--- /dev/null
+This file contains the PCRE man page that describes the regular expressions
+supported by PCRE version 5.0. Note that not all of the features are relevant
+in the context of Exim. In particular, the version of PCRE that is compiled
+with Exim does not include UTF-8 support, there is no mechanism for changing
+the options with which the PCRE functions are called, and features such as
+callout are not accessible.
+-----------------------------------------------------------------------------
+
+PCRE(3) PCRE(3)
+
+
+
+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 descrip-
+ tion 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 alterna-
+ tives 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 recog-
+ nized 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 back-
+ slash, 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 # charac-
+ ter as part of the pattern.
+
+ If you want to remove the special meaning from a sequence of charac-
+ ters, you can do so by putting them between \Q and \E. This is differ-
+ ent from Perl in that $ and @ are handled as literals in \Q...\E
+ sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
+ tion. 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 char-
+ acters 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 dig-
+ its 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 hex-
+ adecimal 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 compli-
+ cated. Outside a character class, PCRE reads it and any following dig-
+ its 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 sin-
+ gle 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 char-
+ acter 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 con-
+ trolled 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 Uni-
+ code character property support is available.
+
+ Unicode character properties
+
+ When PCRE is built with Unicode character property support, three addi-
+ tional 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 prop-
+ erty, 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 sup-
+ ported 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 asser-
+ tion 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 char-
+ acter 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 asser-
+ tions 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, indi-
+ cating 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 argu-
+ ments, you can mimic Perl's /g option, and it is in this kind of imple-
+ mentation 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 argu-
+ ment 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 sub-
+ ject, 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 immedi-
+ ately after and immediately before an internal newline character,
+ respectively, in addition to matching at the start and end of the sub-
+ ject string. For example, the pattern /^abc$/ matches the subject
+ string "def\nabc" (where \n represents a newline character) in multi-
+ line 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_DOL-
+ LAR_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 charac-
+ ter 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 han-
+ dling 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 calcu-
+ late 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 spe-
+ cial. 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 con-
+ sumes 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 charac-
+ ters 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 charac-
+ ter 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 inter-
+ preted 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 circum-
+ flex 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 alterna-
+ tives are within a subpattern (defined below), "succeeds" means match-
+ ing the rest of the main pattern as well as the alternative in the sub-
+ pattern.
+
+
+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 possi-
+ ble to unset these options by preceding the letter with a hyphen, and a
+ combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
+ LESS 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 subpat-
+ tern 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 cur-
+ rent 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 pat-
+ tern
+
+ the ((red|white) (king|queen))
+
+ the captured substrings are "red king", "red", and "king", and are num-
+ bered 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 captur-
+ ing, 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 expres-
+ sions. Furthermore, if an expression is modified, the numbers may
+ change. To help with this difficulty, PCRE supports the naming of sub-
+ patterns, 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 con-
+ venience function for extracting a captured substring by name. For fur-
+ ther 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 num-
+ ber 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 exam-
+ ple, {,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 char-
+ acters, 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 bro-
+ ken.
+
+ 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 (equiv-
+ alent 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 new-
+ lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
+ mization, 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 charac-
+ ter. For this reason, such a pattern is not implicitly anchored.
+
+ When a capturing subpattern is repeated, the value captured is the sub-
+ string 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 itera-
+ tions. 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 dif-
+ ferent number of repeats allows the rest of the pattern to match. Some-
+ times 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 nota-
+ tion 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 con-
+ tains 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 pre-
+ pared 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 charac-
+ ter 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 sub-
+ pattern 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 pat-
+ tern. In other words, the parentheses that are referenced need not be
+ to the left of the reference for numbers less than 10. See the subsec-
+ tion 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 sub-
+ pattern 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 exam-
+ ple,
+
+ ((?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 "Com-
+ ments" 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 sub-
+ patterns. For example, the pattern
+
+ (a|b\1)+
+
+ matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
+ ation 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 sub-
+ patterns 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 semi-
+ colon 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 sev-
+ eral 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 cur-
+ rent 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 impossi-
+ ble 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" pre-
+ ceded by six characters, the first of which are digits and the last
+ three of which are not "999". For example, it doesn't match "123abc-
+ foo". 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 con-
+ ditionally or to choose between two alternative subpatterns, depending
+ on the result of an assertion, or whether a previous capturing subpat-
+ tern 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 alterna-
+ tives 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 con-
+ tains 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 sec-
+ ond 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-pat-
+ tern 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 condi-
+ tion 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 new-
+ line 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 sec-
+ tion.) 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 parenthe-
+ sized 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, keep-
+ ing track of parenthesis numbers can be tricky. It may be more conve-
+ nient 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 documen-
+ tation). 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 pat-
+ tern, PCRE has to obtain extra memory to store data during a recursion,
+ which it does by using pcre_malloc, freeing it via pcre_free after-
+ wards. 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 brack-
+ ets, allowing for arbitrary nesting. Only digits are allowed in nested
+ brackets (that is, when recursing), whereas any characters are permit-
+ ted 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 oper-
+ ates 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 sub-
+ strings that match the same pair of parentheses when there is a repeti-
+ tion.
+
+ 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)abc(?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 alto-
+ gether. 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.
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