLLAR_ENDONLY always, +and PCRE_DOTALL unless REG_NEWLINE is set. + +Before each data line is passed to pcre_exec(), leading and trailing whitespace +is removed, and it is then scanned for \ escapes. The following are recognized: + + \a alarm (= BEL) + \b backspace + \e escape + \f formfeed + \n newline + \r carriage return + \t tab + \v vertical tab + \nnn octal character (up to 3 octal digits) + \xhh hexadecimal character (up to 2 hex digits) + + \A pass the PCRE_ANCHORED option to pcre_exec() + \B pass the PCRE_NOTBOL option to pcre_exec() + \Cdd call pcre_copy_substring() for substring dd after a successful match + (any decimal number less than 32) + \Gdd call pcre_get_substring() for substring dd after a successful match + (any decimal number less than 32) + \L call pcre_get_substringlist() after a successful match + \N pass the PCRE_NOTEMPTY option to pcre_exec() + \Odd set the size of the output vector passed to pcre_exec() to dd + (any number of decimal digits) + \Z pass the PCRE_NOTEOL option to pcre_exec() + +A backslash followed by anything else just escapes the anything else. If the +very last character is a backslash, it is ignored. This gives a way of passing +an empty line as data, since a real empty line terminates the data input. + +If /P was present on the regex, causing the POSIX wrapper API to be used, only +\B, and \Z have any effect, causing REG_NOTBOL and REG_NOTEOL to be passed to +regexec() respectively. + +When a match succeeds, pcretest outputs the list of captured substrings that +pcre_exec() returns, starting with number 0 for the string that matched the +whole pattern. Here is an example of an interactive pcretest run. + + $ pcretest + PCRE version 2.06 08-Jun-1999 + + re> /^abc(\d+)/ + data> abc123 + 0: abc123 + 1: 123 + data> xyz + No match + +If the strings contain any non-printing characters, they are output as \0x +escapes. If the pattern has the /+ modifier, then the output for substring 0 is +followed by the the rest of the subject string, identified by "0+" like this: + + re> /cat/+ + data> cataract + 0: cat + 0+ aract + +If the pattern has the /g or /G modifier, the results of successive matching +attempts are output in sequence, like this: + + re> /\Bi(\w\w)/g + data> Mississippi + 0: iss + 1: ss + 0: iss + 1: ss + 0: ipp + 1: pp + +"No match" is output only if the first match attempt fails. + +If any of \C, \G, or \L are present in a data line that is successfully +matched, the substrings extracted by the convenience functions are output with +C, G, or L after the string number instead of a colon. This is in addition to +the normal full list. The string length (that is, the return from the +extraction function) is given in parentheses after each string for \C and \G. + +Note that while patterns can be continued over several lines (a plain ">" +prompt is used for continuations), data lines may not. However newlines can be +included in data by means of the \n escape. + +If the -p option is given to pcretest, it is equivalent to adding /P to each +regular expression: the POSIX wrapper API is used to call PCRE. None of the +following flags has any effect in this case. + +If the option -d is given to pcretest, it is equivalent to adding /D to each +regular expression: the internal form is output after compilation. + +If the option -i is given to pcretest, it is equivalent to adding /I to each +regular expression: information about the compiled pattern is given after +compilation. + +If the option -m is given to pcretest, it outputs the size of each compiled +pattern after it has been compiled. It is equivalent to adding /M to each +regular expression. For compatibility with earlier versions of pcretest, -s is +a synonym for -m. + +If the -t option is given, each compile, study, and match is run 20000 times +while being timed, and the resulting time per compile or match is output in +milliseconds. Do not set -t with -s, because you will then get the size output +20000 times and the timing will be distorted. If you want to change the number +of repetitions used for timing, edit the definition of LOOPREPEAT at the top of +pcretest.c + +Philip Hazel <ph10@cam.ac.uk> +January 2000 diff --git a/doc/perltest.txt b/doc/perltest.txt new file mode 100644 index 0000000..6c38ebe --- /dev/null +++ b/doc/perltest.txt @@ -0,0 +1,23 @@ +The perltest program +-------------------- + +The perltest program tests Perl's regular expressions; it has the same +specification as pcretest, and so can be given identical input, except that +input patterns can be followed only by Perl's lower case modifiers and /+ (as +used by pcretest), which is recognized and handled by the program. + +The data lines are processed as Perl double-quoted strings, so if they contain +" \ $ or @ characters, these have to be escaped. For this reason, all such +characters in testinput1 and testinput3 are escaped so that they can be used +for perltest as well as for pcretest, and the special upper case modifiers such +as /A that pcretest recognizes are not used in these files. The output should +be identical, apart from the initial identifying banner. + +The testinput2 and testinput4 files are not suitable for feeding to perltest, +since they do make use of the special upper case modifiers and escapes that +pcretest uses to test some features of PCRE. The first of these files also +contains malformed regular expressions, in order to check that PCRE diagnoses +them correctly. + +Philip Hazel <ph10@cam.ac.uk> +January 2000 diff --git a/doc/pgrep.1 b/doc/pgrep.1 new file mode 100644 index 0000000..d9e9b57 --- /dev/null +++ b/doc/pgrep.1 @@ -0,0 +1,76 @@ +.TH PGREP 1 +.SH NAME +pgrep - a grep with Perl-compatible regular expressions. +.SH SYNOPSIS +.B pgrep [-Vchilnsvx] pattern [file] ... + + +.SH DESCRIPTION +\fBpgrep\fR searches files for character patterns, in the same way as other +grep commands do, but it uses the PCRE regular expression library to support +patterns that are compatible with the regular expressions of Perl 5. See +\fBpcre(3)\fR for a full description of syntax and semantics. + +If no files are specified, \fBpgrep\fR reads the standard input. By default, +each line that matches the pattern is copied to the standard output, and if +there is more than one file, the file name is printed before each line of +output. However, there are options that can change how \fBpgrep\fR behaves. + +Lines are limited to BUFSIZ characters. BUFSIZ is defined in \fB<stdio.h>\fR. +The newline character is removed from the end of each line before it is matched +against the pattern. + + +.SH OPTIONS +.TP 10 +\fB-V\fR +Write the version number of the PCRE library being used to the standard error +stream. +.TP +\fB-c\fR +Do not print individual lines; instead just print a count of the number of +lines that would otherwise have been printed. If several files are given, a +count is printed for each of them. +.TP +\fB-h\fR +Suppress printing of filenames when searching multiple files. +.TP +\fB-i\fR +Ignore upper/lower case distinctions during comparisons. +.TP +\fB-l\fR +Instead of printing lines from the files, just print the names of the files +containing lines that would have been printed. Each file name is printed +once, on a separate line. +.TP +\fB-n\fR +Precede each line by its line number in the file. +.TP +\fB-s\fR +Work silently, that is, display nothing except error messages. +The exit status indicates whether any matches were found. +.TP +\fB-v\fR +Invert the sense of the match, so that lines which do \fInot\fR match the +pattern are now the ones that are found. +.TP +\fB-x\fR +Force the pattern to be anchored (it must start matching at the beginning of +the line) and in addition, require it to match the entire line. This is +equivalent to having ^ and $ characters at the start and end of each +alternative branch in the regular expression. + + +.SH SEE ALSO +\fBpcre(3)\fR, Perl 5 documentation + + +.SH DIAGNOSTICS +Exit status is 0 if any matches were found, 1 if no matches were found, and 2 +for syntax errors or inacessible files (even if matches were found). + + +.SH AUTHOR +Philip Hazel <ph10@cam.ac.uk> +.br +Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pgrep.html b/doc/pgrep.html new file mode 100644 index 0000000..54efed6 --- /dev/null +++ b/doc/pgrep.html @@ -0,0 +1,105 @@ +<HTML> +<HEAD> +<TITLE>pgrep specification</TITLE> +</HEAD> +<body bgcolor="#FFFFFF" text="#00005A"> +<H1>pgrep specification</H1> +This HTML document has been generated automatically from the original man page. +If there is any nonsense in it, please consult the man page in case the +conversion went wrong. +<UL> +<LI><A NAME="TOC1" HREF="#SEC1">NAME</A> +<LI><A NAME="TOC2" HREF="#SEC2">SYNOPSIS</A> +<LI><A NAME="TOC3" HREF="#SEC3">DESCRIPTION</A> +<LI><A NAME="TOC4" HREF="#SEC4">OPTIONS</A> +<LI><A NAME="TOC5" HREF="#SEC5">SEE ALSO</A> +<LI><A NAME="TOC6" HREF="#SEC6">DIAGNOSTICS</A> +<LI><A NAME="TOC7" HREF="#SEC7">AUTHOR</A> +</UL> +<LI><A NAME="SEC1" HREF="#TOC1">NAME</A> +<P> +pgrep - a grep with Perl-compatible regular expressions. +</P> +<LI><A NAME="SEC2" HREF="#TOC1">SYNOPSIS</A> +<P> +<B>pgrep [-Vchilnsvx] pattern [file] ...</B> +</P> +<LI><A NAME="SEC3" HREF="#TOC1">DESCRIPTION</A> +<P> +<B>pgrep</B> searches files for character patterns, in the same way as other +grep commands do, but it uses the PCRE regular expression library to support +patterns that are compatible with the regular expressions of Perl 5. See +<B>pcre(3)</B> for a full description of syntax and semantics. +</P> +<P> +If no files are specified, <B>pgrep</B> reads the standard input. By default, +each line that matches the pattern is copied to the standard output, and if +there is more than one file, the file name is printed before each line of +output. However, there are options that can change how <B>pgrep</B> behaves. +</P> +<P> +Lines are limited to BUFSIZ characters. BUFSIZ is defined in <B><stdio.h></B>. +The newline character is removed from the end of each line before it is matched +against the pattern. +</P> +<LI><A NAME="SEC4" HREF="#TOC1">OPTIONS</A> +<P> +<B>-V</B> +Write the version number of the PCRE library being used to the standard error +stream. +</P> +<P> +<B>-c</B> +Do not print individual lines; instead just print a count of the number of +lines that would otherwise have been printed. If several files are given, a +count is printed for each of them. +</P> +<P> +<B>-h</B> +Suppress printing of filenames when searching multiple files. +</P> +<P> +<B>-i</B> +Ignore upper/lower case distinctions during comparisons. +</P> +<P> +<B>-l</B> +Instead of printing lines from the files, just print the names of the files +containing lines that would have been printed. Each file name is printed +once, on a separate line. +</P> +<P> +<B>-n</B> +Precede each line by its line number in the file. +</P> +<P> +<B>-s</B> +Work silently, that is, display nothing except error messages. +The exit status indicates whether any matches were found. +</P> +<P> +<B>-v</B> +Invert the sense of the match, so that lines which do <I>not</I> match the +pattern are now the ones that are found. +</P> +<P> +<B>-x</B> +Force the pattern to be anchored (it must start matching at the beginning of +the line) and in addition, require it to match the entire line. This is +equivalent to having ^ and $ characters at the start and end of each +alternative branch in the regular expression. +</P> +<LI><A NAME="SEC5" HREF="#TOC1">SEE ALSO</A> +<P> +<B>pcre(3)</B>, Perl 5 documentation +</P> +<LI><A NAME="SEC6" HREF="#TOC1">DIAGNOSTICS</A> +<P> +Exit status is 0 if any matches were found, 1 if no matches were found, and 2 +for syntax errors or inacessible files (even if matches were found). +</P> +<LI><A NAME="SEC7" HREF="#TOC1">AUTHOR</A> +<P> +Philip Hazel <ph10@cam.ac.uk> +<BR> +Copyright (c) 1997-1999 University of Cambridge. diff --git a/doc/pgrep.txt b/doc/pgrep.txt new file mode 100644 index 0000000..bcd08c0 --- /dev/null +++ b/doc/pgrep.txt @@ -0,0 +1,86 @@ +NAME + pgrep - a grep with Perl-compatible regular expressions. + + + +SYNOPSIS + pgrep [-Vchilnsvx] pattern [file] ... + + + +DESCRIPTION + pgrep searches files for character patterns, in the same way + as other grep commands do, but it uses the PCRE regular + expression library to support patterns that are compatible + with the regular expressions of Perl 5. See pcre(3) for a + full description of syntax and semantics. + + If no files are specified, pgrep reads the standard input. + By default, each line that matches the pattern is copied to + the standard output, and if there is more than one file, the + file name is printed before each line of output. However, + there are options that can change how pgrep behaves. + + Lines are limited to BUFSIZ characters. BUFSIZ is defined in + <stdio.h>. The newline character is removed from the end of + each line before it is matched against the pattern. + + + +OPTIONS + -V Write the version number of the PCRE library being + used to the standard error stream. + + -c Do not print individual lines; instead just print + a count of the number of lines that would other- + wise have been printed. If several files are + given, a count is printed for each of them. + + -h Suppress printing of filenames when searching mul- + tiple files. + + -i Ignore upper/lower case distinctions during com- + parisons. + + -l Instead of printing lines from the files, just + print the names of the files containing lines that + would have been printed. Each file name is printed + once, on a separate line. + + -n Precede each line by its line number in the file. + + -s Work silently, that is, display nothing except + error messages. The exit status indicates whether + any matches were found. + + -v Invert the sense of the match, so that lines which + do not match the pattern are now the ones that are + found. + + -x Force the pattern to be anchored (it must start + matching at the beginning of the line) and in + addition, require it to match the entire line. + This is equivalent to having ^ and $ characters at + the start and end of each alternative branch in + the regular expression. + + + +SEE ALSO + pcre(3), Perl 5 documentation + + + + + +DIAGNOSTICS + Exit status is 0 if any matches were found, 1 if no matches + were found, and 2 for syntax errors or inacessible files + (even if matches were found). + + + +AUTHOR + Philip Hazel <ph10@cam.ac.uk> + Copyright (c) 1997-1999 University of Cambridge. + |