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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.

       Another special sequence that may appear at the start of a pattern or in combination  with
       (*UTF8) is:


       This  has  the  same effect as setting the PCRE_UCP option: it causes sequences such as \d
       and \w to use Unicode properties to determine character types, instead of recognizing only
       characters with codes less than 128 via a lookup table.

       If  a  pattern  starts  with  (*NO_START_OPT),  it  has  the  same  effect  as setting the
       PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are also some more
       of  these  special  sequences  that  are concerned with the handling of newlines; they are
       described below.

       The remainder of this document discusses the patterns that are supported by PCRE when  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 functions.

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

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

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


       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 character that
       is not a number or a letter, it takes away any special meaning that  character  may  have.
       This  use  of  backslash  as an escape character applies both inside and outside character

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

       In UTF-8 mode, only ASCII numbers and letters have any special meaning after a  backslash.
       All  other  characters  (in  particular,  those whose codepoints are greater than 127) are
       treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option, 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 classes.  An isolated
       \E that is not preceded by \Q is ignored.

   Non-printing characters

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

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any ASCII character
         \e        escape (hex 1B)
         \f        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  (z
       is  7A), but \c{ becomes hex 3B ({ is 7B), while \c; becomes hex 7B (; is 3B). If the byte
       following \c has a value greater than 127, a compile-time error  occurs.  This  locks  out
       non-ASCII  characters  in  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC
       mode, all byte values are valid. A lower case letter is converted to upper case, and  then
       the 0xc0 bits are flipped.)

       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 zero.

       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

       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 example:

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

   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

   Absolute and relative subroutine calls

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

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal 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

       There is also the single sequence \N, which matches a non-newline character.  This is  the
       same as the "." metacharacter when PCRE_DOTALL is not set.

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

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

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

       By  default, in UTF-8 mode, characters with values greater than 128 never match \d, \s, or
       \w, and always match \D, \S, and \W. These sequences retain their original  meanings  from
       before  UTF-8  support  was  available, mainly for efficiency reasons. However, if PCRE is
       compiled with Unicode property support, and the PCRE_UCP option is set, the  behaviour  is
       changed so that Unicode properties are used to determine character types, as follows:

         \d  any character that \p{Nd} matches (decimal digit)
         \s  any character that \p{Z} matches, plus HT, LF, FF, CR
         \w  any character that \p{L} or \p{N} matches, plus underscore

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

       The sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10.  In
       contrast  to  the  other  sequences,  which  match only ASCII characters by default, these
       always match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is set.
       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

   Newline sequences

       Outside  a character class, by default, the escape sequence \R matches any Unicode newline
       sequence. 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:


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

   Unicode character properties

       When PCRE is built with  Unicode  character  property  support,  three  additional  escape
       sequences that match characters with specific properties are available.  When 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, "Any", which matches any character (including  newline),  and
       some  special PCRE properties (described in the next section).  Other Perl properties such
       as "InMusicalSymbols" are not currently supported by PCRE.  Note  that  \P{Any}  does  not
       match any characters, so always causes a match failure.

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


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

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


       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 by
       default, though you can make them do so by setting the PCRE_UCP option for  pcre_compile()
       or by starting the pattern with (*UCP).

   PCRE's additional properties

       As  well  as  the  standard  Unicode  properties  described  in the previous section, PCRE
       supports four more that make it possible to convert traditional escape sequences  such  as
       \w  and  \s  and  POSIX  character classes to use Unicode properties. PCRE uses these non-
       standard, non-Perl properties internally when PCRE_UCP is set. They are:

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

       Xan matches characters that have either the L (letter) or the  N  (number)  property.  Xps
       matches  the characters tab, linefeed, vertical tab, formfeed, or carriage return, and any
       other character that has the Z (separator) property.  Xsp is the same as Xps, except  that
       vertical tab is excluded. Xwd matches the same characters as Xan, plus underscore.

   Resetting the match start

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


       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

   Simple assertions

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

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

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

       A word boundary is a position in the subject string where the current  character  and  the
       previous  character  do not both match \w or \W (i.e. one matches \w and the other matches
       \W), or the start or end of the  string  if  the  first  or  last  character  matches  \w,
       respectively.  In  UTF-8  mode,  the  meanings  of \w and \W can be changed by setting the
       PCRE_UCP option. When this is done, it also affects \b and \B. Neither PCRE nor Perl has a
       separate  "start  of  word"  or  "end  of word" metasequence. However, whatever follows \b
       normally determines which it is. For example, the fragment \ba matches "a" at the start of
       a word.

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

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

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

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


       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

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


       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.

       The  escape  sequence  \N  behaves  like  a  dot,  except  that  it is not affected by the
       PCRE_DOTALL option. In other words, it matches any character except one that signifies the
       end of a line.


       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, the rest of the string may start with a  malformed
       UTF-8 character. For this reason, the \C escape sequence is best avoided.

       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 mechanism.

       When caseless matching is set, any letters in a class represent both their upper case  and
       lower  case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a
       caseless [^aeiou] does not match "A", whereas a caseful version would. In 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 escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,  \V,  \w,  and  \W  may
       appear  in  a  character  class,  and add the characters that they match to the class. For
       example, [\dABCDEF] matches any hexadecimal digit. In  UTF-8  mode,  the  PCRE_UCP  option
       affects  the  meanings  of  \d, \s, \w and their upper case partners, just as it does when
       they appear outside a character class, as  described  in  the  section  entitled  "Generic
       character  types" above. The escape sequence \b has a different meaning inside a character
       class; it matches the backspace character. The sequences  \B,  \N,  \R,  and  \X  are  not
       special  inside  a character class. Like any other unrecognized escape sequences, they are
       treated as the literal characters "B", "N", "R", and "X" by default, but cause an error if
       the PCRE_EXTRA option is set.

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

       The  only  metacharacters  that  are recognized in character classes are backslash, hyphen
       (only where it can be interpreted as specifying a range), circumflex (only at the  start),
       opening  square bracket (only when it can be interpreted as introducing a POSIX class name
       - 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 and space
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

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

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


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

       By default, in UTF-8 mode, characters with values greater than 128 do not match any of the
       POSIX character classes. However, if the PCRE_UCP option is passed to pcre_compile(), some
       of the classes are changed so that Unicode character properties are used. This is achieved
       by replacing the POSIX classes by other sequences, as follows:

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

       Negated  versions,  such  as  [:^alpha:] use \P instead of \p. The other POSIX classes are
       unchanged, and match only characters with code points less than 128.


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


       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

       When  one  of  these  option  changes  occurs at top level (that is, not inside subpattern
       parentheses), the change applies to the remainder of the  pattern  that  follows.  If  the
       change is placed right at the start of a pattern, PCRE extracts it into the global options
       (and it will therefore show up in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern (see below for a description of subpatterns)  affects
       only that part of the subpattern that follows it, so


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


       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
       are also the (*UTF8) and (*UCP) leading sequences that  can  be  used  to  set  UTF-8  and
       Unicode  property  modes;  they  are  equivalent to setting the PCRE_UTF8 and the PCRE_UCP
       options, respectively.


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

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

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

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

       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 "Saturday".


       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 parentheses  that  follow  the  subpattern  start  after  the
       highest  number  used  in  any  branch.  The  following  example  is  taken  from the Perl
       documentation. The numbers underneath show in which buffer the captured  content  will  be

         # 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

       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 or \pL 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 example:


       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 characters.

       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 (but see also the section entitled
       "Defining subpatterns for use by reference only" below). Items other than subpatterns that
       have a {0} quantifier are omitted from the compiled pattern.

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

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

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


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

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


       to the string

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

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

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


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


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

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

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

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

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

       However,  there 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 line


       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 digits.

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


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


       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

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

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

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

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

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


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

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

         (sens|respons)e and \1ibility

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


       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

       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,


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

   Lookbehind assertions

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


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

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

       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 permitted.

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

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


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

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

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

   Checking for a used subpattern by number

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

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

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

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

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

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

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

   Checking for a used subpattern by name

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


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

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

         abc #comment \n still comment

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


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

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

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

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

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

       A  special  item  that consists of (? followed by a number greater than zero and a closing
       parenthesis is a recursive 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

       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-

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

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

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

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

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

       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

       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 different:


       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 it 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 above.

       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 call.


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

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

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


       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 tested.

       The  new  verbs  make  use  of  what was previously invalid syntax: an opening parenthesis
       followed by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some  may
       take  either  form,  with  differing behaviour, depending on whether or not an argument is
       present. An name is a sequence of letters, digits, and underscores. If the name is  empty,
       that is, if the closing parenthesis immediately follows the colon, the effect is as if the
       colon were not there. Any number of these verbs may occur in a pattern.

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

   Verbs that act immediately

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


       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

         (*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 pattern:


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

   Recording which path was taken

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

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

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

       When a match succeeds, the name of the last-encountered (*MARK)  is  passed  back  to  the
       caller via the pcre_extra data structure, as described in the section on pcre_extra in the
       pcreapi documentation. No data is returned for a partial match.  Here  is  an  example  of
       pcretest  output,  where  the /K modifier requests the retrieval and outputting of (*MARK)

          0: XY
         MK: A
          0: XZ
         MK: B

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

       A name may also be returned after a failed match if the final  path  through  the  pattern
       involves  (*MARK).  However,  unless  (*MARK)  used in conjunction with (*COMMIT), this is
       unlikely to happen for an unanchored pattern because, as the starting point  for  matching
       is  advanced,  the  final  check  is  often with an empty string, causing a failure before
       (*MARK) is reached. For example:

         No match

       There are three potential starting points for this match (starting with X,  starting  with
       P, and with an empty string). If the pattern is anchored, the result is different:

         No match, mark = B

       PCRE's  start-of-match  optimizations  can also interfere with this. For example, if, as a
       result of a call to pcre_study(), it knows the minimum  subject  length  for  a  match,  a
       shorter subject will not be scanned at all.

       Note  that  similar anomalies (though different in detail) exist in Perl, no doubt for the
       same reasons. The use of (*MARK) data after a failed match of an unanchored pattern is not
       recommended, unless (*COMMIT) is involved.

   Verbs that act after backtracking

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

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


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


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

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

          0: abc
         No match

       PCRE knows that any match must start with "a", so the optimization skips along the subject
       to "a" before running the first match attempt, which succeeds. When  the  optimization  is
       disabled  by  the  \Y  escape  in  the  second subject, the match starts at "x" and so the
       (*COMMIT) causes it to fail without trying any other starting points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position in the subject if  the
       rest  of  the pattern does not match. If the pattern is unanchored, the normal "bumpalong"
       advance to the next starting character then happens. Backtracking can occur  as  usual  to
       the left of (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
       if there is no match to the right, backtracking cannot cross (*PRUNE).  In  simple  cases,
       the  use  of  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
       but there are some uses of (*PRUNE) that cannot  be  expressed  in  any  other  way.   The
       behaviour  of  (*PRUNE:NAME)  is  the  same  as  (*MARK:NAME)(*PRUNE) when the match fails
       completely; the name is passed back if this is the final attempt.  (*PRUNE:NAME) does  not
       pass  back  a  name  if  the  match succeeds. In an anchored pattern (*PRUNE) has the same
       effect as (*COMMIT).


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


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


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

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next alternation in the innermost enclosing  group  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. The behaviour of (*THEN:NAME) is exactly the
       same as (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not directly  inside
       an alternation, it acts like (*PRUNE).

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

       If more than one is present in a pattern, the "stongest" one wins. For  example,  consider
       this pattern, where A, B, etc. are complex pattern fragments:


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


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


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


       Last updated: 21 November 2010
       Copyright (c) 1997-2010 University of Cambridge.