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diff --git a/srclib/pcre/doc/pcrepattern.3 b/srclib/pcre/doc/pcrepattern.3 new file mode 100644 index 0000000000..6f6a21ab1a --- /dev/null +++ b/srclib/pcre/doc/pcrepattern.3 @@ -0,0 +1,1456 @@ +.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 books, some of which have copious examples. +Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers +regular expressions in great detail. This description of PCRE's regular +expressions is intended as reference material. +.P +The original operation of PCRE was on strings of one-byte characters. However, +there is now also support for UTF-8 character strings. To use this, you must +build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with +the PCRE_UTF8 option. How this affects 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\fP +.\" +page. +.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 +.sp + The quick brown fox +.sp +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 +\fImetacharacters\fP, which do not stand for themselves but instead are +interpreted in some special way. +.P +There are two different sets of metacharacters: those that are recognized +anywhere in the pattern except within square brackets, and those that are +recognized in square brackets. Outside square brackets, the metacharacters are +as follows: +.sp + \e 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 +.sp +Part of a pattern that is in square brackets is called a "character class". In +a character class the only metacharacters are: +.sp + \e general escape character + ^ negate the class, but only if the first character + - indicates character range +.\" JOIN + [ POSIX character class (only if followed by POSIX + syntax) + ] terminates the character class +.sp +The following sections describe the use of each of the metacharacters. +. +.SH BACKSLASH +.rs +.sp +The backslash character has several uses. Firstly, if it is followed by a +non-alphanumeric 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 +For example, if you want to match a * character, you write \e* in the pattern. +This escaping action applies whether or not the following character would +otherwise be interpreted as a metacharacter, so it is always safe to precede a +non-alphanumeric with backslash to specify that it stands for itself. In +particular, if you want to match a backslash, you write \e\e. +.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 +If you want to remove the special meaning from a sequence of characters, you +can do so by putting them between \eQ and \eE. This is different from Perl in +that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in +Perl, $ and @ cause variable interpolation. Note the following examples: +.sp + Pattern PCRE matches Perl matches +.sp +.\" JOIN + \eQabc$xyz\eE abc$xyz abc followed by the + contents of $xyz + \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz + \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz +.sp +The \eQ...\eE sequence is recognized both inside and outside character classes. +. +. +.\" HTML <a name="digitsafterbackslash"></a> +.SS "Non-printing characters" +.rs +.sp +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: +.sp + \ea alarm, that is, the BEL character (hex 07) + \ecx "control-x", where x is any character + \ee escape (hex 1B) + \ef formfeed (hex 0C) + \en newline (hex 0A) + \er carriage return (hex 0D) + \et tab (hex 09) + \eddd character with octal code ddd, or backreference + \exhh character with hex code hh + \ex{hhh..} character with hex code hhh... (UTF-8 mode only) +.sp +The precise effect of \ecx 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 \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex +7B. +.P +After \ex, 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 \ex{ 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 \ex{ and }, or if there is no +terminating }, this form of escape is not recognized. Instead, the initial +\ex will be interpreted as a basic hexadecimal escape, with no following +digits, giving a character whose value is zero. +.P +Characters whose value is less than 256 can be defined by either of the two +syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the +way they are handled. For example, \exdc is exactly the same as \ex{dc}. +.P +After \e0 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 \e0\ex\e07 specifies two binary zeros followed by a BEL character +(code value 7). Make sure you supply two digits after the initial zero if the +pattern character that follows is itself an octal digit. +.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 \fIback reference\fP. A description of how this works is given +.\" HTML <a href="#backreferences"> +.\" </a> +later, +.\" +following the discussion of +.\" HTML <a href="#subpattern"> +.\" </a> +parenthesized subpatterns. +.\" +.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: +.sp + \e040 is another way of writing a space +.\" JOIN + \e40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \e7 is always a back reference +.\" JOIN + \e11 might be a back reference, or another way of + writing a tab + \e011 is always a tab + \e0113 is a tab followed by the character "3" +.\" JOIN + \e113 might be a back reference, otherwise the + character with octal code 113 +.\" JOIN + \e377 might be a back reference, otherwise + the byte consisting entirely of 1 bits +.\" JOIN + \e81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" +.sp +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 +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 \eb is interpreted as the +backspace character (hex 08), and the sequence \eX is interpreted as the +character "X". Outside a character class, these sequences have different +meanings +.\" HTML <a href="#uniextseq"> +.\" </a> +(see below). +.\" +. +. +.SS "Generic character types" +.rs +.sp +The third use of backslash is for specifying generic character types. The +following are always recognized: +.sp + \ed any decimal digit + \eD any character that is not a decimal digit + \es any whitespace character + \eS any character that is not a whitespace character + \ew any "word" character + \eW any "non-word" character +.sp +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 +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 +For compatibility with Perl, \es does not match the VT character (code 11). +This makes it different from the the POSIX "space" class. The \es characters +are HT (9), LF (10), FF (12), CR (13), and space (32). +.P +A "word" character is an underscore or any character less than 256 that is a +letter or digit. The definition of letters and digits is controlled by PCRE's +low-valued 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\fP +.\" +page). For example, in the "fr_FR" (French) locale, some character codes +greater than 128 are used for accented letters, and these are matched by \ew. +.P +In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or +\ew, and always match \eD, \eS, and \eW. This is true even when Unicode +character property support is available. +. +. +.\" HTML <a name="uniextseq"></a> +.SS Unicode character properties +.rs +.sp +When PCRE is built with Unicode character property support, three additional +escape sequences to match generic character types are available when UTF-8 mode +is selected. They are: +.sp + \ep{\fIxx\fP} a character with the \fIxx\fP property + \eP{\fIxx\fP} a character without the \fIxx\fP property + \eX an extended Unicode sequence +.sp +The property names represented by \fIxx\fP above are limited to the +Unicode general category properties. Each character has exactly one such +property, specified by a two-letter abbreviation. For compatibility with Perl, +negation can be specified by including a circumflex between the opening brace +and the property name. For example, \ep{^Lu} is the same as \eP{Lu}. +.P +If only one letter is specified with \ep or \eP, it includes all the properties +that start with that letter. In this case, in the absence of negation, the +curly brackets in the escape sequence are optional; these two examples have +the same effect: +.sp + \ep{L} + \epL +.sp +The following property codes are supported: +.sp + C Other + Cc Control + Cf Format + Cn Unassigned + Co Private use + Cs Surrogate +.sp + L Letter + Ll Lower case letter + Lm Modifier letter + Lo Other letter + Lt Title case letter + Lu Upper case letter +.sp + M Mark + Mc Spacing mark + Me Enclosing mark + Mn Non-spacing mark +.sp + N Number + Nd Decimal number + Nl Letter number + No Other number +.sp + P Punctuation + Pc Connector punctuation + Pd Dash punctuation + Pe Close punctuation + Pf Final punctuation + Pi Initial punctuation + Po Other punctuation + Ps Open punctuation +.sp + S Symbol + Sc Currency symbol + Sk Modifier symbol + Sm Mathematical symbol + So Other symbol +.sp + Z Separator + Zl Line separator + Zp Paragraph separator + Zs Space separator +.sp +Extended properties such as "Greek" or "InMusicalSymbols" are not supported by +PCRE. +.P +Specifying caseless matching does not affect these escape sequences. For +example, \ep{Lu} always matches only upper case letters. +.P +The \eX escape matches any number of Unicode characters that form an extended +Unicode sequence. \eX is equivalent to +.sp + (?>\ePM\epM*) +.sp +That is, it matches a character without the "mark" property, followed by zero +or more characters with the "mark" property, and treats the sequence as an +atomic group +.\" HTML <a href="#atomicgroup"> +.\" </a> +(see below). +.\" +Characters with the "mark" property are typically accents that affect the +preceding character. +.P +Matching characters by Unicode property is not fast, because PCRE has to search +a structure that contains data for over fifteen thousand characters. That is +why the traditional escape sequences such as \ed and \ew do not use Unicode +properties in PCRE. +. +. +.\" HTML <a name="smallassertions"></a> +.SS "Simple assertions" +.rs +.sp +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 +.\" HTML <a href="#bigassertions"> +.\" </a> +below. +.\" +The backslashed +assertions are: +.sp + \eb matches at a word boundary + \eB matches when not at a word boundary + \eA matches at start of subject + \eZ matches at end of subject or before newline at end + \ez matches at end of subject + \eG matches at first matching position in subject +.sp +These assertions may not appear in character classes (but note that \eb has a +different meaning, namely the backspace character, inside a character class). +.P +A word boundary is a position in the subject string where the current character +and the previous character do not both match \ew or \eW (i.e. one matches +\ew and the other matches \eW), or the start or end of the string if the +first or last character matches \ew, respectively. +.P +The \eA, \eZ, and \ez assertions differ from the traditional circumflex and +dollar (described in the next section) 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. These three assertions are not affected by the +PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the +circumflex and dollar metacharacters. However, if the \fIstartoffset\fP +argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start +at a point other than the beginning of the subject, \eA can never match. The +difference between \eZ and \ez is that \eZ matches before a newline that is the +last character of the string as well as at the end of the string, whereas \ez +matches only at the end. +.P +The \eG assertion is true only when the current matching position is at the +start point of the match, as specified by the \fIstartoffset\fP argument of +\fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is +non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate +arguments, you can mimic Perl's /g option, and it is in this kind of +implementation where \eG can be useful. +.P +Note, however, that PCRE's interpretation of \eG, 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. +.P +If all the alternatives of a pattern begin with \eG, 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 that is true only if the current matching point is +at the start of the subject string. If the \fIstartoffset\fP argument of +\fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE +option is unset. Inside a character class, circumflex has an entirely different +meaning +.\" HTML <a href="#characterclass"> +.\" </a> +(see below). +.\" +.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 +A dollar character is an assertion that 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 +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 \eZ assertion. +.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 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\enabc" (where \en +represents a newline character) 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\fP argument of \fBpcre_exec()\fP +is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is +set. +.P +Note that the sequences \eA, \eZ, and \ez can be used to match the start and +end of the subject in both modes, and if all branches of a pattern start with +\eA 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) 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 \eC matches any one byte, both +in and out of UTF-8 mode. Unlike a dot, it can match 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, the \eC escape sequence is +best avoided. +.P +PCRE does not allow \eC to appear in lookbehind assertions +.\" HTML <a href="#lookbehind"> +.\" </a> +(described below), +.\" +because in UTF-8 mode this would make it impossible to calculate the length of +the lookbehind. +. +. +.\" HTML <a name="characterclass"></a> +.SH "SQUARE BRACKETS AND CHARACTER CLASSES" +.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. +.P +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. +.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 that +are in the class by enumerating those that are not. A class that starts with a +circumflex is not an assertion: it still consumes a character from the subject +string, and therefore it fails if the current pointer is at the end of the +string. +.P +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 \ex{ escaping mechanism. +.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. When running in UTF-8 mode, PCRE supports the concept of +case for characters with values greater than 128 only when it is compiled with +Unicode property support. +.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 +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 +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-\e]46] is interpreted as a class containing a range +followed by two other characters. The octal or hexadecimal representation of +"]" can also be used to end a range. +.P +Ranges operate in the collating sequence of character values. They can also be +used for characters specified numerically, for example [\e000-\e037]. In UTF-8 +mode, ranges can include characters whose values are greater than 255, for +example [\ex{100}-\ex{2ff}]. +.P +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 +[][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character +tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E +characters in both cases. In UTF-8 mode, PCRE supports the concept of case for +characters with values greater than 128 only when it is compiled with Unicode +property support. +.P +The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear +in a character class, and add the characters that they match to the class. For +example, [\edABCDEF] 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 [^\eW_] matches any letter or digit, but not underscore. +.P +The only metacharacters that are recognized in character classes are backslash, +hyphen (only where it can be interpreted as specifying a range), circumflex +(only at the start), opening square bracket (only when it can be interpreted as +introducing a POSIX class name - see the next section), and the terminating +closing square bracket. However, escaping other non-alphanumeric characters +does no harm. +. +. +.SH "POSIX CHARACTER CLASSES" +.rs +.sp +Perl supports the POSIX notation for character classes. This uses names +enclosed by [: and :] within the enclosing square brackets. PCRE also supports +this notation. For example, +.sp + [01[:alpha:]%] +.sp +matches "0", "1", any alphabetic character, or "%". The supported class names +are +.sp + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \ed) + 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 \es) + upper upper case letters + word "word" characters (same as \ew) + xdigit hexadecimal digits +.sp +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 \es, which does not include VT (for Perl +compatibility). +.P +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, +.sp + [12[:^digit:]] +.sp +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. +.P +In UTF-8 mode, characters with values greater than 128 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 +.sp + gilbert|sullivan +.sp +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 +.\" HTML <a href="#subpattern"> +.\" </a> +(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 +.sp + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED +.sp +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 +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()\fP function). +.P +An option change within a subpattern affects only that part of the current +pattern that follows it, so +.sp + (a(?i)b)c +.sp +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, +.sp + (a(?i)b|c) +.sp +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 +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 to put it at the start. +. +. +.\" HTML <a name="subpattern"></a> +.SH SUBPATTERNS +.rs +.sp +Subpatterns are delimited by parentheses (round brackets), which can be nested. +Turning part of a pattern into a subpattern does two things: +.sp +1. It localizes a set of alternatives. For example, the pattern +.sp + cat(aract|erpillar|) +.sp +matches one of the words "cat", "cataract", or "caterpillar". Without the +parentheses, it would match "cataract", "erpillar" or the empty string. +.sp +2. It sets up the subpattern as a capturing subpattern. This means that, when +the whole pattern matches, that portion of the subject string that matched the +subpattern is passed back to the caller via the \fIovector\fP argument of +\fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting +from 1) to obtain numbers for the capturing subpatterns. +.P +For example, if the string "the red king" is matched against the pattern +.sp + the ((red|white) (king|queen)) +.sp +the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3, respectively. +.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 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 +.sp + the ((?:red|white) (king|queen)) +.sp +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. +.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 +.sp + (?i:saturday|sunday) + (?:(?i)saturday|sunday) +.sp +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 this +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. +.P +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. There is also a convenience function for +extracting a captured substring by name. For further details see the +.\" HREF +\fBpcreapi\fP +.\" +documentation. +. +. +.SH REPETITION +.rs +.sp +Repetition is specified by quantifiers, which can follow any of the following +items: +.sp + a literal data character + the . metacharacter + the \eC escape sequence + the \eX escape sequence (in UTF-8 mode with Unicode properties) + an escape such as \ed that matches a single character + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion) +.sp +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: +.sp + z{2,4} +.sp +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 +.sp + [aeiou]{3,} +.sp +matches at least 3 successive vowels, but may match many more, while +.sp + \ed{8} +.sp +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 +In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual +bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of +which is represented by a two-byte sequence. Similarly, when Unicode property +support is available, \eX{3} matches three Unicode extended sequences, each of +which may be several bytes long (and they may be of different lengths). +.P +The quantifier {0} is permitted, causing the expression to behave as if the +previous item and the quantifier were not present. +.P +For convenience (and historical compatibility) the three most common +quantifiers have single-character abbreviations: +.sp + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} +.sp +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: +.sp + (a?)* +.sp +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 +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 /* and */ +and within the comment, individual * and / characters may appear. An attempt to +match C comments by applying the pattern +.sp + /\e*.*\e*/ +.sp +to the string +.sp + /* first comment */ not comment /* second comment */ +.sp +fails, because it matches the entire string owing to the greediness of the .* +item. +.P +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 +.sp + /\e*.*?\e*/ +.sp +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 +.sp + \ed??\ed +.sp +which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. +.P +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. +.P +When a parenthesized subpattern is quantified with a minimum repeat count that +is greater than 1 or with a limited maximum, more memory is required for the +compiled pattern, in proportion to the size of the minimum or maximum. +.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, 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 \eA. +.P +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. +.P +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: +.sp + (.*)abc\e1 +.sp +If the subject is "xyz123abc123" the match point is the fourth character. For +this reason, such a pattern is not implicitly anchored. +.P +When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after +.sp + (tweedle[dume]{3}\es*)+ +.sp +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 +.sp + /(a|(b))+/ +.sp +matches "aba" the value of the second captured substring is "b". +. +. +.\" HTML <a name="atomicgroup"></a> +.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. +.P +Consider, for example, the pattern \ed+foo when applied to the subject line +.sp + 123456bar +.sp +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 \ed+ +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. +.P +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: +.sp + (?>\ed+)foo +.sp +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 +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 +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 \ed+ and \ed+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\ed+) can only match an entire sequence of digits. +.P +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 +.sp + \ed++foo +.sp +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. +.P +The possessive quantifier syntax is an extension to the Perl syntax. It +originates in Sun's Java package. +.P +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 +.sp + (\eD+|<\ed+>)*[!?] +.sp +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 +.sp + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +.sp +it takes a long time before reporting failure. This is because the string can +be divided between the internal \eD+ repeat and the external * repeat in a +large number of ways, and all have to be tried. (The example uses [!?] 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 so that it uses +an atomic group, like this: +.sp + ((?>\eD+)|<\ed+>)*[!?] +.sp +sequences of non-digits cannot be broken, and failure happens quickly. +. +. +.\" HTML <a name="backreferences"></a> +.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. +.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 subsection entitled "Non-printing characters" +.\" HTML <a href="#digitsafterbackslash"> +.\" </a> +above +.\" +for further details of the handling of digits following a backslash. +.P +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 +.sp + (sens|respons)e and \e1ibility +.sp +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, +.sp + ((?i)rah)\es+\e1 +.sp +matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. +.P +Back references to named subpatterns use the Python syntax (?P=name). We could +rewrite the above example as follows: +.sp + (?<p1>(?i)rah)\es+(?P=p1) +.sp +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 +.sp + (a|(bc))\e2 +.sp +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 (see +.\" HTML <a href="#comments"> +.\" </a> +"Comments" +.\" +below) can be used. +.P +A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\e1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern +.sp + (a|b\e1)+ +.sp +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. +. +. +.\" HTML <a name="bigassertions"></a> +.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 \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described +.\" HTML <a href="#smallassertions"> +.\" </a> +above. +.\" +.P +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. +.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. +. +. +.SS "Lookahead assertions" +.rs +.sp +Lookahead assertions start +with (?= for positive assertions and (?! for negative assertions. For example, +.sp + \ew+(?=;) +.sp +matches a word followed by a semicolon, but does not include the semicolon in +the match, and +.sp + foo(?!bar) +.sp +matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern +.sp + (?!foo)bar +.sp +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 the other effect. +.P +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. +. +. +.\" HTML <a name="lookbehind"></a> +.SS "Lookbehind assertions" +.rs +.sp +Lookbehind assertions start with (?<= for positive assertions and (?<! for +negative assertions. For example, +.sp + (?<!foo)bar +.sp +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 +.sp + (?<=bullock|donkey) +.sp +is permitted, but +.sp + (?<!dogs?|cats?) +.sp +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 +.sp + (?<=ab(c|de)) +.sp +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: +.sp + (?<=abc|abde) +.sp +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. +.P +PCRE does not allow the \eC 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. The \eX escape, which can match different numbers +of bytes, is also not permitted. +.P +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 +.sp + abcd$ +.sp +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 +.sp + ^.*abcd$ +.sp +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 +.sp + ^(?>.*)(?<=abcd) +.sp +or, equivalently, using the possessive quantifier syntax, +.sp + ^.*+(?<=abcd) +.sp +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. +. +. +.SS "Using multiple assertions" +.rs +.sp +Several assertions (of any sort) may occur in succession. For example, +.sp + (?<=\ed{3})(?<!999)foo +.sp +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\fP 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 +.sp + (?<=\ed{3}...)(?<!999)foo +.sp +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 +Assertions can be nested in any combination. For example, +.sp + (?<=(?<!foo)bar)baz +.sp +matches an occurrence of "baz" that is preceded by "bar" which in turn is not +preceded by "foo", while +.sp + (?<=\ed{3}(?!999)...)foo +.sp +is another pattern that matches "foo" preceded by three digits and any three +characters that are not "999". +. +. +.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 +.sp + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) +.sp +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 +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: +.sp + ( \e( )? [^()]+ (?(1) \e) ) +.sp +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 +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. +.P +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: +.sp + (?(?=[^a-z]*[a-z]) + \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) +.sp +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. +. +. +.\" HTML <a name="comments"></a> +.SH COMMENTS +.rs +.sp +The sequence (?# marks the start of a comment that 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 +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 provides a 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: +.sp + $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; +.sp +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. +.P +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. +.P +For example, this PCRE pattern solves the nested parentheses problem (assume +the PCRE_EXTENDED option is set so that white space is ignored): +.sp + \e( ( (?>[^()]+) | (?R) )* \e) +.sp +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. +.P +If this were part of a larger pattern, you would not want to recurse the entire +pattern, so instead you could use this: +.sp + ( \e( ( (?>[^()]+) | (?1) )* \e) ) +.sp +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: +.sp + (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) ) +.sp +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 +.sp + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() +.sp +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. +.P +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 +the next section and the +.\" HREF +\fBpcrecallout\fP +.\" +documentation). If the pattern above is matched against +.sp + (ab(cd)ef) +.sp +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 +.sp + \e( ( ( (?>[^()]+) | (?R) )* ) \e) + ^ ^ + ^ ^ +.sp +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\fP, freeing it via \fBpcre_free\fP afterwards. If no +memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. +.P +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. +.sp + < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > +.sp +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 +.sp + (sens|respons)e and \e1ibility +.sp +matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If instead the pattern +.sp + (sens|respons)e and (?1)ibility +.sp +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. +.P +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\fP. +By default, this variable contains NULL, which disables all calling out. +.P +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: +.sp + (?C1)\dabc(?C2)def +.sp +If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are +automatically installed before each item in the pattern. They are all numbered +255. +.P +During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is +set), the external function is called. It is provided with the number of the +callout, the position in the pattern, and, optionally, one item of data +originally supplied by the caller of \fBpcre_exec()\fP. The callout function +may cause matching to proceed, to backtrack, or to fail altogether. A complete +description of the interface to the callout function is given in the +.\" HREF +\fBpcrecallout\fP +.\" +documentation. +.P +.in 0 +Last updated: 09 September 2004 +.br +Copyright (c) 1997-2004 University of Cambridge. |