From 7fd9fe181150f166a098eaf4e006f878c28cb770 Mon Sep 17 00:00:00 2001 From: Gluzskiy Alexandr Date: Mon, 15 Feb 2010 05:51:01 +0300 Subject: sort --- Utilities/PCRE/man/html/pcrepattern.3.html | 1268 ++++++++++++++++++++++++++++ 1 file changed, 1268 insertions(+) create mode 100644 Utilities/PCRE/man/html/pcrepattern.3.html (limited to 'Utilities/PCRE/man/html/pcrepattern.3.html') diff --git a/Utilities/PCRE/man/html/pcrepattern.3.html b/Utilities/PCRE/man/html/pcrepattern.3.html new file mode 100644 index 0000000..11bb198 --- /dev/null +++ b/Utilities/PCRE/man/html/pcrepattern.3.html @@ -0,0 +1,1268 @@ + + + + + +PCRE(3) manual page + + +Table of Contents

+ +

Name

+PCRE - Perl-compatible regular expressions +

Pcre Regular Expression Details

+ +

+The syntax and semantics of the regular expressions supported by PCRE are +described below. Regular expressions are also described in the Perl documentation +and in a number of 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.

+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 pcre_compile() 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 section on UTF-8 support in the main pcre 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 metacharacters, which do not stand for themselves but instead are +interpreted in some special way.

+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:

+ \ 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 metacharacters 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 metacharacters. + +

Backslash

+

+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.

+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 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 \\.

+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. + +

Non-printing characters

+

+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 character 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 pattern character that follows is +itself an octal digit.

+The handling of a backslash followed by a digit other +than 0 is complicated. Outside a character class, PCRE reads it and any +following digits as a decimal number. If the number is less than 10, or +if there have been at least that many previous capturing left parentheses +in the expression, the entire sequence is taken as a back reference. A description +of how this works is given later, following the discussion of parenthesized +subpatterns.

+Inside a character class, or if the decimal number is greater +than 9 and there have not been that many capturing subpatterns, PCRE re-reads +up to three octal digits 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), and the sequence \X is interpreted as the character "X". Outside +a character class, these sequences have different meanings (see below). + +

Generic character types

+

+The third use of backslash is for specifying +generic character types. The following are always recognized:

+ \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.

+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.

+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 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 "Locale support" + in the pcreapi 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 \w.

+In UTF-8 mode, characters with values greater than 128 +never match \d, \s, or \w, and always match \D, \S, and \W. This is true even +when Unicode character property support is available. +

Unicode character +properties

+

+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:

+ \p{xx} a character with +the xx property
+ \P{xx} a character without the xx property
+ \X an extended Unicode sequence
+

+The property names represented by xx 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, \p{^Lu} is the same as \P{Lu}.

+If only one letter +is specified with \p or \P, 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: +

+ \p{L}
+ \pL
+

+The following property codes are supported:

+ C Other
+ Cc Control
+ Cf Format
+ Cn Unassigned
+ Co Private use
+ Cs Surrogate
+

+ L Letter
+ Ll Lower case letter
+ Lm Modifier letter
+ Lo Other letter
+ Lt Title case letter
+ Lu Upper case letter
+

+ M Mark
+ Mc Spacing mark
+ Me Enclosing mark
+ Mn Non-spacing mark
+

+ N Number
+ Nd Decimal number
+ Nl Letter number
+ No Other number
+

+ P Punctuation
+ Pc Connector punctuation
+ Pd Dash punctuation
+ Pe Close punctuation
+ Pf Final punctuation
+ Pi Initial punctuation
+ Po Other punctuation
+ Ps Open punctuation
+

+ S Symbol
+ Sc Currency symbol
+ Sk Modifier symbol
+ Sm Mathematical symbol
+ So Other symbol
+

+ Z Separator
+ Zl Line separator
+ Zp Paragraph separator
+ Zs Space separator
+

+Extended properties such as "Greek" or "InMusicalSymbols" are not supported +by PCRE.

+Specifying caseless matching does not affect these escape sequences. +For example, \p{Lu} always matches only upper case letters.

+The \X escape +matches any number of Unicode characters that form an extended Unicode +sequence. \X is equivalent to

+ (?>\PM\pM*)
+

+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 (see below). Characters with the "mark" property are +typically accents that affect the preceding character.

+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 \d and \w do not use Unicode properties +in PCRE. +

Simple assertions

+

+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 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 startoffset argument of pcre_exec() 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 startoffset argument of pcre_exec(). It differs from +\A when the value of startoffset is non-zero. By calling pcre_exec() 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. +

Circumflex and Dollar

+

+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 startoffset argument of pcre_exec() +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 +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.

+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" (where \n 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 startoffset +argument of pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored +if PCRE_MULTILINE is set.

+Note that the sequences \A, \Z, and \z can be used +to match the start and end of the subject in both modes, and if all branches +of a pattern start with \A it is always anchored, whether PCRE_MULTILINE +is set or not. +

Full Stop (period, Dot)

+

+Outside a character class, a dot +in the pattern matches any one character in the subject, including a non-printing +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. +

Matching a Single Byte

+ +

+Outside a character class, the escape sequence \C 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 \C escape sequence +is best avoided.

+PCRE does not allow \C to appear in lookbehind assertions + (described below), because in UTF-8 mode this would make it impossible +to calculate the length of the lookbehind. +

Square Brackets and Character +Classes

+

+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 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.

+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. 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.

+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 class containing a range followed by two +other 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 in non-UTF-8 mode, if character tables for the "fr_FR" locale are in +use, [\xc8-\xcb] 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.

+The character +types \d, \D, \p, \P, \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.

+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. +

Posix Character +Classes

+

+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,

+ [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 128 do not match any of the POSIX character +classes. +

Vertical Bar

+

+Vertical bar characters are used to separate alternative +patterns. For example, the pattern

+ gilbert|sullivan
+

+matches either "gilbert" or "sullivan". Any number of alternatives may +appear, and an empty alternative is permitted (matching the empty string). +The matching process tries each alternative in turn, from left to right, +and the first one that succeeds is used. If the alternatives are within +a subpattern (defined below), "succeeds" means matching the rest of +the main pattern as well as the alternative in the subpattern. +

Internal +Option Setting

+

+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 pcre_fullinfo() 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 to put it +at the start. +

Subpatterns

+

+Subpatterns are delimited by parentheses (round +brackets), which can be nested. Turning part of a pattern into 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. 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 ovector argument +of pcre_exec(). Opening parentheses are counted from left to right (starting +from 1) to obtain numbers for 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". + +

Named Subpatterns

+

+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.

+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 pcreapi documentation. +

Repetition

+

+Repetition is specified +by quantifiers, which can follow any of the following items:

+ a literal +data character
+ the . metacharacter
+ the \C escape sequence
+ the \X escape sequence (in UTF-8 mode with Unicode properties)
+ an escape such as \d that matches a single character
+ 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. Similarly, when Unicode property support is available, +\X{3} matches three Unicode extended sequences, each of which may be several +bytes long (and they may be of different lengths).

+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 /* and */ +and within the comment, individual * and / characters may appear. An attempt +to match C comments by applying the pattern

+ /\*.*\*/
+

+to the string

+ /* first comment */ 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 memory 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". +

Atomic +Grouping and Possessive Quantifiers

+

+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+)foo
+

+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++foo
+

+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 internal \D+ 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:

+ ((?>\D+)|<\d+>)*[!?]
+

+sequences of non-digits cannot be broken, and failure happens quickly. + +

Back References

+

+Outside a character class, a backslash followed by a +digit greater than 0 (and possibly further digits) is a back reference +to a capturing subpattern earlier (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 subsection entitled "Non-printing +characters" 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 "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 (see "Comments" below) 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. +

Assertions

+

+An assertion is a test on the characters +following or preceding the current matching point that does not actually +consume any characters. The simple assertions coded as \b, \B, \A, \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.

+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. +

Lookahead assertions

+

+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 the other 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

+

+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. The \X +escape, which can match different numbers of bytes, is also not permitted. +

+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, using the possessive quantifier syntax,

+ ^.*+(?<=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. +

Using +multiple assertions

+

+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 not match "foo" preceded by six characters, +the first of which are digits and the last three of which are not "999". +For example, it doesn’t match "123abcfoo". A pattern to do that is

+ (?<=\d{3}...)(?<!999)foo
+

+This time the first assertion looks at the preceding six characters, checking +that the first three are digits, and then the second assertion checks that +the preceding three characters are not "999".

+Assertions can be nested in +any combination. For example,

+ (?<=(?<!foo)bar)baz
+

+matches an occurrence of "baz" that is preceded by "bar" which in turn +is not preceded by "foo", while

+ (?<=\d{3}(?!999)...)foo
+

+is another pattern that matches "foo" preceded by three digits and any +three characters that are not "999". +

Conditional Subpatterns

+

+It is possible +to cause the matching process to obey a subpattern conditionally or to +choose between two alternative subpatterns, depending on the result of +an assertion, or whether a previous capturing subpattern matched or not. +The two possible forms of conditional subpattern are

+ (?(condition)yes-pattern)
+ (?(condition)yes-pattern|no-pattern)
+

+If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern +(if present) is used. If there are more than two alternatives in the subpattern, +a compile-time error occurs.

+There are 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. +

Comments

+ +

+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.

+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. +

Recursive Patterns

+

+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:

+ $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:

+ (?P<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 the next section and the pcrecallout + 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 pcre_malloc, freeing it via pcre_free 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. +

Subpatterns As Subroutines

+

+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. +

Callouts

+

+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 pcre_callout. +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
+

+If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are +automatically installed before each item in the pattern. They are all numbered +255.

+During matching, when PCRE reaches a callout point (and pcre_callout +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 pcre_exec(). 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 pcrecallout + documentation.

+ Last updated: 09 September 2004
+Copyright (c) 1997-2004 University of Cambridge.

+ +

+Table of Contents

+

+ + -- cgit v1.2.3