pcrepattern

PCREPATTERN(3)              Library Functions Manual              PCREPATTERN(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS

       The syntax and semantics of the regular expressions that are supported by
       PCRE are described in detail below. There is a quick-reference syntax
       summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE also supports some alternative
       regular expression syntax (which does not conflict with the Perl syntax)
       in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

       Perl's regular expressions are described in its own documentation, and
       regular expressions in general are covered 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.

       This document discusses the patterns that are supported by PCRE when one
       its main matching functions, pcre_exec() (8-bit) or pcre[16|32]_exec()
       (16- or 32-bit), is used. PCRE also has alternative matching functions,
       pcre_dfa_exec() and pcre[16|32_dfa_exec(), which match using a different
       algorithm that is not Perl-compatible. Some of the features discussed
       below are not available when DFA matching is used. The advantages and
       disadvantages of the alternative functions, and how they differ from the
       normal functions, are discussed in the pcrematching page.

SPECIAL START-OF-PATTERN ITEMS

       A number of options that can be passed to pcre_compile() can also be set
       by special items at the start of a pattern. These are not Perl-
       compatible, but are provided to make these options accessible to pattern
       writers who are not able to change the program that processes the
       pattern. Any number of these items may appear, but they must all be
       together right at the start of the pattern string, and the letters must
       be in upper case.

   UTF support

       The original operation of PCRE was on strings of one-byte characters.
       However, there is now also support for UTF-8 strings in the original
       library, an extra library that supports 16-bit and UTF-16 character
       strings, and a third library that supports 32-bit and UTF-32 character
       strings. To use these features, PCRE must be built to include appropriate
       support. When using UTF strings you must either call the compiling
       function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the
       pattern must start with one of these special sequences:

         (*UTF8)
         (*UTF16)
         (*UTF32)
         (*UTF)

       (*UTF) is a generic sequence that can be used with any of the libraries.
       Starting a pattern with such a sequence is equivalent to setting the
       relevant option. How setting a UTF mode affects pattern matching is
       mentioned in several places below. There is also a summary of features in
       the pcreunicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
       option is set at compile time, (*UTF) etc. are not allowed, and their
       appearance causes an error.

   Unicode property support

       Another special sequence that may appear at the start of a pattern is
       (*UCP).  This has the same effect as setting the PCRE_UCP option: it
       causes sequences such as \d and \w to use Unicode properties to determine
       character types, instead of recognizing only characters with codes less
       than 128 via a lookup table.

   Disabling auto-possessification

       If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
       setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE
       from making quantifiers possessive when what follows cannot match the
       repeated item. For example, by default a+b is treated as a++b. For more
       details, see the pcreapi documentation.

   Disabling start-up optimizations

       If a pattern starts with (*NO_START_OPT), it has the same effect as
       setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
       time. This disables several optimizations for quickly reaching "no match"
       results. For more details, see the pcreapi documentation.

   Newline conventions

       PCRE supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a single LF (linefeed)
       character, the two-character sequence CRLF, any of the three preceding,
       or any Unicode newline sequence. The pcreapi page has further discussion
       about newlines, and shows how to set the newline convention in the
       options arguments for the compiling and matching functions.

       It is also possible to specify a newline convention by starting a pattern
       string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to the compiling
       function. For example, on a Unix system where LF is the default newline
       sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

       The newline convention affects where the circumflex and dollar assertions
       are true. It also affects the interpretation of the dot metacharacter
       when PCRE_DOTALL is not set, and the behaviour of \N. However, it does
       not affect what the \R escape sequence matches. By default, this is any
       Unicode newline sequence, for Perl compatibility. However, this can be
       changed; see the description of \R in the section entitled "Newline
       sequences" below. A change of \R setting can be combined with a change of
       newline convention.

   Setting match and recursion limits

       The caller of pcre_exec() can set a limit on the number of times the
       internal match() function is called and on the maximum depth of recursive
       calls. These facilities are provided to catch runaway matches that are
       provoked by patterns with huge matching trees (a typical example is a
       pattern with nested unlimited repeats) and to avoid running out of system
       stack by too much recursion. When one of these limits is reached,
       pcre_exec() gives an error return. The limits can also be set by items at
       the start of the pattern of the form

         (*LIMIT_MATCH=d)
         (*LIMIT_RECURSION=d)

       where d is any number of decimal digits. However, the value of the
       setting must be less than the value set (or defaulted) by the caller of
       pcre_exec() for it to have any effect. In other words, the pattern writer
       can lower the limits set by the programmer, but not raise them. If there
       is more than one setting of one of these limits, the lower value is used.

EBCDIC CHARACTER CODES

       PCRE can be compiled to run in an environment that uses EBCDIC as its
       character code rather than ASCII or Unicode (typically a mainframe
       system). In the sections below, character code values are ASCII or
       Unicode; in an EBCDIC environment these characters may have different
       code values, and there are no code points greater than 255.

CHARACTERS AND METACHARACTERS

       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. When
       caseless matching is specified (the PCRE_CASELESS option), letters are
       matched independently of case. In a UTF mode, PCRE always understands the
       concept of case for characters whose values are less than 128, so
       caseless matching is always possible. For characters with higher values,
       the concept of case is supported if PCRE is compiled with Unicode
       property support, but not otherwise.  If you want to use caseless
       matching for characters 128 and above, you must ensure that PCRE is
       compiled with Unicode property support as well as with UTF support.

       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 within 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
       character that is not a number or a letter, 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 \\.

       In a UTF mode, only ASCII numbers and letters have any special meaning
       after a backslash. All other characters (in particular, those whose
       codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option, most white space
       in the pattern (other than in a character class), and characters between
       a # outside a character class and the next newline, inclusive, are
       ignored. An escaping backslash can be used to include a white space 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.  An isolated \E that is not preceded by \Q is ignored. If \Q is
       not followed by \E later in the pattern, the literal interpretation
       continues to the end of the pattern (that is, \E is assumed at the end).
       If the isolated \Q is inside a character class, this causes an error,
       because the character class is not terminated.

   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 often easier to use one of the following escape sequences
       than the binary character it represents.  In an ASCII or Unicode
       environment, these escapes are as follows:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any ASCII character
         \e        escape (hex 1B)
         \f        form feed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \0dd      character with octal code 0dd
         \ddd      character with octal code ddd, or back reference
         \o{ddd..} character with octal code ddd..
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
         \uhhhh    character with hex code hhhh (JavaScript mode only)

       The precise effect of \cx on ASCII characters 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 \cA to \cZ become hex 01 to hex 1A
       (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex
       7B (; is 3B). If the data item (byte or 16-bit value) following \c has a
       value greater than 127, a compile-time error occurs. This locks out non-
       ASCII characters in all modes.

       When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t generate
       the appropriate EBCDIC code values. The \c escape is processed as
       specified for Perl in the perlebcdic document. The only characters that
       are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any
       other character provokes a compile-time error. The sequence \c@ encodes
       character code 0; after \c the letters (in either case) encode characters
       1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex
       1B to hex 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F).

       Thus, apart from \c?, these escapes generate the same character code
       values as they do in an ASCII environment, though the meanings of the
       values mostly differ. For example, \cG always generates code value 7,
       which is BEL in ASCII but DEL in EBCDIC.

       The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but
       because 127 is not a control character in EBCDIC, Perl makes it generate
       the APC character. Unfortunately, there are several variants of EBCDIC.
       In most of them the APC character has the value 255 (hex FF), but in the
       one Perl calls POSIX-BC its value is 95 (hex 5F). If certain other
       characters have POSIX-BC values, PCRE makes \c? generate 95; otherwise it
       generates 255.

       After \0 up to two further octal digits are read. If there are fewer than
       two digits, just those that are present are used. Thus the sequence
       \0\x\015 specifies two binary zeros followed by a CR character (code
       value 13). Make sure you supply two digits after the initial zero if the
       pattern character that follows is itself an octal digit.

       The escape \o must be followed by a sequence of octal digits, enclosed in
       braces. An error occurs if this is not the case. This escape is a recent
       addition to Perl; it provides way of specifying character code points as
       octal numbers greater than 0777, and it also allows octal numbers and
       back references to be unambiguously specified.

       For greater clarity and unambiguity, it is best to avoid following \ by a
       digit greater than zero. Instead, use \o{} or \x{} to specify character
       numbers, and \g{} to specify back references. The following paragraphs
       describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is
       complicated, and Perl has changed in recent releases, causing PCRE also
       to change. Outside a character class, PCRE reads the digit and any
       following digits as a decimal number. If the number is less than 8, 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 following \ is greater
       than 7 and there have not been that many capturing subpatterns, PCRE
       handles \8 and \9 as the literal characters "8" and "9", and otherwise
       re-reads up to three octal digits following the backslash, using them to
       generate a data character.  Any subsequent digits stand for themselves.
       For example:

         \040   is another way of writing an ASCII 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 value 255 (decimal)
         \81    is either a back reference, or the two
                   characters "8" and "1"

       Note that octal values of 100 or greater that are specified using this
       syntax must not be introduced by a leading zero, because no more than
       three octal digits are ever read.

       By default, after \x that is not followed by {, from zero to two
       hexadecimal digits are read (letters can be in upper or lower case). Any
       number of hexadecimal digits may appear between \x{ and }. If a character
       other than a hexadecimal digit appears between \x{ and }, or if there is
       no terminating }, an error occurs.

       If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
       as just described only when it is followed by two hexadecimal digits.
       Otherwise, it matches a literal "x" character. In JavaScript mode,
       support for code points greater than 256 is provided by \u, which must be
       followed by four hexadecimal digits; otherwise it matches a literal "u"
       character.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x (or by \u in JavaScript mode). There is no difference
       in the way they are handled. For example, \xdc is exactly the same as
       \x{dc} (or \u00dc in JavaScript mode).

   Constraints on character values

       Characters that are specified using octal or hexadecimal numbers are
       limited to certain values, as follows:

         8-bit non-UTF mode    less than 0x100
         8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
         16-bit non-UTF mode   less than 0x10000
         16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
         32-bit non-UTF mode   less than 0x100000000
         32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint

       Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
       "surrogate" codepoints), and 0xffef.

   Escape sequences in character classes

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside a character
       class, \b is interpreted as the backspace character (hex 08).

       \N is not allowed in a character class. \B, \R, and \X are not special
       inside a character class. Like other unrecognized escape sequences, they
       are treated as the literal characters "B", "R", and "X" by default, but
       cause an error if the PCRE_EXTRA option is set. Outside a character
       class, these sequences have different meanings.

   Unsupported escape sequences

       In Perl, the sequences \l, \L, \u, and \U are recognized by its string
       handler and used to modify the case of following characters. By default,
       PCRE does not support these escape sequences. However, if the
       PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and \u
       can be used to define a character by code point, as described in the
       previous section.

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative number, optionally
       enclosed in braces, is an absolute or relative back reference. A named
       back reference can be coded as \g{name}. Back references are discussed
       later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

       For compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.  Note that \g{...} (Perl syntax) and \g<...>
       (Oniguruma syntax) are not synonymous. The former is a back reference;
       the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline
       character.  This is the same as the "." metacharacter when PCRE_DOTALL is
       not set. Perl also uses \N to match characters by name; PCRE does not
       support this.

       Each pair of lower and upper case escape sequences partitions the
       complete set of characters into two disjoint sets. Any given character
       matches one, and only one, of each pair. The 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, because there is no character to match.

       For compatibility with Perl, \s did not used to match the VT character
       (code 11), which made it different from the the POSIX "space" class.
       However, Perl added VT at release 5.18, and PCRE followed suit at release
       8.34. The default \s characters are now HT (9), LF (10), VT (11), FF
       (12), CR (13), and space (32), which are defined as white space in the
       "C" locale. This list may vary if locale-specific matching is taking
       place. For example, in some locales the "non-breaking space" character
       (\xA0) is recognized as white space, and in others the VT character is
       not.

       A "word" character is an underscore or any character that is a letter or
       digit.  By default, 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 a French locale such as "fr_FR" in Unix-like systems, or
       "french" in Windows, some character codes greater than 127 are used for
       accented letters, and these are then matched by \w. The use of locales
       with Unicode is discouraged.

       By default, characters whose code points are greater than 127 never match
       \d, \s, or \w, and always match \D, \S, and \W, although this may vary
       for characters in the range 128-255 when locale-specific matching is
       happening.  These escape sequences retain their original meanings from
       before Unicode support was available, mainly for efficiency reasons. If
       PCRE is compiled with Unicode property support, and the PCRE_UCP option
       is set, the behaviour is changed so that Unicode properties are used to
       determine character types, as follows:

         \d  any character that matches \p{Nd} (decimal digit)
         \s  any character that matches \p{Z} or \h or \v
         \w  any character that matches \p{L} or \p{N}, plus underscore

       The upper case escapes match the inverse sets of characters. Note that \d
       matches only decimal digits, whereas \w matches any Unicode digit, as
       well as any Unicode letter, and underscore. Note also that PCRE_UCP
       affects \b, and \B because they are defined in terms of \w and \W.
       Matching these sequences is noticeably slower when PCRE_UCP is set.

       The sequences \h, \H, \v, and \V are features that were added to Perl at
       release 5.10. In contrast to the other sequences, which match only ASCII
       characters by default, these always match certain high-valued code
       points, whether or not PCRE_UCP is set. The horizontal space characters
       are:

         U+0009     Horizontal tab (HT)
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
       256 are relevant.

   Newline sequences

       Outside a character class, by default, the escape sequence \R matches any
       Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
       following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR followed by LF, or one of the single characters LF (linefeed, U+000A),
       VT (vertical tab, U+000B), FF (form feed, U+000C), CR (carriage return,
       U+000D), or NEL (next line, U+0085). The two-character sequence is
       treated as a single unit that cannot be split.

       In other modes, two additional characters whose codepoints are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph
       separator, U+2029).  Unicode character property support is not needed for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set of Unicode line endings) by setting the option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbreviation for "backslash R".) This can be made the default
       when PCRE is built; if this is the case, the other behaviour can be
       requested via the PCRE_BSR_UNICODE option.  It is also possible to
       specify these settings by starting a pattern string with one of the
       following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling
       function, but they can themselves be overridden by options given to a
       matching function. Note that these special settings, which are not Perl-
       compatible, are recognized only at the very start of a pattern, and that
       they must be in upper case. If more than one of them is present, the last
       one is used. They can be combined with a change of newline convention;
       for example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
       (*UCP) special sequences. Inside a character class, \R is treated as an
       unrecognized escape sequence, and so matches the letter "R" by default,
       but causes an error if PCRE_EXTRA is set.

   Unicode character properties

       When PCRE is built with Unicode character property support, three
       additional escape sequences that match characters with specific
       properties are available.  When in 8-bit non-UTF-8 mode, these sequences
       are of course limited to testing characters whose codepoints are less
       than 256, but they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       a Unicode extended grapheme cluster

       The property names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches any
       character (including newline), and some special PCRE properties
       (described in the next section).  Other Perl properties such as
       "InMusicalSymbols" are not currently supported by PCRE. Note that \P{Any}
       does not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts. A
       character from one of these sets can be matched using a script name. For
       example:

         \p{Greek}
         \P{Han}

       Those that are not part of an identified script are lumped together as
       "Common". The current list of scripts is:

       Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali,
       Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian,
       Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hieroglyphs,
       Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek,
       Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,
       Imperial_Aramaic, Inherited, Inscriptional_Pahlavi,
       Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
       Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu,
       Linear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic,
       Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive,
       Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean,
       New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian,
       Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya,
       Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician,
       Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian,
       Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog,
       Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai,
       Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.

       Each character has exactly one Unicode general category 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
       general category 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 general category 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

       The special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter that is not
       classified as a modifier or "other".

       The Cs (Surrogate) property applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in Unicode strings and so
       cannot be tested by PCRE, unless UTF validity checking has been turned
       off (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
       PCRE_NO_UTF32_CHECK in the pcreapi page). Perl does not support the Cs
       property.

       The long synonyms for property names that Perl supports (such as
       \p{Letter}) are not supported by PCRE, nor is it permitted to prefix any
       of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned)
       property.  Instead, this property is assumed for any code point that is
       not in the Unicode table.

       Specifying caseless matching does not affect these escape sequences. For
       example, \p{Lu} always matches only upper case letters. This is different
       from the behaviour of current versions of Perl.

       Matching characters by Unicode property is not fast, because PCRE has to
       do a multistage table lookup in order to find a character's property.
       That is why the traditional escape sequences such as \d and \w do not use
       Unicode properties in PCRE by default, though you can make them do so by
       setting the PCRE_UCP option or by starting the pattern with (*UCP).

   Extended grapheme clusters

       The \X escape matches any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below).  Up to and including release 8.31, PCRE matched an earlier,
       simpler definition that was equivalent to

         (?>\PM\pM*)

       That is, it matched a character without the "mark" property, followed by
       zero or more characters with the "mark" property. Characters with the
       "mark" property are typically non-spacing accents that affect the
       preceding character.

       This simple definition was extended in Unicode to include more
       complicated kinds of composite character by giving each character a
       grapheme breaking property, and creating rules that use these properties
       to define the boundaries of extended grapheme clusters. In releases of
       PCRE later than 8.31, \X matches one of these clusters.

       \X always matches at least one character. Then it decides whether to add
       additional characters according to the following rules for ending a
       cluster:

       1. End at the end of the subject string.

       2. Do not end between CR and LF; otherwise end after any control
       character.

       3. Do not break Hangul (a Korean script) syllable sequences. Hangul
       characters are of five types: L, V, T, LV, and LVT. An L character may be
       followed by an L, V, LV, or LVT character; an LV or V character may be
       followed by a V or T character; an LVT or T character may be followed
       only by a T character.

       4. Do not end before extending characters or spacing marks. Characters
       with the "mark" property always have the "extend" grapheme breaking
       property.

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

       As well as the standard Unicode properties described above, PCRE supports
       four more that make it possible to convert traditional escape sequences
       such as \w and \s to use Unicode properties. PCRE uses these non-
       standard, non-Perl properties internally when PCRE_UCP is set. However,
       they may also be used explicitly. These properties are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the N (number)
       property. Xps matches the characters tab, linefeed, vertical tab, form
       feed, or carriage return, and any other character that has the Z
       (separator) property.  Xsp is the same as Xps; it used to exclude
       vertical tab, for Perl compatibility, but Perl changed, and so PCRE
       followed at release 8.34. Xwd matches the same characters as Xan, plus
       underscore.

       There is another non-standard property, Xuc, which matches any character
       that can be represented by a Universal Character Name in C++ and other
       programming languages. These are the characters $, @, ` (grave accent),
       and all characters with Unicode code points greater than or equal to
       U+00A0, except for the surrogates U+D800 to U+DFFF. Note that most base
       (ASCII) characters are excluded. (Universal Character Names are of the
       form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. Note that the
       Xuc property does not match these sequences but the characters that they
       represent.)

   Resetting the match start

       The escape sequence \K causes any previously matched characters not to be
       included in the final matched sequence. For example, the pattern:

         foo\Kbar

       matches "foobar", but reports that it has matched "bar". This feature is
       similar to a lookbehind assertion (described below).  However, in this
       case, the part of the subject before the real match does not have to be
       of fixed length, as lookbehind assertions do. The use of \K does not
       interfere with the setting of captured substrings.  For example, when the
       pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl documents that the use of \K within assertions is "not well
       defined". In PCRE, \K is acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions. Note that when a
       pattern such as (?=ab\K) matches, the reported start of the match can be
       greater than the end of the match.

   Simple assertions

       The final 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 the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning; it matches the
       backspace character. If any other of these assertions appears in a
       character class, by default it matches the corresponding literal
       character (for example, \B matches the letter B). However, if the
       PCRE_EXTRA option is set, an "invalid escape sequence" error is generated
       instead.

       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. In a UTF mode,
       the meanings of \w and \W can be changed by setting the PCRE_UCP option.
       When this is done, it also affects \b and \B. Neither PCRE nor Perl has a
       separate "start of word" or "end of word" metasequence. However, whatever
       follows \b normally determines which it is. For example, the fragment \ba
       matches "a" at the start of a word.

       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 at the end of the string as well as at the very end,
       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

       The circumflex and dollar metacharacters are zero-width assertions. That
       is, they test for a particular condition being true without consuming any
       characters from the subject string.

       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.)

       The 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 at the end of the string (by default). Note, however, that it
       does not actually match the newline. 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, a circumflex matches
       immediately after internal newlines as well as at the start of the
       subject string. It does not match after a newline that ends the string. A
       dollar matches before any newlines in the string, as well as at the very
       end, when PCRE_MULTILINE is set. When newline is specified as the two-
       character sequence CRLF, isolated CR and LF characters do not indicate
       newlines.

       For example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) 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 or not PCRE_MULTILINE is set.

FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one character
       in the subject string except (by default) a character that signifies the
       end of a line.

       When a line ending is defined as a single character, dot never matches
       that character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed by LF, but otherwise it
       matches all characters (including isolated CRs and LFs). When any Unicode
       line endings are being recognized, dot does not match CR or LF or any of
       the other line ending characters.

       The behaviour of dot with regard to newlines can be changed. If the
       PCRE_DOTALL option is set, a dot matches any one character, without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of circumflex
       and dollar, the only relationship being that they both involve newlines.
       Dot has no special meaning in a character class.

       The escape sequence \N behaves like a dot, except that it is not affected
       by the PCRE_DOTALL option. In other words, it matches any character
       except one that signifies the end of a line. Perl also uses \N to match
       characters by name; PCRE does not support this.

MATCHING A SINGLE DATA UNIT

       Outside a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. In the 8-bit library, one data
       unit is one byte; in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a dot, \C always matches line-
       ending characters. The feature is provided in Perl in order to match
       individual bytes in UTF-8 mode, but it is unclear how it can usefully be
       used. Because \C breaks up characters into individual data units,
       matching one unit with \C in a UTF mode means that the rest of the string
       may start with a malformed UTF character. This has undefined results,
       because PCRE assumes that it is dealing with valid UTF strings (and by
       default it checks this at the start of processing unless the
       PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option is
       used).

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

       In general, the \C escape sequence is best avoided. However, one way of
       using it that avoids the problem of malformed UTF characters is to use a
       lookahead to check the length of the next character, as in this pattern,
       which could be used with a UTF-8 string (ignore white space and line
       breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A group that starts with (?| resets the capturing parentheses numbers in
       each alternative (see "Duplicate Subpattern Numbers" below). The
       assertions at the start of each branch check the next UTF-8 character for
       values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
       character's individual bytes are then captured by the appropriate number
       of groups.

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 by default.  However, if the PCRE_JAVASCRIPT_COMPAT option is
       set, a lone closing square bracket causes a compile-time error. 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 a UTF
       mode, the character may be more than one data unit long. 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 (UTF-16, UTF-32) mode, characters with values greater than 255
       (0xffff) can be included in a class as a literal string of data units, 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. In a UTF mode, PCRE always
       understands the concept of case for characters whose values are less than
       128, so caseless matching is always possible. For characters with higher
       values, the concept of case is supported if PCRE is compiled with Unicode
       property support, but not otherwise.  If you want to use caseless
       matching in a UTF mode for characters 128 and above, you must ensure that
       PCRE is compiled with Unicode property support as well as with UTF
       support.

       Characters that might indicate line breaks are never treated in any
       special way when matching character classes, whatever line-ending
       sequence is in use, and whatever setting of the PCRE_DOTALL and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       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, or immediately after a range. For example, [b-d-
       z] matches letters in the range b to d, a hyphen character, or z.

       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.

       An error is generated if a POSIX character class (see below) or an escape
       sequence other than one that defines a single character appears at a
       point where a range ending character is expected. For example, [z-\xff]
       is valid, but [A-\d] and [A-[:digit:]] are not.

       Ranges operate in the collating sequence of character values. They can
       also be used for characters specified numerically, for example
       [\000-\037]. Ranges can include any characters that are valid for the
       current mode.

       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 a non-UTF mode, if
       character tables for a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF modes, PCRE supports the
       concept of case for characters with values greater than 128 only when it
       is compiled with Unicode property support.

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
       \w, and \W may appear in a character class, and add the characters that
       they match to the class. For example, [\dABCDEF] matches any hexadecimal
       digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s,
       \w and their upper case partners, just as it does when they appear
       outside a character class, as described in the section entitled "Generic
       character types" above. The escape sequence \b has a different meaning
       inside a character class; it matches the backspace character. The
       sequences \B, \N, \R, and \X are not special inside a character class.
       Like any other unrecognized escape sequences, they are treated as the
       literal characters "B", "N", "R", and "X" by default, but cause an error
       if the PCRE_EXTRA option is set.

       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, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       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, or for a special
       compatibility feature - see the next two sections), 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 and space
         space    white space (the same as \s from PCRE 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR
       (13), and space (32). If locale-specific matching is taking place, the
       list of space characters may be different; there may be fewer or more of
       them. "Space" used to be different to \s, which did not include VT, for
       Perl compatibility.  However, Perl changed at release 5.18, and PCRE
       followed at release 8.34.  "Space" and \s now match the same set of
       characters.

       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.

       By default, characters with values greater than 128 do not match any of
       the POSIX character classes. However, if the PCRE_UCP option is passed to
       pcre_compile(), some of the classes are changed so that Unicode character
       properties are used. This is achieved by replacing certain POSIX classes
       by other sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of \p. Three other
       POSIX classes are handled specially in UCP mode:

       [:graph:] This matches characters that have glyphs that mark the page
                 when printed. In Unicode property terms, it matches all
                 characters with the L, M, N, P, S, or Cf properties, except
                 for:

                   U+061C           Arabic Letter Mark
                   U+180E           Mongolian Vowel Separator
                   U+2066 - U+2069  Various "isolate"s


       [:print:] This matches the same characters as [:graph:] plus space
                 characters that are not controls, that is, characters with the
                 Zs property.

       [:punct:] This matches all characters that have the Unicode P
                 (punctuation) property, plus those characters whose code points
                 are less than 128 that have the S (Symbol) property.

       The other POSIX classes are unchanged, and match only characters with
       code points less than 128.

COMPATIBILITY FEATURE FOR WORD BOUNDARIES

       In the POSIX.2 compliant library that was included in 4.4BSD Unix, the
       ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" and
       "end of word". PCRE treats these items as follows:

         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such as
       [a[:<:]b] provokes error for an unrecognized POSIX class name. This
       support is not compatible with Perl. It is provided to help migrations
       from other environments, and is best not used in any new patterns. Note
       that \b matches at the start and the end of a word (see "Simple
       assertions" above), and in a Perl-style pattern the preceding or
       following character normally shows which is wanted, without the need for
       the assertions that are used above in order to give exactly the POSIX
       behaviour.

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 (which are Perl-compatible) can be changed from
       within the pattern by a sequence of Perl option letters enclosed between
       "(?" and ")".  The option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possible
       to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets PCRE_CASELESS
       and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
       permitted. If a letter appears both before and after the hyphen, the
       option is unset.

       The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
       can be changed in the same way as the Perl-compatible options by using
       the characters J, U and X respectively.

       When one of these option changes occurs at top level (that is, not inside
       subpattern parentheses), the change applies to the remainder of the
       pattern that follows. An option change within a subpattern (see below for
       a description of subpatterns) affects only that part of the subpattern
       that follows it, so

         (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).  By this means, options can be made to have different settings in
       different parts of the pattern. Any changes made in one alternative do
       carry on into subsequent branches within the same subpattern. For
       example,

         (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though when matching "C" the first
       branch is abandoned before the option setting. This is because the
       effects of option settings happen at compile time. There would be some
       very weird behaviour otherwise.

       Note: There are other PCRE-specific options that can be set by the
       application when the compiling or matching functions are called. In some
       cases the pattern can contain special leading sequences such as (*CRLF)
       to override what the application has set or what has been defaulted.
       Details are given in the section entitled "Newline sequences" above.
       There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
       sequences that can be used to set UTF and Unicode property modes; they
       are equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the
       PCRE_UCP options, respectively. The (*UTF) sequence is a generic version
       that can be used with any of the libraries. However, the application can
       set the PCRE_NEVER_UTF option, which locks out the use of the (*UTF)
       sequences.

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 "cataract", "caterpillar", or "cat". Without the parentheses, it
       would match "cataract", "erpillar" or an 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 the matching function. (This applies only to the traditional
       matching functions; the DFA matching functions do not support capturing.)

       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.

       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".

DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses the same numbers for its capturing parentheses. Such a subpattern
       starts with (?| and is itself a non-capturing subpattern. For example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of
       capturing parentheses are numbered one. Thus, when the pattern matches,
       you can look at captured substring number one, whichever alternative
       matched. This construct is useful when you want to capture part, but not
       all, of one of a number of alternatives. Inside a (?| group, parentheses
       are numbered as usual, but the number is reset at the start of each
       branch. The numbers of any capturing parentheses that follow the
       subpattern start after the highest number used in any branch. The
       following example is taken from the Perl documentation. The numbers
       underneath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A back reference to a numbered subpattern uses the most recent value that
       is set for that number by any subpattern. The following pattern matches
       "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a numbered subpattern always refers to
       the first one in the pattern with the given number. The following pattern
       matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       If a condition test for a subpattern's having matched refers to a non-
       unique number, the test is true if any of the subpatterns of that number
       have matched.

       An alternative approach to using this "branch reset" feature is to use
       duplicate named subpatterns, as described in the next section.

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. This
       feature was not added to Perl until release 5.10. Python had the feature
       earlier, and PCRE introduced it at release 4.0, using the Python syntax.
       PCRE now supports both the Perl and the Python syntax. Perl allows
       identically numbered subpatterns to have different names, but PCRE does
       not.

       In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
       (?'name'...) as in Perl, or (?P<name>...) as in Python. References to
       capturing parentheses from other parts of the pattern, such as back
       references, recursion, and conditions, can be made by name as well as by
       number.

       Names consist of up to 32 alphanumeric characters and underscores, but
       must start with a non-digit. Named capturing parentheses are still
       allocated numbers as well as names, exactly as if the names were not
       present. 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.

       By default, a name must be unique within a pattern, but it is possible to
       relax this constraint by setting the PCRE_DUPNAMES option at compile
       time. (Duplicate names are also always permitted for subpatterns with the
       same number, set up as described in the previous section.) Duplicate
       names can be useful for patterns where only one instance of the named
       parentheses can match. Suppose you want to match the name of a weekday,
       either as a 3-letter abbreviation or as the full name, and in both cases
       you want to extract the abbreviation. This pattern (ignoring the line
       breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There are five capturing substrings, but only one is ever set after a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The convenience function for extracting the data by name returns the
       substring for the first (and in this example, the only) subpattern of
       that name that matched. This saves searching to find which numbered
       subpattern it was.

       If you make a back reference to a non-unique named subpattern from
       elsewhere in the pattern, the subpatterns to which the name refers are
       checked in the order in which they appear in the overall pattern. The
       first one that is set is used for the reference. For example, this
       pattern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":

         (?:(?<n>foo)|(?<n>bar))\k<n>


       If you make a subroutine call to a non-unique named subpattern, the one
       that corresponds to the first occurrence of the name is used. In the
       absence of duplicate numbers (see the previous section) this is the one
       with the lowest number.

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a subpattern has matched, or
       to check for recursion, all subpatterns with the same name are tested. If
       the condition is true for any one of them, the overall condition is true.
       This is the same behaviour as testing by number. For further details of
       the interfaces for handling named subpatterns, see the pcreapi
       documentation.

       Warning: You cannot use different names to distinguish between two
       subpatterns with the same number because PCRE uses only the numbers when
       matching. For this reason, an error is given at compile time if different
       names are given to subpatterns with the same number. However, you can
       always give the same name to subpatterns with the same number, even when
       PCRE_DUPNAMES is not set.

REPETITION

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

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (including assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       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 modes, quantifiers apply to characters rather than to individual
       data units. Thus, for example, \x{100}{2} matches two characters, each of
       which is represented by a two-byte sequence in a UTF-8 string. Similarly,
       \X{3} matches three Unicode extended grapheme clusters, each of which may
       be several data units 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. This may be useful
       for subpatterns that are referenced as subroutines from elsewhere in the
       pattern (but see also the section entitled "Defining subpatterns for use
       by reference only" below). Items other than subpatterns that have a {0}
       quantifier are omitted from the compiled pattern.

       For convenience, 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 that 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 dot 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 are some cases where the optimization cannot be used. When
       .*  is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where a
       later one succeeds. 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.

       Another case where implicit anchoring is not applied is when the leading
       .* is inside an atomic group. Once again, a match at the start may fail
       where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking control
       verbs (*PRUNE) and (*SKIP) also disable this optimization.

       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 ("greedy") and minimizing ("ungreedy" or "lazy")
       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 gives 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

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       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 of a possessive quantifier and the equivalent atomic group,
       though there may be a performance difference; possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
       Jeffrey Friedl originated the idea (and the name) in the first edition of
       his book. Mike McCloskey liked it, so implemented it when he built Sun's
       Java package, and PCRE copied it from there. It ultimately found its way
       into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain simple
       pattern constructs. For example, the sequence A+B is treated as A++B
       because there is no point in backtracking into a sequence of A's when B
       must follow.

       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. A "forward back reference" of
       this type can make sense when a repetition is involved and the subpattern
       to the right has participated in an earlier iteration.

       It is not possible to have a numerical "forward back reference" to a
       subpattern whose number is 10 or more using this syntax because a
       sequence such as \50 is interpreted as a character defined in octal. See
       the subsection entitled "Non-printing characters" above for further
       details of the handling of digits following a backslash. There is no such
       problem when named parentheses are used. A back reference to any
       subpattern is possible using named parentheses (see below).

       Another way of avoiding the ambiguity inherent in the use of digits
       following a backslash is to use the \g escape sequence. This escape must
       be followed by an unsigned number or a negative number, optionally
       enclosed in braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An unsigned number specifies an absolute reference without the ambiguity
       that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started capturing
       subpattern before \g, that is, is it equivalent to \2 in this example.
       Similarly, \g{-2} would be equivalent to \1. The use of relative
       references can be helpful in long patterns, and also in patterns that are
       created by joining together fragments that contain references within
       themselves.

       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.

       There are several different ways of writing back references to named
       subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
       \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and named references, is also supported. We could rewrite the above
       example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A subpattern that is referenced by name may appear in the pattern before
       or after the reference.

       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 by default. For example, the pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". However, if the
       PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to
       an unset value matches an empty string.

       Because there may be many capturing parentheses in a pattern, all digits
       following a 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 white space. Otherwise, the \g{ syntax or an empty
       comment (see "Comments" below) can be used.

   Recursive back references

       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.

       Back references of this type cause the group that they reference to be
       treated as an atomic group.  Once the whole group has been matched, a
       subsequent matching failure cannot cause backtracking into the middle of
       the group.

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. If such an 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.
       (Perl sometimes, but not always, does do capturing in negative
       assertions.)

       WARNING: If a positive assertion containing one or more capturing
       subpatterns succeeds, but failure to match later in the pattern causes
       backtracking over this assertion, the captures within the assertion are
       reset only if no higher numbered captures are already set. This is,
       unfortunately, a fundamental limitation of the current implementation,
       and as PCRE1 is now in maintenance-only status, it is unlikely ever to
       change.

       For compatibility with Perl, assertion subpatterns may be repeated;
       though it makes no sense to assert the same thing several times, the side
       effect of capturing parentheses may occasionally be useful. In practice,
       there only three cases:

       (1) If the quantifier is {0}, the assertion is never obeyed during
       matching.  However, it may contain internal capturing parenthesized
       groups that are called from elsewhere via the subroutine mechanism.

       (2) If quantifier is {0,n} where n is greater than zero, it is treated as
       if it were {0,1}. At run time, the rest of the pattern match is tried
       with and without the assertion, the order depending on the greediness of
       the quantifier.

       (3) If the minimum repetition is greater than zero, the quantifier is
       ignored.  The assertion is obeyed just once when encountered during
       matching.

   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.  The backtracking control verb (*FAIL) or (*F) is a
       synonym for (?!).

   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 top-level 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, 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 to PCRE if rewritten to use two
       top-level branches:

         (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be used instead of
       a lookbehind assertion to get round the fixed-length restriction.

       The implementation of lookbehind assertions is, for each alternative, to
       temporarily move the current position back by the fixed length and then
       try to match. If there are insufficient characters before the current
       position, the assertion fails.

       In a UTF mode, PCRE does not allow the \C escape (which matches a single
       data unit even in a UTF mode) to appear in lookbehind assertions, because
       it makes it impossible to calculate the length of the lookbehind. The \X
       and \R escapes, which can match different numbers of data units, are also
       not permitted.

       "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the subpattern matches a fixed-length string.
       Recursion, however, is not supported.

       Possessive quantifiers can be used in conjunction with lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. 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)

       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 specific capturing subpattern
       has already been matched. 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. Each of the two alternatives
       may itself contain nested subpatterns of any form, including conditional
       subpatterns; the restriction to two alternatives applies only at the
       level of the condition. This pattern fragment is an example where the
       alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There are four kinds of condition: references to subpatterns, references
       to recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence of digits, the
       condition is true if a capturing subpattern of that number has previously
       matched. If there is more than one capturing subpattern with the same
       number (see the earlier section about duplicate subpattern numbers), the
       condition is true if any of them have matched. An alternative notation is
       to precede the digits with a plus or minus sign. In this case, the
       subpattern number is relative rather than absolute. The most recently
       opened parentheses can be referenced by (?(-1), the next most recent by
       (?(-2), and so on. Inside loops it can also make sense to refer to
       subsequent groups. The next parentheses to be opened can be referenced as
       (?(+1), and so on. (The value zero in any of these forms is not used; it
       provokes a compile-time error.)

       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 or not the first set
       of parentheses matched. 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 you were embedding this pattern in a larger one, you could use a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This makes the fragment independent of the parentheses in the larger
       pattern.

   Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
       subpattern by name. For compatibility with earlier versions of PCRE,
       which had this facility before Perl, the syntax (?(name)...) is also
       recognized.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If the name used in a condition of this kind is a duplicate, the test is
       applied to all subpatterns of the same name, and is true if any one of
       them has matched.

   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name R, the condition is true if a recursive call to the whole pattern or
       any subpattern has been made. If digits or a name preceded by ampersand
       follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into a subpattern
       whose number or name is given. This condition does not check the entire
       recursion stack. If the name used in a condition of this kind is a
       duplicate, the test is applied to all subpatterns of the same name, and
       is true if any one of them is the most recent recursion.

       At "top level", all these recursion test conditions are false.  The
       syntax for recursive patterns is described below.

   Defining subpatterns for use by reference only

       If the condition is the string (DEFINE), and there is no subpattern with
       the name DEFINE, the condition is always false. In this case, there may
       be only one alternative in the subpattern. It is always skipped if
       control reaches this point in the pattern; the idea of DEFINE is that it
       can be used to define subroutines that can be referenced from elsewhere.
       (The use of subroutines is described below.) For example, a pattern to
       match an IPv4 address such as "192.168.23.245" could be written like this
       (ignore white space and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a another
       group named "byte" is defined. This matches an individual component of an
       IPv4 address (a number less than 256). When matching takes place, this
       part of the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the named group to
       match the four dot-separated components of an IPv4 address, insisting on
       a word boundary at each end.

   Assertion conditions

       If the condition is not in any of the above formats, 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

       There are two ways of including comments in patterns that are processed
       by PCRE. In both cases, the start of the comment must not be in a
       character class, nor in the middle of any other sequence of related
       characters such as (?: or a subpattern name or number. The characters
       that make up a comment play no part in the pattern matching.

       The sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If the
       PCRE_EXTENDED option is set, an unescaped # character also introduces a
       comment, which in this case continues to immediately after the next
       newline character or character sequence in the pattern. Which characters
       are interpreted as newlines is controlled by the options passed to a
       compiling function or by a special sequence at the start of the pattern,
       as described in the section entitled "Newline conventions" above. Note
       that the end of this type of comment is a literal newline sequence in the
       pattern; escape sequences that happen to represent a newline do not
       count. For example, consider this pattern when PCRE_EXTENDED is set, and
       the default newline convention is in force:

         abc #comment \n still comment

       On encountering the # character, pcre_compile() skips along, looking for
       a newline in the pattern. The sequence \n is still literal at this stage,
       so it does not terminate the comment. Only an actual character with the
       code value 0x0a (the default newline) does so.

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.

       For some time, Perl has provided 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 using code interpolation
       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 special syntax for recursion of the entire pattern, and also
       for individual subpattern recursion. After its introduction in PCRE and
       Python, this kind of recursion was subsequently introduced into Perl at
       release 5.10.

       A special item that consists of (? followed by a number greater than zero
       and a closing parenthesis is a recursive subroutine call of the
       subpattern of the given number, provided that it occurs inside that
       subpattern. (If not, it is a non-recursive subroutine call, which is
       described in the next section.) The special item (?R) or (?0) is a
       recursive call of the entire regular expression.

       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. Note the use of a
       possessive quantifier to avoid backtracking into sequences of non-
       parentheses.

       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.
       This is made easier by the use of relative references. Instead of (?1) in
       the pattern above you can write (?-2) to refer to the second most
       recently opened parentheses preceding the recursion. In other words, a
       negative number counts capturing parentheses leftwards from the point at
       which it is encountered.

       It is also possible to refer to subsequently opened parentheses, by
       writing references such as (?+2). However, these cannot be recursive
       because the reference is not inside the parentheses that are referenced.
       They are always non-recursive subroutine calls, as described in the next
       section.

       An alternative approach is to use named parentheses instead. The Perl
       syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one subpattern with the same name, the earliest one
       is used.

       This particular example pattern that we have been looking at contains
       nested unlimited repeats, and so the use of a possessive quantifier 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 a possessive quantifier 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 of capturing parentheses are those from
       the outermost level. If you want to obtain intermediate values, a callout
       function can be used (see below and the pcrecallout documentation). If
       the pattern above is matched against

         (ab(cd)ef)

       the value for the inner capturing parentheses (numbered 2) is "ef", which
       is the last value taken on at the top level. If a capturing subpattern is
       not matched at the top level, its final captured value is unset, even if
       it was (temporarily) set at a deeper level during the matching process.

       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.

   Differences in recursion processing between PCRE and Perl

       Recursion processing in PCRE differs from Perl in two important ways. In
       PCRE (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure. This can be
       illustrated by the following pattern, which purports to match a
       palindromic string that contains an odd number of characters (for
       example, "a", "aba", "abcba", "abcdcba"):

         ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two identical
       characters surrounding a sub-palindrome. In Perl, this pattern works; in
       PCRE it does not if the pattern is longer than three characters. Consider
       the subject string "abcba":

       At the top level, the first character is matched, but as it is not at the
       end of the string, the first alternative fails; the second alternative is
       taken and the recursion kicks in. The recursive call to subpattern 1
       successfully matches the next character ("b"). (Note that the beginning
       and end of line tests are not part of the recursion).

       Back at the top level, the next character ("c") is compared with what
       subpattern 2 matched, which was "a". This fails. Because the recursion is
       treated as an atomic group, there are now no backtracking points, and so
       the entire match fails. (Perl is able, at this point, to re-enter the
       recursion and try the second alternative.) However, if the pattern is
       written with the alternatives in the other order, things are different:

         ^((.)(?1)\2|.)$

       This time, the recursing alternative is tried first, and continues to
       recurse until it runs out of characters, at which point the recursion
       fails. But this time we do have another alternative to try at the higher
       level. That is the big difference: in the previous case the remaining
       alternative is at a deeper recursion level, which PCRE cannot use.

       To change the pattern so that it matches all palindromic strings, not
       just those with an odd number of characters, it is tempting to change the
       pattern to this:

         ^((.)(?1)\2|.?)$

       Again, this works in Perl, but not in PCRE, and for the same reason. When
       a deeper recursion has matched a single character, it cannot be entered
       again in order to match an empty string. The solution is to separate the
       two cases, and write out the odd and even cases as alternatives at the
       higher level:

         ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If you want to match typical palindromic phrases, the pattern has to
       ignore all non-word characters, which can be done like this:

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases such
       as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
       Perl. Note the use of the possessive quantifier *+ to avoid backtracking
       into sequences of non-word characters. Without this, PCRE takes a great
       deal longer (ten times or more) to match typical phrases, and Perl takes
       so long that you think it has gone into a loop.

       WARNING: The palindrome-matching patterns above work only if the subject
       string does not start with a palindrome that is shorter than the entire
       string.  For example, although "abcba" is correctly matched, if the
       subject is "ababa", PCRE finds the palindrome "aba" at the start, then
       fails at top level because the end of the string does not follow. Once
       again, it cannot jump back into the recursion to try other alternatives,
       so the entire match fails.

       The second way in which PCRE and Perl differ in their recursion
       processing is in the handling of captured values. In Perl, when a
       subpattern is called recursively or as a subpattern (see the next
       section), it has no access to any values that were captured outside the
       recursion, whereas in PCRE these values can be referenced. Consider this
       pattern:

         ^(.)(\1|a(?2))

       In PCRE, this pattern matches "bab". The first capturing parentheses
       match "b", then in the second group, when the back reference \1 fails to
       match "b", the second alternative matches "a" and then recurses. In the
       recursion, \1 does now match "b" and so the whole match succeeds. In
       Perl, the pattern fails to match because inside the recursive call \1
       cannot access the externally set value.

SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern call (either by number or by
       name) is used outside the parentheses to which it refers, it operates
       like a subroutine in a programming language. The called subpattern may be
       defined before or after the reference. A numbered reference can be
       absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       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. Another example is given in the discussion of DEFINE above.

       All subroutine calls, whether recursive or not, are always treated as
       atomic groups. That is, once a subroutine has matched some of the subject
       string, it is never re-entered, even if it contains untried alternatives
       and there is a subsequent matching failure. Any capturing parentheses
       that are set during the subroutine call revert to their previous values
       afterwards.

       Processing options such as case-independence are fixed when a subpattern
       is defined, so if it is used as a subroutine, such options cannot be
       changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It matches "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.

ONIGURUMA SUBROUTINE SYNTAX

       For compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a subpattern as a subroutine,
       possibly recursively. Here are two of the examples used above, rewritten
       using this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE supports an extension to Oniguruma: if a number is preceded by a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a back reference; the latter is a subroutine
       call.

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 (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit
       library).  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 a compiling function, callouts
       are automatically installed before each item in the pattern. They are all
       numbered 255. If there is a conditional group in the pattern whose
       condition is an assertion, an additional callout is inserted just before
       the condition. An explicit callout may also be set at this position, as
       in this example:

         (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other types
       of condition.

       During matching, when PCRE reaches a callout point, 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 the matching function. The callout function may cause matching
       to proceed, to backtrack, or to fail altogether.

       By default, PCRE implements a number of optimizations at compile time and
       matching time, and one side-effect is that sometimes callouts are
       skipped. If you need all possible callouts to happen, you need to set
       options that disable the relevant optimizations. More details, and a
       complete description of the interface to the callout function, are given
       in the pcrecallout documentation.

BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
       which are still described in the Perl documentation as "experimental and
       subject to change or removal in a future version of Perl". It goes on to
       say: "Their usage in production code should be noted to avoid problems
       during upgrades." The same remarks apply to the PCRE features described
       in this section.

       The new verbs make use of what was previously invalid syntax: an opening
       parenthesis followed by an asterisk. They are generally of the form
       (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
       differently depending on whether or not a name is present. A name is any
       sequence of characters that does not include a closing parenthesis. The
       maximum length of name is 255 in the 8-bit library and 65535 in the
       16-bit and 32-bit libraries. If the name is empty, that is, if the
       closing parenthesis immediately follows the colon, the effect is as if
       the colon were not there.  Any number of these verbs may occur in a
       pattern.

       Since these verbs are specifically related to backtracking, most of them
       can be used only when the pattern is to be matched using one of the
       traditional matching functions, because these use a backtracking
       algorithm. With the exception of (*FAIL), which behaves like a failing
       negative assertion, the backtracking control verbs cause an error if
       encountered by a DFA matching function.

       The behaviour of these verbs in repeated groups, assertions, and in
       subpatterns called as subroutines (whether or not recursively) is
       documented below.

   Optimizations that affect backtracking verbs

       PCRE contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that a particular
       character must be present. When one of these optimizations bypasses the
       running of a match, any included backtracking verbs will not, of course,
       be processed. You can suppress the start-of-match optimizations by
       setting the PCRE_NO_START_OPTIMIZE option when calling pcre_compile() or
       pcre_exec(), or by starting the pattern with (*NO_START_OPT). There is
       more discussion of this option in the section entitled "Option bits for
       pcre_exec()" in the pcreapi documentation.

       Experiments with Perl suggest that it too has similar optimizations,
       sometimes leading to anomalous results.

   Verbs that act immediately

       The following verbs act as soon as they are encountered. They may not be
       followed by a name.

          (*ACCEPT)

       This verb causes the match to end successfully, skipping the remainder of
       the pattern. However, when it is inside a subpattern that is called as a
       subroutine, only that subpattern is ended successfully. Matching then
       continues at the outer level. If (*ACCEPT) in triggered in a positive
       assertion, the assertion succeeds; in a negative assertion, the assertion
       fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far is
       captured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured
       by the outer parentheses.

         (*FAIL) or (*F)

       This verb causes a matching failure, forcing backtracking to occur. It is
       equivalent to (?!) but easier to read. The Perl documentation notes that
       it is probably useful only when combined with (?{}) or (??{}). Those are,
       of course, Perl features that are not present in PCRE. The nearest
       equivalent is the callout feature, as for example in this pattern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

   Recording which path was taken

       There is one verb whose main purpose is to track how a match was arrived
       at, though it also has a secondary use in conjunction with advancing the
       match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A name is always required with this verb. There may be as many instances
       of (*MARK) as you like in a pattern, and their names do not have to be
       unique.

       When a match succeeds, the name of the last-encountered (*MARK:NAME),
       (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
       caller as described in the section entitled "Extra data for pcre_exec()"
       in the pcreapi documentation. Here is an example of pcretest output,
       where the /K modifier requests the retrieval and outputting of (*MARK)
       data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this example
       it indicates which of the two alternatives matched. This is a more
       efficient way of obtaining this information than putting each alternative
       in its own capturing parentheses.

       If a verb with a name is encountered in a positive assertion that is
       true, the name is recorded and passed back if it is the last-encountered.
       This does not happen for negative assertions or failing positive
       assertions.

       After a partial match or a failed match, the last encountered name in the
       entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XP
         No match, mark = B

       Note that in this unanchored example the mark is retained from the match
       attempt that started at the letter "X" in the subject. Subsequent match
       attempts starting at "P" and then with an empty string do not get as far
       as the (*MARK) item, but nevertheless do not reset it.

       If you are interested in (*MARK) values after failed matches, you should
       probably set the PCRE_NO_START_OPTIMIZE option (see above) to ensure that
       the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching
       continues with what follows, but if there is no subsequent match, causing
       a backtrack to the verb, a failure is forced. That is, backtracking
       cannot pass to the left of the verb. However, when one of these verbs
       appears inside an atomic group or an assertion that is true, its effect
       is confined to that group, because once the group has been matched, there
       is never any backtracking into it. In this situation, backtracking can
       "jump back" to the left of the entire atomic group or assertion.
       (Remember also, as stated above, that this localization also applies in
       subroutine calls.)

       These verbs differ in exactly what kind of failure occurs when
       backtracking reaches them. The behaviour described below is what happens
       when the verb is not in a subroutine or an assertion. Subsequent sections
       cover these special cases.

         (*COMMIT)

       This verb, which may not be followed by a name, causes the whole match to
       fail outright if there is a later matching failure that causes
       backtracking to reach it. Even if the pattern is unanchored, no further
       attempts to find a match by advancing the starting point take place. If
       (*COMMIT) is the only backtracking verb that is encountered, once it has
       been passed pcre_exec() is committed to finding a match at the current
       starting point, or not at all. For example:

         a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as a kind of
       dynamic anchor, or "I've started, so I must finish." The name of the most
       recently passed (*MARK) in the path is passed back when (*COMMIT) forces
       a match failure.

       If there is more than one backtracking verb in a pattern, a different one
       that follows (*COMMIT) may be triggered first, so merely passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note that (*COMMIT) at the start of a pattern is not the same as an
       anchor, unless PCRE's start-of-match optimizations are turned off, as
       shown in this output from pcretest:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         data> xyzabc\Y
         No match

       For this pattern, PCRE knows that any match must start with "a", so the
       optimization skips along the subject to "a" before applying the pattern
       to the first set of data. The match attempt then succeeds. In the second
       set of data, the escape sequence \Y is interpreted by the pcretest
       program. It causes the PCRE_NO_START_OPTIMIZE option to be set when
       pcre_exec() is called.  This disables the optimization that skips along
       to the first character. The pattern is now applied starting at "x", and
       so the (*COMMIT) causes the match to fail without trying any other
       starting points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position in
       the subject if there is a later matching failure that causes backtracking
       to reach it. If the pattern is unanchored, the normal "bumpalong" advance
       to the next starting character then happens. Backtracking can occur as
       usual to the left of (*PRUNE), before it is reached, or when matching to
       the right of (*PRUNE), but if there is no match to the right,
       backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
       is just an alternative to an atomic group or possessive quantifier, but
       there are some uses of (*PRUNE) that cannot be expressed in any other
       way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT).

       The behaviour of (*PRUNE:NAME) is the not the same as
       (*MARK:NAME)(*PRUNE).  It is like (*MARK:NAME) in that the name is
       remembered for passing back to the caller. However, (*SKIP:NAME) searches
       only for names set with (*MARK).

         (*SKIP)

       This verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to the next
       character, but to the position in the subject where (*SKIP) was
       encountered. (*SKIP) signifies that whatever text was matched leading up
       to it cannot be part of a successful match. Consider:

         a+(*SKIP)b

       If the subject is "aaaac...", after the first match attempt fails
       (starting at the first character in the string), the starting point skips
       on to start the next attempt at "c". Note that a possessive quantifier
       does not have the same effect as this example; although it would suppress
       backtracking during the first match attempt, the second attempt would
       start at the second character instead of skipping on to "c".

         (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour is modified. When it
       is triggered, the previous path through the pattern is searched for the
       most recent (*MARK) that has the same name. If one is found, the
       "bumpalong" advance is to the subject position that corresponds to that
       (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
       matching name is found, the (*SKIP) is ignored.

       Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
       ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative when
       backtracking reaches it. That is, it cancels any further backtracking
       within the current alternative. Its name comes from the observation that
       it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on failure, the matcher
       skips to the second alternative and tries COND2, without backtracking
       into COND1. If that succeeds and BAR fails, COND3 is tried. If
       subsequently BAZ fails, there are no more alternatives, so there is a
       backtrack to whatever came before the entire group. If (*THEN) is not
       inside an alternation, it acts like (*PRUNE).

       The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to the caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK).

       A subpattern that does not contain a | character is just a part of the
       enclosing alternative; it is not a nested alternation with only one
       alternative. The effect of (*THEN) extends beyond such a subpattern to
       the enclosing alternative. Consider this pattern, where A, B, etc. are
       complex pattern fragments that do not contain any | characters at this
       level:

         A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However, if the subpattern containing (*THEN) is given an alternative, it
       behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner subpattern. After a
       failure in C, matching moves to (*FAIL), which causes the whole
       subpattern to fail because there are no more alternatives to try. In this
       case, matching does now backtrack into A.

       Note that a conditional subpattern is not considered as having two
       alternatives, because only one is ever used. In other words, the |
       character in a conditional subpattern has a different meaning. Ignoring
       white space, consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .*? is
       ungreedy, it initially matches zero characters. The condition (?=a) then
       fails, the character "b" is matched, but "c" is not. At this point,
       matching does not backtrack to .*? as might perhaps be expected from the
       presence of the | character. The conditional subpattern is part of the
       single alternative that comprises the whole pattern, and so the match
       fails. (If there was a backtrack into .*?, allowing it to match "b", the
       match would succeed.)

       The verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match at the next alternative. (*PRUNE) comes next, failing the match at
       the current starting position, but allowing an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

   More than one backtracking verb

       If more than one backtracking verb is present in a pattern, the one that
       is backtracked onto first acts. For example, consider this pattern, where
       A, B, etc. are complex pattern fragments:

         (A(*COMMIT)B(*THEN)C|ABD)

       If A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN) causes the next alternative (ABD) to be tried. This behaviour is
       consistent, but is not always the same as Perl's. It means that if two or
       more backtracking verbs appear in succession, all the the last of them
       has no effect. Consider this example:

         ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       causes it to be triggered, and its action is taken. There can never be a
       backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

       PCRE differs from Perl in its handling of backtracking verbs in repeated
       groups. For example, consider:

         /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches, but PCRE fails because the
       (*COMMIT) in the second repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL) in an assertion has its normal effect: it forces an immediate
       backtrack.

       (*ACCEPT) in a positive assertion causes the assertion to succeed without
       any further processing. In a negative assertion, (*ACCEPT) causes the
       assertion to fail without any further processing.

       The other backtracking verbs are not treated specially if they appear in
       a positive assertion. In particular, (*THEN) skips to the next
       alternative in the innermost enclosing group that has alternations,
       whether or not this is within the assertion.

       Negative assertions are, however, different, in order to ensure that
       changing a positive assertion into a negative assertion changes its
       result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a
       negative assertion to be true, without considering any further
       alternative branches in the assertion.  Backtracking into (*THEN) causes
       it to skip to the next enclosing alternative within the assertion (the
       normal behaviour), but if the assertion does not have such an
       alternative, (*THEN) behaves like (*PRUNE).

   Backtracking verbs in subroutines

       These behaviours occur whether or not the subpattern is called
       recursively.  Perl's treatment of subroutines is different in some cases.

       (*FAIL) in a subpattern called as a subroutine has its normal effect: it
       forces an immediate backtrack.

       (*ACCEPT) in a subpattern called as a subroutine causes the subroutine
       match to succeed without any further processing. Matching then continues
       after the subroutine call.

       (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
       cause the subroutine match to fail.

       (*THEN) skips to the next alternative in the innermost enclosing group
       within the subpattern that has alternatives. If there is no such group
       within the subpattern, (*THEN) causes the subroutine match to fail.

SEE ALSO

       pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
       pcre16(3), pcre32(3).

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 23 October 2016
       Copyright (c) 1997-2016 University of Cambridge.



PCRE 8.40                        23 October 2016                  PCREPATTERN(3)