Provided by: libpcre3_8.39-12build1_amd64 bug

NAME

       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS


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

       Perl's regular expressions are described in its own documentation, and regular expressions
       in  general are covered in a number of books, some of which have copious examples. Jeffrey
       Friedl's  "Mastering  Regular  Expressions",  published  by   O'Reilly,   covers   regular
       expressions in great detail. This description of PCRE's regular expressions is intended as
       reference material.

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

SPECIAL START-OF-PATTERN ITEMS


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

   UTF support

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

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

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

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

   Unicode property support

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

   Disabling auto-possessification

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

   Disabling start-up optimizations

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

   Newline conventions

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

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

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

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

         (*CR)a.b

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

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

   Setting match and recursion limits

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

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

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

EBCDIC CHARACTER CODES


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

CHARACTERS AND METACHARACTERS


       A  regular  expression  is a pattern that is matched against a subject string from left to
       right. Most characters stand for themselves in a  pattern,  and  match  the  corresponding
       characters in the subject. As a trivial example, the pattern

         The quick brown fox

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

       The  power  of  regular  expressions  comes  from  the ability to include alternatives and
       repetitions in the pattern. These are encoded in the pattern by the use of metacharacters,
       which do not stand for themselves but instead are interpreted in some special way.

       There  are two different sets of metacharacters: those that are recognized anywhere in the
       pattern except within square  brackets,  and  those  that  are  recognized  within  square
       brackets. Outside square brackets, the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part of a pattern that is in square brackets is called a "character class". In a character
       class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The following sections describe the use of each of the metacharacters.

BACKSLASH


       The backslash character has several uses. Firstly, if it is followed by a  character  that
       is  not  a  number or a letter, it takes away any special meaning that character may have.
       This use of backslash as an escape character applies both  inside  and  outside  character
       classes.

       For  example,  if  you  want  to  match  a * character, you write \* in the pattern.  This
       escaping action applies  whether  or  not  the  following  character  would  otherwise  be
       interpreted  as  a  metacharacter, so it is always safe to precede a non-alphanumeric with
       backslash to specify that it stands for itself. In particular, if  you  want  to  match  a
       backslash, you write \\.

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

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

       If you want to remove the special meaning from a sequence of characters, you can do so  by
       putting them between \Q and \E. This is different from Perl in that $ and @ are handled as
       literals in  \Q...\E  sequences  in  PCRE,  whereas  in  Perl,  $  and  @  cause  variable
       interpolation. Note the following examples:

         Pattern            PCRE matches   Perl matches

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

       The \Q...\E sequence is recognized both inside and outside character classes.  An isolated
       \E that is not preceded by \Q is ignored. If \Q  is  not  followed  by  \E  later  in  the
       pattern,  the  literal  interpretation continues to the end of the pattern (that is, \E is
       assumed at the end). If the isolated \Q is inside a character class, this causes an error,
       because the character class is not terminated.

   Non-printing characters

       A  second  use of backslash provides a way of encoding non-printing characters in patterns
       in a visible manner. There is no restriction on the appearance of non-printing characters,
       apart from the binary zero that terminates a pattern, but when a pattern is being prepared
       by text editing, it is often easier to use one of the following escape sequences than  the
       binary  character it represents.  In an ASCII or Unicode environment, these escapes are as
       follows:

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

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

       When  PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t generate the appropriate
       EBCDIC code values. The \c escape is processed as specified for  Perl  in  the  perlebcdic
       document.  The  only characters that are allowed after \c are A-Z, a-z, or one of @, [, \,
       ], ^, _, or ?. Any other character provokes a compile-time error. The sequence \@  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.
       Unfortunately, there are several variants of EBCDIC. In most of them the APC character has
       the value 255 (hex FF), but in the one Perl calls POSIX-BC its value is 95  (hex  5F).  If
       certain  other  characters  have  POSIX-BC values, PCRE makes \? generate 95; otherwise it
       generates 255.

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

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

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

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

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

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

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

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

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

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

   Constraints on character values

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

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

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

   Escape sequences in character classes

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

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

   Unsupported escape sequences

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

   Absolute and relative back references

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

   Absolute and relative subroutine calls

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

   Generic character types

       Another use of backslash is for specifying generic character types:

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

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

       Each  pair  of  lower  and  upper  case  escape  sequences  partitions the complete set of
       characters into two disjoint sets. Any given character matches one, and only one, of  each
       pair.  The sequences can appear both inside and outside character classes. They each match
       one character of the appropriate type. If the current matching point is at the end of  the
       subject string, all of them fail, because there is no character to match.

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

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

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

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

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

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

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

       The vertical space characters are:

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

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

   Newline sequences

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

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

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

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

       It  is  possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set
       of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF either at compile time  or
       when  the  pattern is matched. (BSR is an 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
       starting a pattern string with one of the following sequences:

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

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

         (*ANY)(*BSR_ANYCRLF)

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

   Unicode character properties

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

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

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

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

         \p{Greek}
         \P{Han}

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

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

       Each  character  has  exactly  one  Unicode general category property, specified by a two-
       letter abbreviation. For compatibility with Perl, negation can be specified by including a
       circumflex  between  the  opening brace and the property name. For example, \p{^Lu} is the
       same as \P{Lu}.

       If only one letter is specified with \p or  \P,  it  includes  all  the  general  category
       properties  that  start  with  that  letter. In this case, in the absence of negation, the
       curly brackets in the escape sequence are optional;  these  two  examples  have  the  same
       effect:

         \p{L}
         \pL

       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

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

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

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

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

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

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

   Extended grapheme clusters

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

         (?>\PM\pM*)

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

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

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

       1. End at the end of the subject string.

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

       3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five
       types: L, V, T, LV, and LVT. An L character may be  followed  by  an  L,  V,  LV,  or  LVT
       character;  an  LV  or  V  character  may  be  followed by a V or T character; an LVT or T
       character may be 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
       represented  by  a  Universal Character Name in C++ and other programming languages. These
       are the characters $, @, ` (grave accent), and all characters  with  Unicode  code  points
       greater  than  or  equal  to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that
       most base (ASCII) characters are excluded. (Universal Character  Names  are  of  the  form
       \uHHHH  or  \UHHHHHHHH where H is a hexadecimal digit. Note that the Xuc property does not
       match these sequences but the characters that they represent.)

   Resetting the match start

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

         foo\Kbar

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

         (foo)\Kbar

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

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

   Simple assertions

       The final use of backslash is for certain simple  assertions.  An  assertion  specifies  a
       condition  that  has  to  be  met  at a particular point in a match, without consuming any
       characters from the subject string. The use of subpatterns for more complicated assertions
       is described below.  The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

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

       A word boundary is a position in the subject string where the current  character  and  the
       previous  character  do not both match \w or \W (i.e. one matches \w and the other matches
       \W), or the start or end of the  string  if  the  first  or  last  character  matches  \w,
       respectively.  In  a  UTF  mode,  the  meanings of \w and \W can be changed by setting the
       PCRE_UCP option. When this is done, it also affects \b and \B. Neither PCRE nor Perl has a
       separate  "start  of  word"  or  "end  of word" metasequence. However, whatever follows \b
       normally determines which it is. For example, the fragment \ba matches "a" at the start of
       a word.

       The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described
       in the next section) in that they only ever match at the very start and end of the subject
       string,  whatever  options  are  set.  Thus, they are independent of multiline mode. These
       three assertions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which  affect
       only  the  behaviour  of  the  circumflex  and  dollar  metacharacters.  However,  if  the
       startoffset argument of pcre_exec() is non-zero, indicating that matching is to start at a
       point  other than the beginning of the subject, \A can never match. The difference between
       \Z and \z is that \Z matches before a newline at the end of the string as well as  at  the
       very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at the start point of
       the match, as specified by the startoffset argument of pcre_exec().  It  differs  from  \A
       when  the  value  of  startoffset  is non-zero. By calling pcre_exec() multiple times with
       appropriate arguments, you can mimic  Perl's  /g  option,  and  it  is  in  this  kind  of
       implementation where \G can be useful.

       Note,  however,  that  PCRE's  interpretation of \G, as the start of the current match, is
       subtly different from Perl's, which defines it as the end of the previous match. In  Perl,
       these  can  be  different  when the previously matched string was empty. Because PCRE does
       just one match at a time, it cannot reproduce this behaviour.

       If all the alternatives of a pattern begin with \G, the  expression  is  anchored  to  the
       starting  match  position,  and  the  "anchored"  flag  is  set  in  the  compiled regular
       expression.

CIRCUMFLEX AND DOLLAR


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

       Outside a character class, in the default matching mode, the circumflex  character  is  an
       assertion  that  is true only if the current matching point is at the start of the subject
       string. If the startoffset argument of pcre_exec() is non-zero, circumflex can never match
       if  the  PCRE_MULTILINE  option  is  unset.  Inside  a  character class, circumflex has an
       entirely different meaning (see below).

       Circumflex need not be the first character of the pattern if a number of alternatives  are
       involved,  but it should be the first thing in each alternative in which it appears if the
       pattern is ever  to  match  that  branch.  If  all  possible  alternatives  start  with  a
       circumflex,  that  is,  if  the  pattern  is constrained to match only at the start of the
       subject, it is said to be an "anchored" pattern. (There are also other constructs that can
       cause a pattern to be anchored.)

       The dollar character is an assertion that is true only if the current matching point is at
       the end of the subject string, or immediately before a newline at the end  of  the  string
       (by  default). Note, however, that it does not actually match the newline. Dollar need not
       be the last character of the pattern if a number of  alternatives  are  involved,  but  it
       should  be  the last item in any branch in which it appears. Dollar has no special meaning
       in a character class.

       The meaning of dollar can be changed so that it matches  only  at  the  very  end  of  the
       string,  by  setting  the PCRE_DOLLAR_ENDONLY option at compile time. This does not affect
       the \Z assertion.

       The meanings of the circumflex and dollar characters are  changed  if  the  PCRE_MULTILINE
       option  is  set.  When  this  is the case, a circumflex matches immediately after internal
       newlines as well as at the start of the subject string. It does not match after a  newline
       that  ends  the  string. A dollar matches before any newlines in the string, as well as at
       the very end, when PCRE_MULTILINE is set. When newline is specified as  the  two-character
       sequence CRLF, isolated CR and LF characters do not indicate newlines.

       For  example,  the  pattern  /^abc$/  matches  the  subject  string  "def\nabc"  (where \n
       represents a newline) in multiline mode, but not otherwise.  Consequently,  patterns  that
       are  anchored  in  single  line mode because all branches start with ^ are not anchored in
       multiline mode, and a match for circumflex is possible when the  startoffset  argument  of
       pcre_exec()  is  non-zero.  The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
       set.

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

FULL STOP (PERIOD, DOT) AND \N


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

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

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

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

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

MATCHING A SINGLE DATA UNIT


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

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

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

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

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

SQUARE BRACKETS AND CHARACTER CLASSES


       An opening square bracket introduces a character class, terminated  by  a  closing  square
       bracket.  A  closing square bracket on its own is not special by default.  However, if the
       PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square bracket causes a  compile-time
       error.  If a closing square bracket is required as a member of the class, it should be the
       first data character in the class (after an initial circumflex,  if  present)  or  escaped
       with a backslash.

       A  character class matches a single character in the subject. In a UTF mode, the character
       may be more than one data unit long. A matched character must be in the set of  characters
       defined  by the class, unless the first character in the class definition is a circumflex,
       in which case the subject character must not be in the set defined  by  the  class.  If  a
       circumflex  is  actually  required  as  a  member of the class, ensure it is not the first
       character, or escape it with a backslash.

       For example, the character class [aeiou] matches any  lower  case  vowel,  while  [^aeiou]
       matches  any  character  that  is not a lower case vowel. Note that a circumflex is just a
       convenient notation for specifying the characters that are in  the  class  by  enumerating
       those  that  are  not. A class that starts with a circumflex is not an assertion; it still
       consumes a character from the subject string,  and  therefore  it  fails  if  the  current
       pointer is at the end of the string.

       In  UTF-8  (UTF-16,  UTF-32) mode, characters with values greater than 255 (0xffff) can be
       included in a class as a literal string of data  units,  or  by  using  the  \x{  escaping
       mechanism.

       When  caseless matching is set, any letters in a class represent both their upper case and
       lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and  a
       caseless [^aeiou] does not match "A", whereas a caseful version would. In a UTF mode, PCRE
       always understands the concept of case for characters whose values are less than  128,  so
       caseless  matching  is  always possible. For characters with higher values, the concept of
       case is supported if PCRE is compiled with Unicode property support,  but  not  otherwise.
       If  you want to use caseless matching in a UTF mode for characters 128 and above, you must
       ensure that PCRE is compiled with Unicode property support as well as with UTF support.

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

       The  minus  (hyphen) character can be used to specify a range of characters in a character
       class. For example, [d-m] matches any letter between  d  and  m,  inclusive.  If  a  minus
       character  is  required  in  a  class,  it must be escaped with a backslash or appear in a
       position where it cannot be interpreted as indicating a range, typically as the  first  or
       last  character  in  the class, or immediately after a range. For example, [b-d-z] matches
       letters in the range b to d, a hyphen character, or z.

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

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

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

       If  a  range  that  includes letters is used when caseless matching is set, it matches the
       letters in either case. For example, [W-c]  is  equivalent  to  [][\\^_`wxyzabc],  matched
       caselessly,  and  in  a  non-UTF mode, if character tables for a French locale are in use,
       [\xc8-\xcb] matches accented E characters in both cases. In UTF modes, PCRE  supports  the
       concept  of case for characters with values greater than 128 only when it is compiled with
       Unicode property support.

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

       A  circumflex  can  conveniently  be used with the upper case character types to specify a
       more restricted set of characters than the matching lower case  type.   For  example,  the
       class  [^\W_]  matches  any  letter  or  digit,  but not underscore, whereas [\w] includes
       underscore. A positive character class should be read as "something OR something  OR  ..."
       and a negative class as "NOT something AND NOT something AND NOT ...".

       The  only  metacharacters  that  are recognized in character classes are backslash, hyphen
       (only where it can be interpreted as specifying a range), circumflex (only at the  start),
       opening square bracket (only when it can be interpreted as introducing a POSIX class name,
       or for a special compatibility feature - see the next two sections), and  the  terminating
       closing square bracket. However, escaping other non-alphanumeric characters does no harm.

POSIX CHARACTER CLASSES


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

         [01[:alpha:]%]

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

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

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

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

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX syntax [.ch.]
       and  [=ch=] where "ch" is a "collating element", but these are not supported, and an error
       is given if they are encountered.

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

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

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

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

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

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

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

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

COMPATIBILITY FEATURE FOR WORD BOUNDARIES


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

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

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

VERTICAL BAR


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

         gilbert|sullivan

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

INTERNAL OPTION SETTING


       The  settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options
       (which are Perl-compatible) can be changed from within the pattern by a sequence  of  Perl
       option letters enclosed between "(?" and ")".  The option letters are

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

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

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

       When  one  of  these  option  changes  occurs at top level (that is, not inside subpattern
       parentheses), the change applies to the remainder of the  pattern  that  follows.  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
       abandoned before the option setting. This is because the effects of option settings happen
       at compile time. There would be some very weird behaviour otherwise.

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

SUBPATTERNS


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

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

         cat(aract|erpillar|)

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

       2. It sets up the subpattern as a capturing subpattern. This means that,  when  the  whole
       pattern  matches, that portion of the subject string that matched the subpattern is passed
       back to the caller via the ovector argument of the matching function. (This  applies  only
       to  the  traditional  matching  functions;  the  DFA  matching  functions  do  not support
       capturing.)

       Opening parentheses are counted from left to right (starting from 1) to obtain numbers for
       the  capturing  subpatterns.  For example, if the string "the red king" is matched against
       the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are numbered 1, 2,  and  3,
       respectively.

       The  fact  that  plain  parentheses fulfil two functions is not always helpful.  There are
       often times when a grouping subpattern is required without a capturing requirement. If  an
       opening parenthesis is followed by a question mark and a colon, the subpattern does not do
       any capturing, and is not counted when computing the number of  any  subsequent  capturing
       subpatterns. For example, if the string "the white queen" is matched against the pattern

         the ((?:red|white) (king|queen))

       the  captured  substrings  are  "white  queen"  and "queen", and are numbered 1 and 2. The
       maximum number of capturing subpatterns is 65535.

       As a convenient shorthand, if any option settings are required at  the  start  of  a  non-
       capturing  subpattern, the option letters may appear between the "?" and the ":". Thus the
       two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are tried from left to
       right,  and  options  are  not reset until the end of the subpattern is reached, an option
       setting in one branch does  affect  subsequent  branches,  so  the  above  patterns  match
       "SUNDAY" as well as "Saturday".

DUPLICATE SUBPATTERN NUMBERS


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

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

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

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

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

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

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

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

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

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

NAMED SUBPATTERNS


       Identifying  capturing  parentheses  by  number is simple, but it can be very hard to keep
       track of the numbers in complicated regular expressions. Furthermore, if an expression  is
       modified,  the  numbers may change. To help with this difficulty, PCRE supports the naming
       of subpatterns. This feature was not added to Perl until  release  5.10.  Python  had  the
       feature  earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now
       supports both the Perl and the Python syntax. Perl allows identically numbered subpatterns
       to have different names, but PCRE does not.

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

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

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

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

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

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

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

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

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

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

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

REPETITION


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

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

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

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

       In  UTF modes, quantifiers apply to characters rather than to individual data units. Thus,
       for example, \x{100}{2} matches two characters, each of which is represented by a two-byte
       sequence  in  a  UTF-8  string.  Similarly,  \X{3} matches three Unicode extended grapheme
       clusters, each of which may be several data units long  (and  they  may  be  of  different
       lengths).

       The  quantifier {0} is permitted, causing the expression to behave as if the previous item
       and the quantifier were  not  present.  This  may  be  useful  for  subpatterns  that  are
       referenced as subroutines from elsewhere in the pattern (but see also the section entitled
       "Defining subpatterns for use by reference only" below). Items other than subpatterns that
       have a {0} quantifier are omitted from the compiled pattern.

       For convenience, the three most common quantifiers have single-character abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It  is  possible  to  construct infinite loops by following a subpattern that can match no
       characters with a quantifier that has no upper limit, for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time for such patterns.
       However, because there are cases where this can be useful, such patterns are now accepted,
       but if any repetition of the subpattern does in fact match  no  characters,  the  loop  is
       forcibly broken.

       By  default,  the quantifiers are "greedy", that is, they match as much as possible (up to
       the maximum number of permitted times), without causing the rest of the pattern  to  fail.
       The  classic  example  of  where  this  gives problems is in trying to match comments in C
       programs. These appear between /* and */ and  within  the  comment,  individual  *  and  /
       characters may appear. An attempt to match C comments by applying the pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness of the .*  item.

       However,  if  a  quantifier  is  followed  by a question mark, it ceases to be greedy, and
       instead matches the minimum number of times possible, so the pattern

         /\*.*?\*/

       does the right thing with the C comments. The meaning of the various  quantifiers  is  not
       otherwise  changed,  just  the  preferred  number  of matches.  Do not confuse this use of
       question mark with its use as a quantifier in its own right. Because it has two  uses,  it
       can sometimes appear doubled, as in

         \d??\d

       which  matches one digit by preference, but can match two if that is the only way the rest
       of the pattern matches.

       If the PCRE_UNGREEDY option is set  (an  option  that  is  not  available  in  Perl),  the
       quantifiers are not greedy by default, but individual ones can be made greedy by following
       them with a question mark. In other words, it inverts the default behaviour.

       When a parenthesized subpattern is quantified with a minimum repeat count that is  greater
       than  1  or  with  a limited maximum, more memory is required for the compiled pattern, in
       proportion to the size of the minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent to Perl's  /s)
       is  set,  thus  allowing  the  dot  to match newlines, the pattern is implicitly anchored,
       because whatever follows will be tried against every character  position  in  the  subject
       string,  so  there  is  no  point  in retrying the overall match at any position after the
       first. PCRE normally treats such a pattern as though it were preceded by \A.

       In cases where it is known that the subject string  contains  no  newlines,  it  is  worth
       setting  PCRE_DOTALL  in  order  to  obtain this optimization, or alternatively using ^ to
       indicate anchoring explicitly.

       However, there are some cases where the optimization cannot be used. When  .*   is  inside
       capturing parentheses that are the subject of a back reference elsewhere in the pattern, a
       match at the start may fail where a later one succeeds. Consider, for example:

         (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth character. For this reason,
       such a pattern is not implicitly anchored.

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

         (?>.*?a)b

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

       When a capturing subpattern is repeated, the value captured is the substring that  matched
       the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has  matched  "tweedledum tweedledee" the value of the captured substring is "tweedledee".
       However, if there are nested capturing subpatterns, the corresponding captured values  may
       have been set in previous iterations. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS


       With  both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure
       of what follows normally causes the repeated item to be re-evaluated to see if a different
       number  of  repeats  allows  the  rest  of the pattern to match. Sometimes it is useful to
       prevent this, either to change the nature of the match, or to cause it fail  earlier  than
       it otherwise might, when the author of the pattern knows there is no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to the subject line

         123456bar

       After  matching  all  6  digits  and then failing to match "foo", the normal action of the
       matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and  so
       on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book)
       provides the means for specifying that once a subpattern has matched, it is not to be  re-
       evaluated in this way.

       If  we  use  atomic grouping for the previous example, the matcher gives up immediately on
       failing to match "foo" the first time. The notation is  a  kind  of  special  parenthesis,
       starting with (?> as in this example:

         (?>\d+)foo

       This  kind  of  parenthesis  "locks  up"  the  part of the pattern it contains once it has
       matched, and a failure further into the pattern is prevented from  backtracking  into  it.
       Backtracking past it to previous items, however, works as normal.

       An  alternative  description  is  that  a  subpattern  of  this type matches the string of
       characters that an identical standalone pattern would match, if anchored  at  the  current
       point in the subject string.

       Atomic  grouping subpatterns are not capturing subpatterns. Simple cases such as the above
       example can be thought of as a maximizing repeat that must swallow everything it can.  So,
       while both \d+ and \d+? are prepared to adjust the number of digits they match in order to
       make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits.

       Atomic groups in general can of course contain arbitrarily  complicated  subpatterns,  and
       can  be nested. However, when the subpattern for an atomic group is just a single repeated
       item, as in the example above, a simpler notation, called a "possessive quantifier" can be
       used.  This  consists  of  an  additional  +  character following a quantifier. Using this
       notation, the previous example can be rewritten as

         \d++foo

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

         (abc|xyz){2,3}+

       Possessive quantifiers are always greedy; the  setting  of  the  PCRE_UNGREEDY  option  is
       ignored.  They  are  a convenient notation for the simpler forms of atomic group. However,
       there is no difference in the meaning of a possessive quantifier and the equivalent atomic
       group,  though  there  may  be  a performance difference; possessive quantifiers should be
       slightly faster.

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

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

       When  a  pattern  contains  an  unlimited  repeat  inside  a subpattern that can itself be
       repeated an unlimited number of times, the use of an atomic group is the only way to avoid
       some failing matches taking a very long time indeed. The pattern

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

       matches  an  unlimited  number  of substrings that either consist of non-digits, or digits
       enclosed in <>, followed by either ! or ?. When it matches, it runs quickly.  However,  if
       it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it  takes  a long time before reporting failure. This is because the string can be divided
       between the internal \D+ repeat and the external * repeat in a large number of  ways,  and
       all  have  to  be tried. (The example uses [!?] rather than a single character at the end,
       because both PCRE and Perl have an optimization that allows for fast failure when a single
       character  is  used. They remember the last single character that is required for a match,
       and fail early if it is not present in the string.) If the pattern is changed so  that  it
       uses an atomic group, like this:

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

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

BACK REFERENCES


       Outside  a  character  class, a backslash followed by a digit greater than 0 (and possibly
       further digits) is a back reference to a capturing subpattern earlier  (that  is,  to  its
       left)  in  the  pattern,  provided  there  have  been  that  many  previous capturing left
       parentheses.

       However, if the decimal number following the backslash is less than 10, it is always taken
       as  a  back  reference, and causes an error only if there are not that many capturing left
       parentheses in the entire pattern. In other words, the  parentheses  that  are  referenced
       need  not  be  to  the  left  of  the  reference for numbers less than 10. A "forward back
       reference" of this type can make sense when a repetition is involved and the subpattern to
       the right has participated in an earlier iteration.

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

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

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

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

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

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

       A back reference matches whatever actually matched the capturing subpattern in the current
       subject  string,  rather than anything matching the subpattern itself (see "Subpatterns as
       subroutines" below for a way of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and  responsibility",  but  not  "sense  and
       responsibility".  If  caseful  matching is in force at the time of the back reference, the
       case of letters is relevant. For example,

         ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH rah", even  though  the  original  capturing
       subpattern is matched caselessly.

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

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

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

       There may be more than one back reference to the same subpattern. If a subpattern has  not
       actually  been  used  in  a  particular  match,  any  back references to it always fail by
       default. For example, the pattern

         (a|(bc))\2

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

       Because there may be many capturing parentheses in  a  pattern,  all  digits  following  a
       backslash  are  taken  as  part  of  a  potential  back  reference number.  If the pattern
       continues with a digit character, some delimiter  must  be  used  to  terminate  the  back
       reference. If the PCRE_EXTENDED option is set, this can be white space. Otherwise, the \g{
       syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

       A back reference that occurs inside the parentheses to which  it  refers  fails  when  the
       subpattern  is first used, so, for example, (a\1) never matches.  However, such references
       can be useful inside repeated subpatterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also  "aba",  "ababbaa"  etc.  At  each  iteration  of  the
       subpattern,  the back reference matches the character string corresponding to the previous
       iteration. In order for this to work, the pattern must be such that  the  first  iteration
       does  not  need to match the back reference. This can be done using alternation, as in the
       example above, or by a quantifier with a minimum of zero.

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

ASSERTIONS


       An assertion is a test on the characters following or preceding the current matching point
       that  does not actually consume any characters. The simple assertions coded as \b, \B, \A,
       \G, \Z, \z, ^ and $ are described above.

       More complicated assertions are coded as subpatterns. There are two kinds: those that look
       ahead  of  the  current  position in the subject string, and those that look behind it. An
       assertion subpattern is matched in the normal way, except  that  it  does  not  cause  the
       current matching position to be changed.

       Assertion  subpatterns  are  not  capturing  subpatterns.  If  such  an assertion contains
       capturing subpatterns within it, these are counted  for  the  purposes  of  numbering  the
       capturing  subpatterns  in  the whole pattern. However, substring capturing is carried out
       only for positive assertions. (Perl sometimes,  but  not  always,  does  do  capturing  in
       negative assertions.)

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

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

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

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

   Lookahead assertions

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

         \w+(?=;)

       matches  a  word followed by a semicolon, but does not include the semicolon in the match,
       and

         foo(?!bar)

       matches any occurrence of "foo" that is not followed by "bar". Note  that  the  apparently
       similar pattern

         (?!foo)bar

       does  not  find  an occurrence of "bar" that is preceded by something other than "foo"; it
       finds any occurrence of "bar" whatsoever, because the assertion  (?!foo)  is  always  true
       when  the next three characters are "bar". A lookbehind assertion is needed to achieve the
       other effect.

       If you want to force a matching failure at some point in a pattern,  the  most  convenient
       way  to  do  it  is with (?!) because an empty string always matches, so an assertion that
       requires there not to be an empty string must always fail.  The backtracking control  verb
       (*FAIL) or (*F) is a synonym for (?!).

   Lookbehind assertions

       Lookbehind  assertions  start  with  (?<=  for  positive  assertions and (?<! for negative
       assertions. For example,

         (?<!foo)bar

       does find an occurrence of "bar" that  is  not  preceded  by  "foo".  The  contents  of  a
       lookbehind assertion are restricted such that all the strings it matches must have a fixed
       length. However, if there are several top-level alternatives, they do not all have to have
       the same fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error  at  compile  time.  Branches  that  match  different length strings are
       permitted only at the top level of a lookbehind assertion. This is an  extension  compared
       with  Perl,  which  requires all branches to match the same length of string. An assertion
       such as

         (?<=ab(c|de))

       is not permitted, because its single top-level branch can match two different lengths, but
       it is acceptable to PCRE if rewritten to use two top-level branches:

         (?<=abc|abde)

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

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

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

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

       Possessive quantifiers can be used in conjunction with lookbehind  assertions  to  specify
       efficient  matching  of  fixed-length  strings  at  the end of subject strings. Consider a
       simple pattern such as

         abcd$

       when applied to a long string that does not match. Because matching proceeds from left  to
       right, PCRE will look for each "a" in the subject and then see if what follows matches the
       rest of the pattern. If the pattern is specified as

         ^.*abcd$

       the initial .* matches the entire string at first, but when this fails (because  there  is
       no  following  "a"),  it  backtracks to match all but the last character, then all but the
       last two characters, and so on. Once again the search for "a" covers  the  entire  string,
       from right to left, so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there  can  be  no backtracking for the .*+ item; it can match only the entire string. The
       subsequent lookbehind assertion does a single test on the  last  four  characters.  If  it
       fails,  the  match  fails immediately. For long strings, this approach makes a significant
       difference to the processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

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

       matches "foo" preceded by three digits that  are  not  "999".  Notice  that  each  of  the
       assertions  is  applied independently at the same point in the subject string. First there
       is a check that the previous three characters are all digits, and then there  is  a  check
       that  the same three characters are not "999".  This pattern does not match "foo" preceded
       by six characters, the first of which are digits and the  last  three  of  which  are  not
       "999". For example, it doesn't match "123abcfoo". A pattern to do that is

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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

CONDITIONAL SUBPATTERNS


       It  is  possible  to  cause  the matching process to obey a subpattern conditionally or to
       choose between two alternative subpatterns, depending on the result of  an  assertion,  or
       whether  a  specific capturing subpattern has already been matched. The two possible forms
       of conditional subpattern are:

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

       If the condition is satisfied, the yes-pattern  is  used;  otherwise  the  no-pattern  (if
       present)  is  used.  If there are more than two alternatives in the subpattern, a compile-
       time error occurs. Each of the two alternatives may itself contain nested  subpatterns  of
       any  form,  including conditional subpatterns; the restriction to two alternatives applies
       only at the level of the  condition.  This  pattern  fragment  is  an  example  where  the
       alternatives are complex:

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

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

   Checking for a used subpattern by number

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

       Consider the following pattern, which contains non-significant white space to make it more
       readable (assume the PCRE_EXTENDED option) and to divide it into three parts for  ease  of
       discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first part matches an optional opening parenthesis, and if that character is present,
       sets it as the first captured substring. The second part matches one  or  more  characters
       that are not parentheses. The third part is a conditional subpattern that tests whether or
       not the first set of parentheses matched. If they did, that is, if subject started with an
       opening  parenthesis,  the  condition  is  true,  and so the yes-pattern is executed and a
       closing  parenthesis  is  required.  Otherwise,  since  no-pattern  is  not  present,  the
       subpattern  matches  nothing.  In  other  words,  this  pattern matches a sequence of non-
       parentheses, optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one, you could use a relative reference:

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

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

   Checking for a used subpattern by name

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

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

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

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

   Checking for pattern recursion

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

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

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

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

   Defining subpatterns for use by reference only

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

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

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

   Assertion conditions

       If the condition is not in any of the above formats, it must be an assertion.  This may be
       a  positive  or  negative  lookahead or lookbehind assertion. Consider this pattern, again
       containing non-significant white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition is a positive lookahead assertion that matches an optional sequence of  non-
       letters  followed  by  a letter. In other words, it tests for the presence of at least one
       letter in the subject. If a letter is found, the subject  is  matched  against  the  first
       alternative;  otherwise  it is matched against the second. This pattern matches strings in
       one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.

COMMENTS


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

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

         abc #comment \n still comment

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

RECURSIVE PATTERNS


       Consider the problem of matching a string in parentheses, allowing  for  unlimited  nested
       parentheses.  Without  the use of recursion, the best that can be done is to use a pattern
       that matches up to some fixed depth of nesting. It is not possible to handle an  arbitrary
       nesting depth.

       For  some  time,  Perl  has provided a facility that allows regular expressions to recurse
       (amongst other things). It does this by interpolating Perl code in the expression  at  run
       time,  and  the  code  can  refer  to  the  expression  itself.  A Perl pattern using code
       interpolation to solve the parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively
       to the pattern in which it appears.

       Obviously,  PCRE  cannot  support  the  interpolation  of  Perl code. Instead, it supports
       special syntax for recursion of the entire pattern, and  also  for  individual  subpattern
       recursion.  After  its  introduction  in  PCRE  and  Python,  this  kind  of recursion was
       subsequently introduced into Perl at release 5.10.

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

       This PCRE pattern solves the nested parentheses problem (assume the  PCRE_EXTENDED  option
       is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First  it  matches  an opening parenthesis. Then it matches any number of substrings which
       can either be a sequence of non-parentheses, or a recursive match of  the  pattern  itself
       (that  is,  a correctly parenthesized substring).  Finally there is a closing parenthesis.
       Note the use of a possessive quantifier to  avoid  backtracking  into  sequences  of  non-
       parentheses.

       If  this  were part of a larger pattern, you would not want to recurse the entire pattern,
       so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We have put the pattern into parentheses, and  caused  the  recursion  to  refer  to  them
       instead of the whole pattern.

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

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

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

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

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

       This particular example pattern that we have been looking  at  contains  nested  unlimited
       repeats, and so the use of a possessive quantifier for matching strings of non-parentheses
       is important when applying the pattern to strings that do not  match.  For  example,  when
       this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields  "no match" quickly. However, if a possessive quantifier is not used, the match
       runs for a very long time indeed because there are so many different  ways  the  +  and  *
       repeats  can  carve  up  the  subject,  and  all  have  to be tested before failure can be
       reported.

       At the end of a match, the values of capturing parentheses are those  from  the  outermost
       level.  If  you  want  to  obtain intermediate values, a callout function can be used (see
       below and the pcrecallout documentation). If the pattern above is matched against

         (ab(cd)ef)

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

       If  there  are  more  than 15 capturing parentheses in a pattern, PCRE has to obtain extra
       memory to store data during a recursion, which it does by using  pcre_malloc,  freeing  it
       via  pcre_free  afterwards.  If  no  memory  can  be  obtained,  the  match fails with the
       PCRE_ERROR_NOMEMORY error.

       Do not confuse the (?R) item with the condition (R), which tests for recursion.   Consider
       this  pattern,  which matches text in angle brackets, allowing for arbitrary nesting. Only
       digits are allowed in nested brackets (that is, when recursing),  whereas  any  characters
       are permitted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In  this  pattern,  (?(R)  is  the  start  of a conditional subpattern, with two different
       alternatives for the recursive and non-recursive  cases.  The  (?R)  item  is  the  actual
       recursive call.

   Differences in recursion processing between PCRE and Perl

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

SUBPATTERNS AS SUBROUTINES


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

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

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and  responsibility",  but  not  "sense  and
       responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is  used,  it  does  match  "sense  and  responsibility" as well as the other two strings.
       Another example is given in the discussion of DEFINE above.

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

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

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

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

ONIGURUMA SUBROUTINE SYNTAX


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

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

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

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

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

CALLOUTS


       Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl  code  to  be
       obeyed  in  the  middle  of matching a regular expression. This makes it possible, amongst
       other things, to extract different substrings that match the same pair of parentheses when
       there is a repetition.

       PCRE  provides  a  similar  feature, but of course it cannot obey arbitrary Perl code. The
       feature is called "callout". The caller of PCRE provides an external function  by  putting
       its entry point in the global variable pcre_callout (8-bit library) or pcre[16|32]_callout
       (16-bit or 32-bit library).  By default, this variable contains NULL, which  disables  all
       calling out.

       Within  a  regular expression, (?C) indicates the points at which the external function is
       to be called. If you want to identify different callout points, you can put a number  less
       than 256 after the letter C. The default value is zero.  For example, this pattern has two
       callout points:

         (?C1)abc(?C2)def

       If  the  PCRE_AUTO_CALLOUT  flag  is  passed  to  a  compiling  function,   callouts   are
       automatically  installed  before  each  item in the pattern. They are all numbered 255. If
       there is a conditional group in the pattern whose condition is an assertion, an additional
       callout is inserted just before the condition. An explicit callout may also be set at this
       position, as in this example:

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

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

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

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

BACKTRACKING CONTROL


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

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

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

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

   Optimizations that affect backtracking verbs

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

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

   Verbs that act immediately

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

          (*ACCEPT)

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

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

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

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

         (*FAIL) or (*F)

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

         a+(?C)(*FAIL)

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

   Recording which path was taken

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

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

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

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

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

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

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

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

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

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

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

   Verbs that act after backtracking

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

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

         (*COMMIT)

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

         a+(*COMMIT)b

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

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

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

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

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

         (*PRUNE) or (*PRUNE:NAME)

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

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

         (*SKIP)

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

         a+(*SKIP)b

       If  the  subject is "aaaac...", after the first match attempt fails (starting at the first
       character in the string), the starting point skips on to start the next  attempt  at  "c".
       Note  that  a possessive 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
       corresponds to that (*MARK) instead of to where (*SKIP) was  encountered.  If  no  (*MARK)
       with a matching name is found, the (*SKIP) is ignored.

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

         (*THEN) or (*THEN:NAME)

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

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

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

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

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

         A (B(*THEN)C) | D

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

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

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

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

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

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

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

   More than one backtracking verb

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

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

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

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

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

   Backtracking verbs in repeated groups

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

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

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

   Backtracking verbs in assertions

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

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

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

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

   Backtracking verbs in subroutines

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

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

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

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

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

SEE ALSO


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

AUTHOR


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

REVISION


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