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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 sup-
       ports 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  expres-
       sions  in great detail. This description of PCRE's regular expressions is intended as ref-
       erence material.

       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 sup-
       port. 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  pat-
       tern  with  such  a sequence is equivalent to setting the relevant option. This feature is
       not Perl-compatible. How setting a UTF mode affects pattern matching is mentioned in  sev-
       eral places below. There is also a summary of features in the pcreunicode page.

       Another  special sequence that may appear at the start of a pattern or in combination with
       (*UTF8), (*UTF16), (*UTF32) or (*UTF) 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.

       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. There are also some more
       of these special sequences that are concerned with the  handling  of  newlines;  they  are
       described below.

       The  remainder of 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-compati-
       ble. 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.

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,  char-
       acter 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.

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  new-
       line. 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.

       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.

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  repe-
       titions  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 brack-
       ets. 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 escap-
       ing  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, white space in the pattern  (other
       than  in  a  character class) and characters between a # outside a character class and the
       next newline 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 interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside and outside character classes.  An isolated
       \E  that  is  not preceded by \Q is ignored. If \Q is not followed by \E later in the pat-
       tern, 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:

         \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)
         \ddd      character with octal code ddd, or back reference
         \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.

       The \c facility was designed for use with ASCII characters, but with the extension to Uni-
       code it is even less useful than it once was. It is, however, recognized when PCRE is com-
       piled  in  EBCDIC  mode,  where  data items are always bytes. In this mode, all values are
       valid after \c. If the next character is a lower case letter, it  is  converted  to  upper
       case. Then the 0xc0 bits of the byte are inverted. Thus \cA becomes hex 01, as in ASCII (A
       is C1), but because the EBCDIC letters are disjoint, \cZ becomes hex 29  (Z  is  E9),  and
       other characters also generate different values.

       By  default,  after  \x,  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  },  but
       the character code is constrained 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 0x80000000
         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" code-
       points), and 0xffef.

       If characters other than hexadecimal digits appear between \x{ and }, or if  there  is  no
       terminating  },  this  form  of  escape is not recognized. Instead, the initial \x will be
       interpreted as a basic hexadecimal escape, with no following digits,  giving  a  character
       whose value is zero.

       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.  Character codes specified by \u in JavaScript mode are constrained in the same
       was as those specified by \x in non-JavaScript mode.

       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).

       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\07 specifies two binary zeros fol-
       lowed by a BEL character (code value 7). Make sure you supply two digits after the initial
       zero if the pattern character that follows is itself an octal digit.

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

       Inside  a  character  class, or if the decimal number is greater than 9 and there have not
       been that many capturing subpatterns, PCRE re-reads up to three octal digits following the
       backslash,  and  uses  them  to generate a data character. Any subsequent digits stand for
       themselves. The value of the character is constrained in the same way as characters speci-
       fied in hexadecimal.  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 a binary zero
                   followed by the two characters "8" and "1"

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

       All the sequences that define a single 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 charac-
       ters "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" charac-
       ter, 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 referenc-
       ing 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  charac-
       ters  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 sub-
       ject string, all of them fail, because there is no character to match.

       For compatibility with Perl, \s does not match the VT character (code 11).  This makes  it
       different  from  the  the  POSIX  "space" class. The \s characters are HT (9), LF (10), FF
       (12), CR (13), and space (32). If "use locale;" is included in a Perl script, \s may match
       the VT character. In PCRE, it never does.

       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 128 are used for accented let-
       ters, and these are then matched by \w. The use of locales with Unicode is discouraged.

       By default, in a UTF mode, characters with values greater than 128 never match \d, \s,  or
       \w,  and  always match \D, \S, and \W. These sequences retain their original meanings from
       before UTF support was available, mainly for efficiency reasons. However, if PCRE is  com-
       piled  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 \p{Nd} matches (decimal digit)
         \s  any character that \p{Z} matches, plus HT, LF, FF, CR
         \w  any character that \p{L} or \p{N} matches, 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 codepoints, whether or not PCRE_UCP is set. The horizon-
       tal 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 particu-
       lar group matches either the two-character sequence CR followed by LF, or one of the  sin-
       gle  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 prop-
       erty 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 abbrevation 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 start-
       ing 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  pat-
       tern,  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 pat-
       tern 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 code-
       points 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 cur-
       rent list of scripts is:

       Arabic, Armenian, Avestan, Balinese, Bamum, Batak,  Bengali,  Bopomofo,  Brahmi,  Braille,
       Buginese,  Buhid, Canadian_Aboriginal, Carian, Chakma, Cham, Cherokee, Common, Coptic, Cu-
       neiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian,
       Glagolitic,  Gothic,  Greek,  Gujarati,  Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,
       Imperial_Aramaic,  Inherited,  Inscriptional_Pahlavi,  Inscriptional_Parthian,   Javanese,
       Kaithi,  Kannada,  Katakana,  Kayah_Li, Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Lin-
       ear_B,  Lisu,  Lycian,  Lydian,  Malayalam,   Mandaic,   Meetei_Mayek,   Meroitic_Cursive,
       Meroitic_Hieroglyphs,  Miao,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,  Ogham, Old_Italic,
       Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya,  Osmanya,  Phags_Pa,  Phoeni-
       cian,  Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sun-
       danese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil,
       Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, Yi.

       Each  character has exactly one Unicode general category property, specified by a two-let-
       ter 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  prop-
       erties  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 sup-
       ported 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.

       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 com-
       posite 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  char-
       acters 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 charac-
       ter; an LV or V character may be followed by a V or T character; an LVT or T character may
       be follwed 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, except  that
       vertical tab is excluded. Xwd matches the :qa same characters as Xan, plus underscore.

   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 asser-
       tions.

   Simple assertions

       The final use of backslash is for certain simple assertions. An assertion specifies a con-
       dition  that has to be met at a particular point in a match, without consuming any charac-
       ters 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,  respec-
       tively.  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  startoff-
       set  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 implemen-
       tation 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  expres-
       sion.

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 circum-
       flex, 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 new-
       lines  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  repre-
       sents  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  multi-
       line  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 sub-
       ject 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 unde-
       fined  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, respec-
       tively. 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 cir-
       cumflex is actually required as a member of the class, ensure it is not the first  charac-
       ter, 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 mecha-
       nism.

       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 char-
       acter 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 char-
       acter in the class.

       It is not possible to have the literal character "]" as the end character of  a  range.  A
       pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed
       by a literal string "46]", so it would match "W46]" or "-46]".  However,  if  the  "]"  is
       escaped with a backslash it is interpreted as the end of range, so [W-\]46] is interpreted
       as a class containing a range followed by two other characters. The octal  or  hexadecimal
       representation of "]" can also be used to end a range.

       Ranges  operate  in  the collating sequence of character values. They can also be used for
       characters specified numerically, for example [\000-\037]. Ranges can include any  charac-
       ters 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 char-
       acter  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  spe-
       cial  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 under-
       score. 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
       - see the next section), and the terminating closing  square  bracket.  However,  escaping
       other non-alphanumeric characters does no harm.

POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names enclosed by [: and
       :] within the enclosing square brackets. PCRE also supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The "space" characters are HT (9), LF (10), VT (11), FF (12), CR  (13),  and  space  (32).
       Notice that this list includes the VT character (code 11). This makes "space" different to
       \s, which does not include VT (for Perl compatibility).

       The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another
       Perl extension is negation, which is indicated by a ^ character after the colon. For exam-
       ple,

         [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, in UTF modes, 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.

VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns. For example,  the  pat-
       tern

         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  respec-
       tively.

       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.  If  the
       change is placed right at the start of a pattern, PCRE extracts it into the global options
       (and it will therefore show up in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern (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  aban-
       doned  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.

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  captur-
       ing.)

       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 maxi-
       mum number of capturing subpatterns is 65535.

       As  a convenient shorthand, if any option settings are required at the start of a non-cap-
       turing 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 "SUN-
       DAY" as well as "Saturday".

DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same num-
       bers 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 num-
       ber 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 under-
       neath 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 fea-
       ture  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. Named capturing  paren-
       theses  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  transla-
       tion  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  con-
       straint  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-let-
       ter  abbreviation or as the full name, and in both cases you want to extract the abbrevia-
       tion. 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 alterna-
       tive  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  pat-
       tern, 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  subpat-
       terns  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 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 clus-
       ters, 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  refer-
       enced  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 pro-
       grams. These appear between /* and */ and within the comment, individual * and  /  charac-
       ters 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 ques-
       tion 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  quanti-
       fiers  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  set-
       ting  PCRE_DOTALL  in order to obtain this optimization, or alternatively using ^ to indi-
       cate 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.  Con-
       sider 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 pre-
       vent 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 charac-
       ters 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 nota-
       tion, 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 con-
       structs. 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 parenthe-
       ses.

       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 refer-
       ence" 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 num-
       ber 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 nega-
       tive 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 ref-
       erence.

       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 back-
       slash  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 sub-
       pattern 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 subpat-
       tern, the back reference matches the character string corresponding to the previous itera-
       tion. 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 exam-
       ple 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  cur-
       rent matching position to be changed.

       Assertion subpatterns are not capturing subpatterns. If such an assertion contains captur-
       ing subpatterns within it, these are counted for the purposes of numbering  the  capturing
       subpatterns  in  the  whole  pattern. However, substring capturing is carried out only for
       positive assertions, because it does not make sense for negative assertions.

       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 asser-
       tion is obeyed just once when encountered during matching.

   Lookahead assertions

       Lookahead  assertions  start  with (?= for positive assertions and (?! for negative asser-
       tions. 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 asser-
       tions. For example,

         (?<!foo)bar

       does  find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbe-
       hind 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  permit-
       ted  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 insuffi-
       cient 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 calcu-
       late the length of the lookbehind. The \X and \R escapes, which can match  different  num-
       bers 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 sim-
       ple 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  asser-
       tions  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 alterna-
       tives 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  dupli-
       cate  subpattern  numbers), the condition is true if any of them have matched. An alterna-
       tive notation is to precede the digits with a plus or minus sign. In this case,  the  sub-
       pattern  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 clos-
       ing 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. However, there is a  possible  ambiguity
       with  this  syntax,  because  subpattern  names may consist entirely of digits. PCRE looks
       first for a named subpattern; if it cannot find one and the name consists entirely of dig-
       its,  PCRE  looks  for a subpattern of that number, which must be greater than zero. Using
       subpattern names that consist entirely of digits is not recommended.

       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 condi-
       tion 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 sub-
       pattern.  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 char-
       acters 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 paren-
       thesis. 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 immedi-
       ately after the next newline character or character sequence in the pattern. Which charac-
       ters  are interpreted as newlines is controlled by the options passed to a compiling func-
       tion 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 lit-
       eral 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 com-
       ment. 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 interpo-
       lation 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 spe-
       cial syntax for recursion of the entire pattern, and also for individual subpattern recur-
       sion.  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-paren-
       theses.

       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 eas-
       ier 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 alter-
       natives 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  sur-
       rounding 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 previ-
       ous 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 recur-
       sion 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  han-
       dling of captured values. In Perl, when a subpattern is called recursively or as a subpat-
       tern (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 out-
       side 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  cap-
       turing 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 exam-
       ple, 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 referenc-
       ing  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  automati-
       cally installed before each item in the pattern. They are all numbered 255.

       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 func-
       tion may cause matching to proceed, to  backtrack,  or  to  fail  altogether.  A  complete
       description  of the interface to the callout function is given in the pcrecallout documen-
       tation.

BACKTRACKING CONTROL

       Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are 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.

       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, which
       use a backtracking algorithm. With the exception of (*FAIL), which behaves like a  failing
       negative assertion, they cause an error if encountered by a DFA matching function.

       If any of these verbs are used in an assertion or in a subpattern that is called as a sub-
       routine (whether or not recursively), their effect is confined to that subpattern; it does
       not  extend  to  the  surrounding  pattern,  with  one exception: the name from a *(MARK),
       (*PRUNE), or (*THEN) that is encountered in a successful positive assertion is passed back
       when  a  match succeeds (compare capturing parentheses in assertions). Note that such sub-
       patterns are processed as anchored at the point where they  are  tested.  Note  also  that
       Perl's treatment of subroutines and assertions is different in some cases.

       The  new verbs make use of what was previously invalid syntax: an opening parenthesis fol-
       lowed by an asterisk. They are generally of the form (*VERB)  or  (*VERB:NAME).  Some  may
       take  either  form,  with  differing behaviour, depending on whether or not an argument 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
       library.  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.

   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 match-
       ing subject, or that a particular character must be present. When one of  these  optimiza-
       tions  suppresses  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  subpat-
       tern  is  ended  successfully. Matching then continues at the outer level. 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  back-
       track 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) 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 (*MARK) is encountered in a positive assertion, its name is recorded and passed back if
       it is the last-encountered. This does not happen for negative assertions.

       After a partial match or a failed match, the name of the last encountered (*MARK)  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, its effect is confined to that group,  because
       once  the  group has been matched, there is never any backtracking into it. In this situa-
       tion, backtracking can "jump back" to the left of the entire atomic group. (Remember also,
       as stated above, that this localization also applies in subroutine calls and assertions.)

       These verbs differ in exactly what kind of failure occurs when backtracking reaches them.

         (*COMMIT)

       This verb, which may not be followed by a name, causes the whole match to fail outright if
       the rest of the pattern does not match. Even if the  pattern  is  unanchored,  no  further
       attempts  to  find  a match by advancing the starting point take place. Once (*COMMIT) 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.

       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 pcretest example:

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

       PCRE knows that any match must start with "a", so the optimization skips along the subject
       to  "a"  before  running the first match attempt, which succeeds. When the optimization is
       disabled by the \Y escape in the second subject, the match starts at "x" and so the (*COM-
       MIT) causes it 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 the
       rest of the pattern does not match. 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.  The behav-
       iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an anchored pattern (*PRUNE)
       has the same effect as (*COMMIT).

         (*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 quantifer 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. If the  following  pattern
       fails  to  match,  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.

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative if the rest of the pattern  does
       not  match. That is, it cancels pending backtracking, but only within the current alterna-
       tive. 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. The behaviour of (*THEN:NAME) is exactly the
       same as (*MARK:NAME)(*THEN).  If (*THEN) is  not  inside  an  alternation,  it  acts  like
       (*PRUNE).

       Note that 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 pat-
       tern, 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 contain-
       ing (*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  also  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.

       If  more  than one such verb is present in a pattern, the "strongest" one wins.  For exam-
       ple, consider this pattern, where A, B, etc. are complex pattern fragments:

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

       Once A has matched, PCRE is committed to this match, at the current starting position.  If
       subsequently  B  matches,  but  C  does  not, the normal (*THEN) action of trying the next
       alternative (that is, D) does not happen because (*COMMIT) overrides.

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: 11 November 2012
       Copyright (c) 1997-2012 University of Cambridge.



PCRE 8.32                                11 November 2012                          PCREPATTERN(3)

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