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       PCRE - Perl-compatible regular expressions


       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.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, PCRE must be built to include UTF-8 support, and  you  must  call
       pcre_compile()  or  pcre_compile2() with the PCRE_UTF8 option. There is
       also a special sequence that can be given at the start of a pattern:


       Starting a pattern with this sequence  is  equivalent  to  setting  the
       PCRE_UTF8  option.  This  feature  is  not Perl-compatible. How setting
       UTF-8 mode affects pattern matching  is  mentioned  in  several  places
       below.  There  is  also  a  summary of UTF-8 features in the section on
       UTF-8 support in the main pcre page.

       The  remainder  of  this  document  discusses  the  patterns  that  are
       supported  by  PCRE  when  its  main matching function, pcre_exec(), is
       used.  From release  6.0,  PCRE  offers  a  second  matching  function,
       pcre_dfa_exec(),  which matches using a different algorithm that is not
       Perl-compatible. Some of the features discussed below are not available
       when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
       alternative function, and how it differs from the normal function,  are
       discussed in the pcrematching page.


       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

       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  pcre_compile()  or
       pcre_compile2().  For example, on a Unix system where LF is the default
       newline sequence, the pattern


       changes the convention to CR. That pattern matches "a\nb" because LF is
       no  longer  a  newline. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start  of  a  pattern,
       and  that  they  must  be  in  upper  case. If more than one of them is
       present, the last one is used.

       The newline convention 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.


       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 UTF-8 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-8

       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
         ]      terminates the character class

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


       The backslash character has several uses. Firstly, if it is followed by
       a  non-alphanumeric  character,  it takes away any special meaning that
       character may have. This  use  of  backslash  as  an  escape  character
       applies both inside and outside character classes.

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

       If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class  and  the  next  newline  are  ignored.  An
       escaping  backslash  can be used to include a whitespace 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

   Non-printing characters

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

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

       The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;
       becomes hex 7B.

       After  \x, from zero to two hexadecimal digits are read (letters can be
       in upper or lower case). Any number of hexadecimal  digits  may  appear
       between  \x{  and  },  but the value of the character code must be less
       than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
       the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
       than the largest Unicode code point, which is 10FFFF.

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

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

       After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those  that  are  present  are used. Thus the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code  value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

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

       Inside a character class, or if the decimal number is  greater  than  9
       and  there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the  backslash,  and  uses  them  to
       generate  a data character. Any subsequent digits stand for themselves.
       In non-UTF-8 mode, the value of a character specified in octal must  be
       less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

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

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class,  the  sequence \b is interpreted as the backspace character (hex
       08), and the sequences \R and \X are interpreted as the characters  "R"
       and  "X", respectively. Outside a character class, these sequences have
       different meanings (see below).

   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. The
       following are always recognized:

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

       Each pair of escape sequences partitions the complete set of characters
       into  two disjoint sets. Any given character matches one, and only one,
       of each pair.

       These character type sequences  can  appear  both  inside  and  outside
       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, since there is no character to match.

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

       In  UTF-8 mode, characters with values greater than 128 never match \d,
       \s, or \w, and always match \D, \S, and \W.  This  is  true  even  when
       Unicode character property support is available. These sequences retain
       their original meanings from before UTF-8 support was available, mainly
       for  efficiency  reasons. Note that this also affects \b, because it is
       defined in terms of \w and \W.

       The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
       the  other  sequences, these do match certain high-valued codepoints in
       UTF-8 mode.  The horizontal space characters are:

         U+0009     Horizontal tab
         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
         U+000B     Vertical tab
         U+000C     Formfeed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

       A "word" character is an underscore or any character less than 256 that
       is  a  letter  or  digit.  The  definition  of  letters  and  digits is
       controlled by PCRE’s low-valued  character  tables,  and  may  vary  if
       locale-specific  matching  is taking place (see "Locale support" in the
       pcreapi page). For example, in a French locale such as "fr_FR" in Unix-
       like systems, or "french" in Windows, some character codes greater than
       128 are used for accented letters, and these are matched by \w. The use
       of locales with Unicode is discouraged.

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
       mode \R is equivalent to the following:


       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 (formfeed, 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 UTF-8 mode, 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  pcre_compile()  or
       pcre_compile2(),  but  they  can  be  overridden  by  options  given to
       pcre_exec() or pcre_dfa_exec(). 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:


       Inside a character class, \R matches the letter "R".

   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 not in 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       an extended Unicode sequence

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

       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:


       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, Bengali, Bopomofo, Braille,
       Buginese,  Buhid,  Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
       Coptic,   Cuneiform,   Cypriot,    Cyrillic,    Deseret,    Devanagari,
       Egyptian_Hieroglyphs,  Ethiopic,  Georgian,  Glagolitic, Gothic, Greek,
       Gujarati,   Gurmukhi,   Han,   Hangul,   Hanunoo,   Hebrew,   Hiragana,
       Imperial_Aramaic,           Inherited,           Inscriptional_Pahlavi,
       Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana,  Kayah_Li,
       Kharoshthi,  Khmer,  Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
       Lydian, Malayalam, Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue,  Nko,
       Ogham,    Old_Italic,   Old_Persian,   Old_South_Arabian,   Old_Turkic,
       Ol_Chiki,  Oriya,  Osmanya,  Phags_Pa,   Phoenician,   Rejang,   Runic,
       Samaritan,   Saurashtra,  Shavian,  Sinhala,  Sundanese,  Syloti_Nagri,
       Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,  Tamil,  Telugu,
       Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, Yi.

       Each  character has exactly one 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:


       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 UTF-8  strings  (see
       RFC  3629)  and  so  cannot  be  tested  by PCRE, unless UTF-8 validity
       checking has been turned off (see the discussion of  PCRE_NO_UTF8_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.

       The \X escape matches any number of Unicode  characters  that  form  an
       extended Unicode sequence. \X is equivalent to


       That  is,  it matches a character without the "mark" property, followed
       by zero or more characters with the "mark"  property,  and  treats  the
       sequence  as  an  atomic group (see below).  Characters with the "mark"
       property are typically accents that  affect  the  preceding  character.
       None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X
       matches any one character.

       Matching characters by Unicode property is not fast, because  PCRE  has
       to  search  a  structure  that  contains data for over fifteen thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE.

   Resetting the match start

       The  escape  sequence  \K,  which  is  a  Perl 5.10 feature, causes any
       previously matched characters not to be included in the  final  matched
       sequence. For example, the pattern:


       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


       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.

   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

       These assertions may not appear in character classes (but note that  \b
       has  a  different  meaning,  namely  the  backspace character, inside a
       character class).

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string if the first or last character matches \w, respectively. Neither
       PCRE nor  Perl  has  a  separte  "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.


       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.)

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

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

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE  option  is  set.  When  this  is the case, 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


       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. In UTF-8 mode, the matched  character  may
       be more than one byte long.

       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.


       Outside a character class, the escape sequence \C matches any one byte,
       both in and out of UTF-8 mode. Unlike a  dot,  it  always  matches  any
       line-ending  characters.  The  feature  is provided in Perl in order to
       match individual bytes in  UTF-8  mode.  Because  it  breaks  up  UTF-8
       characters  into  individual bytes, what remains in the string may be a
       malformed UTF-8 string. For this reason, the \C escape sequence is best

       PCRE  does  not  allow \C to appear in lookbehind assertions (described
       below), because  in  UTF-8  mode  this  would  make  it  impossible  to
       calculate the length of the lookbehind.


       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 UTF-8
       mode, the character may be more than one byte 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 mode, characters with values greater than 255 can be included
       in a class as a literal string of bytes, or by using the  \x{  escaping

       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 UTF-8 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 UTF8-mode for characters 128 and above, you must
       ensure that PCRE is compiled with Unicode property support as  well  as
       with UTF-8 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.

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

       Ranges  operate in the collating sequence of character values. They can
       also  be  used  for  characters  specified  numerically,  for   example
       [\000-\037].  In UTF-8 mode, ranges can include characters whose values
       are greater than 255, for example [\x{100}-\x{2ff}].

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_‘wxyzabc], matched caselessly,  and  in  non-UTF-8  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF-8 mode, PCRE  supports  the
       concept  of  case for characters with values greater than 128 only when
       it is compiled with Unicode property support.

       The character types \d, \D, \p, \P, \s, \S, \w, and \W may also  appear
       in  a  character  class,  and add the characters that they match to the
       class.  For  example,  [\dABCDEF]  matches  any  hexadecimal  digit.  A
       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.

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


       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,


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

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

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

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


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

       In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.


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


       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.


       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 current pattern that follows
       it, so


       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


       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 compile or match 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 is also the (*UTF8) leading sequence that  can  be  used  to  set
       UTF-8 mode; this is equivalent to setting the PCRE_UTF8 option.


       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


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

       2. It sets up the subpattern as  a  capturing  subpattern.  This  means
       that,  when  the  whole  pattern  matches,  that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       ovector  argument  of pcre_exec(). Opening parentheses are counted from
       left to right (starting from 1) to obtain  numbers  for  the  capturing

       For  example,  if  the  string  "the  red  king" is matched against the

         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


       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


       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:


       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 buffers 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":


       In contrast, a recursive or "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":


       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.


       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.
       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:


       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  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  give  the same name to subpatterns with the same number, even
       when PCRE_DUPNAMES is not set.


       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 (in UTF-8 mode with Unicode properties)
         the \R escape sequence
         an escape such as \d that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (unless it is an assertion)
         a recursive or "subroutine" call to a subpattern

       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


       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


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


       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

       In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
       individual bytes. Thus,  for  example,  \x{100}{2}  matches  two  UTF-8
       characters,  each  of  which  is  represented  by  a two-byte sequence.
       Similarly, when Unicode property support is  available,  \X{3}  matches
       three  Unicode  extended  sequences, each of which may be several bytes
       long (and they may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the  previous  item  and  the  quantifier were not present. This may be
       useful  for  subpatterns  that  are  referenced  as  subroutines   from
       elsewhere  in the pattern. 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:


       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

       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


       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


       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

       However, there is one situation 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:


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

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


       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


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


       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


       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:


       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

       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


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


       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


       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


       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:


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


       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

       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, which is a
       feature introduced in Perl 5.10. 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:


       The  sequence  \g{-1}  is  a  reference  to  the  most recently started
       capturing subpattern before \g,  that  is,  is  it  equivalent  to  \2.
       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


       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:


       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


       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 whitespace. 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


       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.


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

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

       Assertion subpatterns are not capturing subpatterns,  and  may  not  be
       repeated,  because  it  makes no sense to assert the same thing several
       times. If any kind of assertion contains capturing  subpatterns  within
       it,  these  are  counted  for  the  purposes of numbering the capturing
       subpatterns in the whole  pattern.   However,  substring  capturing  is
       carried  out  only  for  positive  assertions, because it does not make
       sense for negative assertions.

   Lookahead assertions

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


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


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


       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  Perl  5.10 backtracking control verb
       (*FAIL) or (*F) is essentially a synonym for (?!).

   Lookbehind assertions

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


       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


       is permitted, but


       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 (5.8 and 5.10), which requires
       all branches to match the same length of string. An assertion such as


       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:


       In some cases, the Perl 5.10 escape sequence \K (see above) can be used
       instead  of  a  lookbehind  assertion  to  get  round  the fixed-length

       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.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode)  to  appear  in  lookbehind  assertions,  because  it  makes   it
       impossible  to  calculate  the  length of the lookbehind. The \X and \R
       escapes, which can match different  numbers  of  bytes,  are  also  not

       "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


       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


       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


       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,


       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


       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,


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


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


       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:


       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.

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

   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 been set. 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.  In looping
       constructs it can also make sense to refer to  subsequent  groups  with
       constructs such as (?(+2).

       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 the first
       set  of  parentheses  matched  or not. If they did, that is, if subject
       started with an opening parenthesis, the condition is true, and so  the
       yes-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

   Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?(’name’)...)  to  test  for  a
       used  subpattern  by  name.  For compatibility with earlier versions of
       PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
       also  recognized.  However,  there  is  a  possible ambiguity with this
       syntax, because subpattern names may consist entirely of  digits.  PCRE
       looks  first for a named subpattern; if it cannot find one and the name
       consists entirely of digits,  PCRE  looks  for  a  subpattern  of  that
       number,  which  must  be greater than zero. Using subpattern names that
       consist entirely of digits is not recommended.

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

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

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

   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name  R, the 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  could  be
       written like this (ignore whitespace 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:

         \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.


       The sequence (?# marks the start of a comment that continues up to  the
       next  closing  parenthesis.  Nested  parentheses are not permitted. The
       characters that make up a comment play no part in the pattern  matching
       at all.

       If  the PCRE_EXTENDED option is set, an unescaped # character outside a
       character class introduces a  comment  that  continues  to  immediately
       after the next newline in the pattern.


       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

       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 call of the subpattern of
       the given number, provided that it occurs inside that  subpattern.  (If
       not,  it  is  a  "subroutine"  call,  which  is  described  in the next
       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  (a  Perl
       5.10  feature).   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  "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


       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


       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 value is unset,
       even if it is (temporarily) set at a deeper level.

       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.

   Recursion difference from Perl

       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"):


       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


       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

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


       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:


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


       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.


       If the syntax for a recursive subpattern reference (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:


       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

       Like recursive subpatterns, a subroutine 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. Any capturing parentheses that
       are set during the subroutine call  revert  to  their  previous  values

       When  a  subpattern is used as a subroutine, processing options such as
       case-independence are fixed when the subpattern is defined. They cannot
       be changed for different calls. For example, consider this pattern:


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


       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:


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


       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

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

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


       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the  number
       of  the callout, the position in the pattern, and, optionally, one item
       of data originally supplied by the caller of pcre_exec().  The  callout
       function  may  cause  matching  to  proceed,  to  backtrack, or to fail
       altogether. A complete description of  the  interface  to  the  callout
       function is given in the pcrecallout documentation.


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

       Since these verbs are specifically related  to  backtracking,  most  of
       them  can  be  used  only  when  the  pattern  is  to  be matched using
       pcre_exec(), which uses a backtracking algorithm. With the exception of
       (*FAIL), which behaves like a failing negative assertion, they cause an
       error if encountered by pcre_dfa_exec().

       If any of these verbs are used in an assertion or subroutine subpattern
       (including  recursive  subpatterns),  their  effect is confined to that
       subpattern; it does not extend to the surrounding  pattern.  Note  that
       such  subpatterns are processed as anchored at the point where they are

       The new verbs make use  of  what  was  previously  invalid  syntax:  an
       opening  parenthesis  followed  by  an  asterisk.  In  Perl,  they  are
       generally of the form (*VERB:ARG) but PCRE does not support the use  of
       arguments,  so  its  general  form is just (*VERB). Any number of these
       verbs may occur in a pattern. There are two kinds:

   Verbs that act immediately

       The following verbs act as soon as they are encountered:


       This verb causes the match to end successfully, skipping the  remainder
       of  the pattern. When inside a recursion, only the innermost pattern is
       ended immediately. If (*ACCEPT) is inside  capturing  parentheses,  the
       data  so  far  is  captured. (This feature was added to PCRE at release
       8.00.) For example:


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

         (*FAIL) or (*F)

       This  verb  causes the match to fail, 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


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

   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,  a
       failure  is  forced.   The verbs differ in exactly what kind of failure


       This verb causes the whole match to fail outright if the  rest  of  the
       pattern  does  not match. Even if the pattern is unanchored, no further
       attempts to find a match by advancing the starting  point  take  place.
       Once  (*COMMIT)  has been passed, pcre_exec() is committed to finding a
       match at the current starting point, or not at all. For example:


       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."


       This  verb causes the match to fail at the current position if the rest
       of the pattern does not match. If the pattern is unanchored, the normal
       "bumpalong"  advance  to  the  next  starting  character  then happens.
       Backtracking can occur as usual  to  the  left  of  (*PRUNE),  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.


       This  verb  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:


       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


       This  verb  causes  a  skip  to the next alternation if the rest of the
       pattern does not match. That is, it cancels pending  backtracking,  but
       only   within   the  current  alternation.  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 (*THEN) is used outside  of  any  alternation,  it  acts
       exactly like (*PRUNE).


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


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


       Last updated: 06 March 2010
       Copyright (c) 1997-2010 University of Cambridge.