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diff --git a/doc/pcrepattern.3 b/doc/pcrepattern.3 new file mode 100644 index 0000000..cef74e1 --- /dev/null +++ b/doc/pcrepattern.3 @@ -0,0 +1,1231 @@ +.TH PCRE 3 +.SH NAME +PCRE - Perl-compatible regular expressions +.SH PCRE REGULAR EXPRESSION DETAILS +.rs +.sp +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, covers them in great detail. The description here is intended as +reference documentation. + +The basic operation of PCRE is on strings of bytes. However, there is also +support for UTF-8 character strings. To use this support you must build PCRE to +include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8 +option. How this affects the pattern matching is mentioned in several places +below. There is also a summary of UTF-8 features in the +.\" HTML <a href="pcre.html#utf8support"> +.\" </a> +section on UTF-8 support +.\" +in the main +.\" HREF +\fBpcre\fR +.\" +page. + +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 string (or line, in multiline mode) + $ assert end of string (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 + also "possessive 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 + [ POSIX character class (only if followed by POSIX + syntax) + ] terminates the character class + +The following sections describe the use of each of the meta-characters. + +.SH BACKSLASH +.rs +.sp +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 escaping action 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 backslash 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. + +If you want to remove the special meaning from a sequence of characters, you +can do so by putting them between \\Q and \\E. This is different from Perl in +that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in +Perl, $ and @ cause variable interpolation. Note the following examples: + + Pattern PCRE matches Perl matches + + \\Qabc$xyz\\E abc$xyz abc followed by the + contents of $xyz + \\Qabc\\$xyz\\E abc\\$xyz abc\\$xyz + \\Qabc\\E\\$\\Qxyz\\E abc$xyz abc$xyz + +The \\Q...\\E sequence is recognized both inside and outside character classes. + +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) + \\ddd character with octal code ddd, or backreference + \\xhh character with hex code hh + \\x{hhh..} character with hex code hhh... (UTF-8 mode only) + +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, from zero to two hexadecimal digits are read (letters can be in +upper or lower case). In UTF-8 mode, any number of hexadecimal digits may +appear between \\x{ and }, but the value of the character code must be less +than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters +other than hexadecimal digits appear between \\x{ and }, or if there is no +terminating }, this form of escape is not recognized. Instead, the initial +\\x will be interpreted as a basic hexadecimal escape, with no following +digits, giving a byte whose value is zero. + +Characters whose value is less than 256 can be defined by either of the two +syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the +way they are handled. For example, \\xdc is exactly the same as \\x{dc}. + +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 +(code value 7). 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 might be a back reference, otherwise the + character with octal code 113 + \\377 might be a back reference, otherwise + the 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 or a single UTF-8 character +(in UTF-8 mode) 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. + +In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or +\\w, and always match \\D, \\S, and \\W. + +For compatibility with Perl, \\s does not match the VT character (code 11). +This makes it different from the the POSIX "space" class. The \\s characters +are HT (9), LF (10), FF (12), CR (13), and space (32). + +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 +.\" HTML <a href="pcreapi.html#localesupport"> +.\" </a> +"Locale support" +.\" +in the +.\" HREF +\fBpcreapi\fR +.\" +page). 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 matches at a word boundary + \\B matches when not at a word boundary + \\A matches at start of subject + \\Z matches at end of subject or before newline at end + \\z matches at end of subject + \\G matches at first matching position in subject + +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. Thus, they are independent of +multiline mode. + +They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the +\fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that +matching is to start at a point other than the beginning of the subject, \\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. + +The \\G assertion is true only when the current matching position is at the +start point of the match, as specified by the \fIstartoffset\fR argument of +\fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is +non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate +arguments, you can mimic Perl's /g option, and it is in this kind of +implementation where \\G can be useful. + +Note, however, that PCRE's interpretation of \\G, as the start of the current +match, is subtly different from Perl's, which defines it as the end of the +previous match. In Perl, these can be different when the previously matched +string was empty. Because PCRE does just one match at a time, it cannot +reproduce this behaviour. + +If all the alternatives of a pattern begin with \\G, the expression is anchored +to the starting match position, and the "anchored" flag is set in the compiled +regular expression. + +.SH CIRCUMFLEX AND DOLLAR +.rs +.sp +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 if the PCRE_MULTILINE +option is unset. 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 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 newline 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 it is always anchored, whether PCRE_MULTILINE is set or not. + +.SH FULL STOP (PERIOD, DOT) +.rs +.sp +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. +In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one +byte long, except (by default) for newline. If the PCRE_DOTALL option is set, +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 MATCHING A SINGLE BYTE +.rs +.sp +Outside a character class, the escape sequence \\C matches any one byte, both +in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The +feature is provided in Perl in order to match individual bytes in UTF-8 mode. +Because it breaks up UTF-8 characters into individual bytes, what remains in +the string may be a malformed UTF-8 string. For this reason it is best avoided. + +PCRE does not allow \\C to appear in lookbehind assertions (see below), because +in UTF-8 mode it makes it impossible to calculate the length of the lookbehind. + +.SH SQUARE BRACKETS +.rs +.sp +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. In UTF-8 mode, the +character may occupy more than one byte. A matched character must be in the set +of characters defined by the class, unless the first character in the class +definition 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. + +In UTF-8 mode, characters with values greater than 255 can be included in a +class as a literal string of bytes, or by using the \\x{ escaping mechanism. + +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. PCRE does not support the concept of case for characters +with values greater than 255. + +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 the collating sequence of character values. They can also be +used for characters specified numerically, for example [\\000-\\037]. In UTF-8 +mode, ranges can include characters whose values are greater than 255, for +example [\\x{100}-\\x{2ff}]. + +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 POSIX CHARACTER CLASSES +.rs +.sp +Perl supports the POSIX notation for character classes, which uses names +enclosed by [: and :] within the enclosing square brackets. PCRE also supports +this notation. For example, + + [01[:alpha:]%] + +matches "0", "1", any alphabetic character, or "%". The supported class names +are + + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \\d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits + space white space (not quite the same as \\s) + upper upper case letters + word "word" characters (same as \\w) + xdigit hexadecimal digits + +The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and +space (32). Notice that this list includes the VT character (code 11). This +makes "space" different to \\s, which does not include VT (for Perl +compatibility). + +The name "word" is a Perl extension, and "blank" is a GNU extension from Perl +5.8. Another Perl extension is negation, which is indicated by a ^ character +after the colon. For example, + + [12[:^digit:]] + +matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX +syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not +supported, and an error is given if they are encountered. + +In UTF-8 mode, characters with values greater than 255 do not match any of +the POSIX character classes. + +.SH VERTICAL BAR +.rs +.sp +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 +.rs +.sp +The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and +PCRE_EXTENDED options 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. + +When an option change occurs at top level (that is, not inside subpattern +parentheses), the change applies to the remainder of the pattern that follows. +If the change is placed right at the start of a pattern, PCRE extracts it into +the global options (and it will therefore show up in data extracted by the +\fBpcre_fullinfo()\fR function). + +An option change within a subpattern affects only that part of the current +pattern 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 +.rs +.sp +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, respectively. + +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 a question mark +and a colon, 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 capturing subpatterns is 65535, and the maximum depth +of nesting 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 NAMED SUBPATTERNS +.rs +.sp +Identifying capturing parentheses by number is simple, but it can be very hard +to keep track of the numbers in complicated regular expressions. Furthermore, +if an expression is modified, the numbers may change. To help with the +difficulty, PCRE supports the naming of subpatterns, something that Perl does +not provide. The Python syntax (?P<name>...) is used. Names consist of +alphanumeric characters and underscores, and must be unique within a pattern. + +Named capturing parentheses are still allocated numbers as well as names. The +PCRE API provides function calls for extracting the name-to-number translation +table from a compiled pattern. For further details see the +.\" HREF +\fBpcreapi\fR +.\" +documentation. + +.SH REPETITION +.rs +.sp +Repetition is specified by quantifiers, which can follow any of the following +items: + + a literal data character + the . metacharacter + the \\C escape sequence + escapes such as \\d that match single characters + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion) + +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. + +In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual +bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of +which is represented by a two-byte sequence. + +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 owing to the greediness of the .* +item. + +However, if a quantifier is followed by a question mark, 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), +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, 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 normally 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 in order to obtain this optimization, or +alternatively using ^ to indicate anchoring explicitly. + +However, there is one situation where the optimization cannot be used. When .* +is inside capturing parentheses that are the subject of a backreference +elsewhere in the pattern, a match at the start may fail, and a later one +succeed. Consider, for example: + + (.*)abc\\1 + +If the subject is "xyz123abc123" the match point is the fourth character. For +this reason, such a pattern is not implicitly anchored. + +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 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS +.rs +.sp +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. "Atomic grouping" +(a term taken from Jeffrey Friedl's book) provides the means for specifying +that once a subpattern has matched, it is not to be re-evaluated in this way. + +If we use atomic grouping for the previous example, the matcher would give up +immediately on failing to match "foo" the first time. The notation is a 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. + +Atomic grouping 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. + +Atomic groups in general can of course contain arbitrarily complicated +subpatterns, and can be nested. However, when the subpattern for an atomic +group is just a single repeated item, as in the example above, a simpler +notation, called a "possessive quantifier" can be used. This consists of an +additional + character following a quantifier. Using this notation, the +previous example can be rewritten as + + \\d++bar + +Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY +option is ignored. They are a convenient notation for the simpler forms of +atomic group. However, there is no difference in the meaning or processing of a +possessive quantifier and the equivalent atomic group. + +The possessive quantifier syntax is an extension to the Perl syntax. It +originates in Sun's Java package. + +When a pattern contains an unlimited repeat inside a subpattern that can itself +be repeated an unlimited number of times, the use of an atomic group is the +only way to avoid some failing matches taking a very long time indeed. The +pattern + + (\\D+|<\\d+>)*[!?] + +matches an unlimited number of substrings that either consist of non-digits, or +digits enclosed in <>, followed by either ! or ?. When it matches, it runs +quickly. However, if it is applied to + + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + +it takes a long time before reporting failure. This is because the string can +be divided between the two repeats in a large number of ways, and all have to +be tried. (The example used [!?] rather than a single character at the end, +because both PCRE and Perl have an optimization that allows for fast failure +when a single character is used. They remember the last single character that +is required for a match, and fail early if it is not present in the string.) +If the pattern is changed to + + ((?>\\D+)|<\\d+>)*[!?] + +sequences of non-digits cannot be broken, and failure happens quickly. + +.SH BACK REFERENCES +.rs +.sp +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 +(that is, 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 (see +.\" HTML <a href="#subpatternsassubroutines"> +.\" </a> +"Subpatterns as subroutines" +.\" +below for a way of doing that). 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, 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. + +Back references to named subpatterns use the Python syntax (?P=name). We could +rewrite the above example as follows: + + (?<p1>(?i)rah)\\s+(?P=p1) + +There may be more than one back reference to the same subpattern. If a +subpattern has not actually been used in a particular match, 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 +many capturing parentheses in a pattern, all digits following the backslash are +taken as part of a potential back reference number. If the pattern continues +with a digit character, 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", "ababbaa" 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 +.rs +.sp +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, \\G, \\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. + +If you want to force a matching failure at some point in a pattern, the most +convenient way to do it is with (?!) because an empty string always matches, so +an assertion that requires there not to be an empty string must always fail. + +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 (at least for 5.8), 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. + +PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode) +to appear in lookbehind assertions, because it makes it impossible to calculate +the length of the lookbehind. + +Atomic groups 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 that does not match. 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$ + +the initial .* matches the entire string at first, but when this fails (because +there is no following "a"), 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) + +or, equivalently, + + ^.*+(?<=abcd) + +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. + +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, and 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. + +.SH CONDITIONAL SUBPATTERNS +.rs +.sp +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 three kinds of condition. If the text between the parentheses +consists of a sequence of digits, the condition is satisfied if the capturing +subpattern of that number has previously matched. The number must be greater +than zero. 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 the string (R), it is satisfied if a recursive call to the +pattern or subpattern has been made. At "top level", the condition is false. +This is a PCRE extension. Recursive patterns are described in the next section. + +If the condition is not a sequence of digits or (R), 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 +.rs +.sp +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 RECURSIVE PATTERNS +.rs +.sp +Consider the problem of matching a string in parentheses, allowing for +unlimited nested parentheses. Without the use of recursion, the best that can +be done is to use a pattern that matches up to some fixed depth of nesting. It +is not possible to handle an arbitrary nesting depth. Perl has provided an +experimental facility that allows regular expressions to recurse (amongst other +things). It does this by interpolating Perl code in the expression at run time, +and the code can refer to the expression itself. A Perl pattern to solve the +parentheses problem can be created like this: + + $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x; + +The (?p{...}) item interpolates Perl code at run time, and in this case refers +recursively to the pattern in which it appears. Obviously, PCRE cannot support +the interpolation of Perl code. Instead, it supports some special syntax for +recursion of the entire pattern, and also for individual subpattern recursion. + +The special item that consists of (? followed by a number greater than zero and +a closing parenthesis is a recursive call of the subpattern of the given +number, provided that it occurs inside that subpattern. (If not, it is a +"subroutine" call, which is described in the next section.) The special item +(?R) is a recursive call of the entire regular expression. + +For example, this PCRE pattern solves the nested parentheses problem (assume +the PCRE_EXTENDED option is set so that white space is ignored): + + \\( ( (?>[^()]+) | (?R) )* \\) + +First it matches an opening parenthesis. Then it matches any number of +substrings which can either be a sequence of non-parentheses, or a recursive +match of the pattern itself (that is a correctly parenthesized substring). +Finally there is a closing parenthesis. + +If this were part of a larger pattern, you would not want to recurse the entire +pattern, so instead you could use this: + + ( \\( ( (?>[^()]+) | (?1) )* \\) ) + +We have put the pattern into parentheses, and caused the recursion to refer to +them instead of the whole pattern. In a larger pattern, keeping track of +parenthesis numbers can be tricky. It may be more convenient to use named +parentheses instead. For this, PCRE uses (?P>name), which is an extension to +the Python syntax that PCRE uses for named parentheses (Perl does not provide +named parentheses). We could rewrite the above example as follows: + + (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) ) + +This particular example pattern contains nested unlimited repeats, and so the +use of atomic grouping for matching strings of non-parentheses is important +when applying the pattern to strings that do not match. For example, when this +pattern is applied to + + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() + +it yields "no match" quickly. However, if atomic grouping is not used, +the match runs for a very long time indeed because there are so many different +ways the + and * repeats can carve up the subject, and all have to be tested +before failure can be reported. + +At the end of a match, the values set for any capturing subpatterns are those +from the outermost level of the recursion at which the subpattern value is set. +If you want to obtain intermediate values, a callout function can be used (see +below and the +.\" HREF +\fBpcrecallout\fR +.\" +documentation). If the pattern above is matched against + + (ab(cd)ef) + +the value for the capturing parentheses is "ef", which is the last value taken +on at the top level. If additional parentheses are added, giving + + \\( ( ( (?>[^()]+) | (?R) )* ) \\) + ^ ^ + ^ ^ + +the string they capture is "ab(cd)ef", the contents of the top level +parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE +has to obtain extra memory to store data during a recursion, which it does by +using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no +memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. + +Do not confuse the (?R) item with the condition (R), which tests for recursion. +Consider this pattern, which matches text in angle brackets, allowing for +arbitrary nesting. Only digits are allowed in nested brackets (that is, when +recursing), whereas any characters are permitted at the outer level. + + < (?: (?(R) \\d++ | [^<>]*+) | (?R)) * > + +In this pattern, (?(R) is the start of a conditional subpattern, with two +different alternatives for the recursive and non-recursive cases. The (?R) item +is the actual recursive call. + +.\" HTML <a name="subpatternsassubroutines"></a> +.SH SUBPATTERNS AS SUBROUTINES +.rs +.sp +If the syntax for a recursive subpattern reference (either by number or by +name) is used outside the parentheses to which it refers, it operates like a +subroutine in a programming language. An earlier example pointed out that the +pattern + + (sens|respons)e and \\1ibility + +matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If instead the pattern + + (sens|respons)e and (?1)ibility + +is used, it does match "sense and responsibility" as well as the other two +strings. Such references must, however, follow the subpattern to which they +refer. + +.SH CALLOUTS +.rs +.sp +Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl +code to be obeyed in the middle of matching a regular expression. This makes it +possible, amongst other things, to extract different substrings that match the +same pair of parentheses when there is a repetition. + +PCRE provides a similar feature, but of course it cannot obey arbitrary Perl +code. The feature is called "callout". The caller of PCRE provides an external +function by putting its entry point in the global variable \fIpcre_callout\fR. +By default, this variable contains NULL, which disables all calling out. + +Within a regular expression, (?C) indicates the points at which the external +function is to be called. If you want to identify different callout points, you +can put a number less than 256 after the letter C. The default value is zero. +For example, this pattern has two callout points: + + (?C1)\dabc(?C2)def + +During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is +set), the external function is called. It is provided with the number of the +callout, and, optionally, one item of data originally supplied by the caller of +\fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to +fail altogether. A complete description of the interface to the callout +function is given in the +.\" HREF +\fBpcrecallout\fR +.\" +documentation. + +.in 0 +Last updated: 03 February 2003 +.br +Copyright (c) 1997-2003 University of Cambridge. |