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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.

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

SPECIAL START-OF-PATTERN ITEMS

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

   UTF support

       The original operation of PCRE was on strings of one-byte characters.  However,  there  is
       now also support for UTF-8 strings in the original library, an extra library that supports
       16-bit and UTF-16 character strings, and a third library that supports 32-bit  and  UTF-32
       character  strings.  To use these features, PCRE must be built to include appropriate 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. How setting a UTF
       mode affects pattern matching is mentioned in several places below. There is also  a  sum‐
       mary of features in the pcreunicode page.

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

   Unicode property support

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

   Disabling auto-possessification

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

   Disabling start-up optimizations

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

   Newline conventions

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

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

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

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

         (*CR)a.b

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

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

   Setting match and recursion limits

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

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

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

EBCDIC CHARACTER CODES

       PCRE can be compiled to run in an environment that  uses  EBCDIC  as  its  character  code
       rather  than ASCII or Unicode (typically a mainframe system). In the sections below, 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.

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, most white space in the pattern
       (other than in a character class), and characters between a # outside  a  character  class
       and the next newline, inclusive, are ignored. An escaping backslash can be used to include
       a white space or # character as part of the pattern.

       If you want to remove the special meaning from a sequence of characters, you can do so  by
       putting them between \Q and \E. This is different from Perl in that $ and @ are handled as
       literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable  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.  In an ASCII or Unicode environment, these escapes are as
       follows:

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

       The precise effect of \cx on ASCII characters is as follows: if x is a lower case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cA
       to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is  7B),  and
       \c;  becomes  hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c has a
       value greater than 127, a compile-time error occurs. This locks out  non-ASCII  characters
       in all modes.

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

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

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

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

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

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

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

       Inside a character class, or if the decimal number following \ is greater than 7 and there
       have not been that many capturing subpatterns, PCRE handles \8 and \9 as the literal char‐
       acters  "8"  and  "9", and otherwise re-reads up to three octal digits following the back‐
       slash, using them to generate a data character.  Any subsequent  digits  stand  for  them‐
       selves. For example:

         \040   is another way of writing an ASCII space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the value 255 (decimal)
         \81    is either a back reference, or the two
                   characters "8" and "1"

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

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

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

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

   Constraints on character values

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

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

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

   Escape sequences in character classes

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

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

       A  "word"  character  is  an  underscore  or  any character that is a letter or digit.  By
       default, the definition of letters and digits is controlled by PCRE's low-valued character
       tables,  and may vary if locale-specific matching is taking place (see "Locale support" in
       the pcreapi page). For example, in a French locale such as "fr_FR" in  Unix-like  systems,
       or  "french"  in Windows, some character codes greater than 127 are used for accented let‐
       ters, and these are then matched by \w. The use of locales with Unicode is discouraged.

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

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

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

       The  sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10. In
       contrast to the other sequences, which match  only  ASCII  characters  by  default,  these
       always match certain high-valued code points, whether or not PCRE_UCP is set. The 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, Bassa_Vah, Batak, Bengali,  Bopomofo,  Brahmi,
       Braille,  Buginese,  Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham,
       Cherokee, Common, Coptic, Cuneiform, Cypriot,  Cyrillic,  Deseret,  Devanagari,  Duployan,
       Egyptian_Hieroglyphs,  Elbasan,  Ethiopic,  Georgian,  Glagolitic, Gothic, Grantha, Greek,
       Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,  Imperial_Aramaic,  Inherited,
       Inscriptional_Pahlavi,   Inscriptional_Parthian,   Javanese,  Kaithi,  Kannada,  Katakana,
       Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A,  Lin‐
       ear_B,  Lisu,  Lycian,  Lydian,  Mahajani,  Malayalam,  Mandaic, Manichaean, Meetei_Mayek,
       Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian,  Mro,  Myan‐
       mar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Per‐
       mic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong,  Palmyrene,
       Pau_Cin_Hau,  Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra,
       Sharada, Shavian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri,  Syriac,  Taga‐
       log,  Tagbanwa,  Tai_Le,  Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan,
       Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.

       Each character has exactly one Unicode general category property, specified by a  two-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. This is different from the behaviour of current
       versions of Perl.

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

   Extended grapheme clusters

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

         (?>\PM\pM*)

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

       This  simple  definition was extended in Unicode to include more complicated kinds of 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; it  used  to
       exclude  vertical  tab,  for Perl compatibility, but Perl changed, and so PCRE followed at
       release 8.34. Xwd matches the same characters as Xan, plus underscore.

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

   Resetting the match start

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

         foo\Kbar

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

         (foo)\Kbar

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

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

   Simple assertions

       The final use of backslash is for certain simple assertions. An assertion specifies a 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, or immediately after a range. For example, [b-d-z] matches letters  in
       the range b to d, a hyphen character, or z.

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

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

       Ranges operate in the collating sequence of character values. They can also  be  used  for
       characters  specified numerically, for example [\000-\037]. Ranges can include any 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,
       or  for  a special compatibility feature - see the next two sections), and the terminating
       closing square bracket. However, escaping other non-alphanumeric characters does no harm.

POSIX CHARACTER CLASSES

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

         [01[:alpha:]%]

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

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (the same as \s from PCRE 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

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

       The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another
       Perl extension is negation, which is indicated by a ^ character after the colon. For 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, characters with values greater than 128 do not match any of the POSIX charac‐
       ter classes. However, if the PCRE_UCP option is passed  to  pcre_compile(),  some  of  the
       classes  are  changed  so  that Unicode character properties are used. This is achieved by
       replacing certain POSIX classes by other sequences, as follows:

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

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

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

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

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

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

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

COMPATIBILITY FEATURE FOR WORD BOUNDARIES

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

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

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

VERTICAL BAR

       Vertical  bar  characters are used to separate alternative patterns. For example, the 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. However, the  application  can
       set the PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.

SUBPATTERNS

       Subpatterns  are  delimited by parentheses (round brackets), which can be nested.  Turning
       part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches "cataract", "caterpillar", or "cat".  Without  the  parentheses,  it  would  match
       "cataract", "erpillar" or an empty string.

       2.  It  sets  up the subpattern as a capturing subpattern. This means that, when the whole
       pattern matches, that portion of the subject string that matched the subpattern is  passed
       back  to  the caller via the ovector argument of the matching function. (This applies only
       to the traditional matching functions; the DFA matching functions do not  support  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: (?...) or (?'name'...) as
       in Perl, or (?P...) as in Python. References to  capturing  parentheses  from  other
       parts  of  the pattern, such as back references, recursion, and conditions, can be made by
       name as well as by number.

       Names consist of up to 32 alphanumeric characters and underscores, but must start  with  a
       non-digit.  Named  capturing  parentheses  are  still  allocated numbers as well as names,
       exactly as if the names were not  present.  The  PCRE  API  provides  function  calls  for
       extracting  the  name-to-number translation table from a compiled pattern. There is also a
       convenience function for extracting a captured substring by name.

       By default, a name must be unique within a pattern, but it is possible to relax this  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:

         (?Mon|Fri|Sun)(?:day)?|
         (?Tue)(?:sday)?|
         (?Wed)(?:nesday)?|
         (?Thu)(?:rsday)?|
         (?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  subpatterns  to  which  the name refers are checked in the order in which they
       appear in the overall pattern. The first one that is set is used for  the  reference.  For
       example, this pattern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":

         (?:(?foo)|(?bar))\k

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

       If you use a named reference in a condition test (see the section about conditions below),
       either to check whether a subpattern has matched, or to check for recursion,  all  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 always give the same name to subpatterns with the same number, even  when
       PCRE_DUPNAMES is not set.

REPETITION

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

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

       The  general  repetition  quantifier  specifies  a minimum and maximum number of permitted
       matches, by giving the two numbers in curly brackets (braces), separated by a  comma.  The
       numbers  must  be less than 65536, and the first must be less than or equal to the second.
       For example:

         z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character.  If
       the  second  number  is omitted, but the comma is present, there is no upper limit; if the
       second number and the comma are both omitted, the quantifier specifies an exact number  of
       required matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches  exactly  8  digits.  An  opening curly bracket that appears in a position where a
       quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken
       as  a  literal  character.  For example, {,6} is not a quantifier, but a literal string of
       four characters.

       In UTF modes, quantifiers apply to characters rather than to individual data units.  Thus,
       for example, \x{100}{2} matches two characters, each of which is represented by a two-byte
       sequence in a UTF-8 string. Similarly, \X{3} matches three Unicode extended grapheme 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 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:

         (?(?i)rah)\s+\k
         (?'p1'(?i)rah)\s+\k{p1}
         (?P(?i)rah)\s+(?P=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. (Perl sometimes, but not always, does do capturing in negative asser‐
       tions.)

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

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

         (? \( )?    [^()]+    (?() \) )

       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) (? 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:

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

       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:

         (? \( ( (?>[^()]+) | \g )* \) )
         (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. If there is  a
       conditional group in the pattern whose condition is an assertion, an additional callout is
       inserted just before the condition. An explicit callout may also be set at this  position,
       as in this example:

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

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

       During matching, when PCRE reaches a callout point, the external function is called. It is
       provided with the number of the callout, the position in the pattern, and, optionally, one
       item of data originally supplied by the caller of the matching function. The callout func‐
       tion may cause matching to proceed, to backtrack, or to fail altogether.

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

BACKTRACKING CONTROL

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

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

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

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

   Optimizations that affect backtracking verbs

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

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

   Verbs that act immediately

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

          (*ACCEPT)

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

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

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

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

         (*FAIL) or (*F)

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

         a+(?C)(*FAIL)

       A  match  with  the string "aaaa" always fails, but the callout is taken before each 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:NAME), (*PRUNE:NAME), or
       (*THEN:NAME) on the matching path is passed back to the caller as described in the section
       entitled  "Extra data for pcre_exec()" in the pcreapi documentation. Here is an example of
       pcretest output, where the /K modifier requests the retrieval and  outputting  of  (*MARK)
       data:

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

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

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

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

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

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

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

   Verbs that act after backtracking

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

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

         (*COMMIT)

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

         a+(*COMMIT)b

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

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

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

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

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

         (*PRUNE) or (*PRUNE:NAME)

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

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

         (*SKIP)

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

         a+(*SKIP)b

       If the subject is "aaaac...", after the first match attempt fails (starting at  the  first
       character  in  the  string), the starting point skips on to start the next attempt at "c".
       Note that a possessive 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. When it is triggered,  the
       previous  path  through  the  pattern is searched for the most recent (*MARK) that has the
       same name. If one is found, the "bumpalong" advance is to the subject position that corre‐
       sponds  to  that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
       matching name is found, the (*SKIP) is ignored.

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

         (*THEN) or (*THEN:NAME)

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

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

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

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

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

         A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does not backtrack  into  A;
       instead it moves to the next alternative, that is, D.  However, if the subpattern 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 that a conditional subpattern is not considered as having two  alternatives,  because
       only  one  is ever used. In other words, the | character in a conditional subpattern has a
       different meaning. Ignoring white space, consider:

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

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

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

   More than one backtracking verb

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

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

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

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

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

   Backtracking verbs in repeated groups

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

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

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

   Backtracking verbs in assertions

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

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

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

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

   Backtracking verbs in subroutines

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

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

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

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

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

SEE ALSO

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

AUTHOR

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

REVISION

       Last updated: 14 June 2015
       Copyright (c) 1997-2015 University of Cambridge.

PCRE 8.38                                  14 June 2015                            PCREPATTERN(3)

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