PCREPATTERN(3)             Library Functions Manual             PCREPATTERN(3)

       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  syn-
       tax 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 syn-
       tax) 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 Ex-
       pressions", published by O'Reilly, covers regular expressions in  great
       detail.  This  description of PCRE's regular expressions is intended as
       reference material.

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


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

   UTF support

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


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

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

   Unicode property support

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

   Disabling auto-possessification

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

   Disabling start-up optimizations

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

   Newline conventions

       PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, 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  pat-
       tern string with one of the following five sequences:

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

       These override the default and the options given to the compiling func-
       tion. For example, on a Unix system where LF is the default newline se-
       quence, the pattern


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

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

   Setting match and recursion limits

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


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


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


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

         The quick brown fox

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

       The  power of regular expressions comes from the ability to include al-
       ternatives 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  recog-
       nized  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 classes.

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

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

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

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

         Pattern            PCRE matches   Perl matches

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

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

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters 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 se-
       quences than the binary character it represents.  In an ASCII  or  Uni-
       code environment, these escapes are as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Constraints on character values

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

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

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

   Escape sequences in character classes

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

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

   Unsupported escape sequences

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

   Absolute and relative back references

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

   Absolute and relative subroutine calls

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

   Generic character types

       Another use of backslash is for specifying generic character types:

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

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

       Each  pair of lower and upper case escape sequences partitions the com-
       plete 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

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

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

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

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

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

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

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

       The vertical space characters are:

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

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

   Newline sequences

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


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

       In  other modes, two additional characters whose codepoints are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator,  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  abbreviation for "backslash R".) This can be made the de-
       fault when PCRE is built; if this is the case, the other behaviour  can
       be  requested  via the PCRE_BSR_UNICODE option.  It is also possible to
       specify these settings by starting a pattern string  with  one  of  the
       following sequences:

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

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


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

   Unicode character properties

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

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

       The property names represented by xx above are limited to  the  Unicode
       script names, the general category properties, "Any", which matches any
       character (including newline), and some special  PCRE  properties  (de-
       scribed  in  the next section).  Other Perl properties such as "InMusi-
       calSymbols" 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, Bassa_Vah, Batak,  Bengali,
       Bopomofo,  Brahmi,  Braille, Buginese, Buhid, Canadian_Aboriginal, Car-
       ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei-
       form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero-
       glyphs,  Elbasan,  Ethiopic,  Georgian,  Glagolitic,  Gothic,  Grantha,
       Greek,  Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Im-
       perial_Aramaic,     Inherited,     Inscriptional_Pahlavi,      Inscrip-
       tional_Parthian,   Javanese,   Kaithi,   Kannada,  Katakana,  Kayah_Li,
       Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha,  Limbu,  Lin-
       ear_A,  Linear_B,  Lisu,  Lycian, Lydian, Mahajani, Malayalam, Mandaic,
       Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hi-
       eroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue,
       Nko,  Ogham,  Ol_Chiki,  Old_Italic,   Old_North_Arabian,   Old_Permic,
       Old_Persian,   Old_South_Arabian,   Old_Turkic,   Oriya,  Osmanya,  Pa-
       hawh_Hmong,    Palmyrene,    Pau_Cin_Hau,     Phags_Pa,     Phoenician,
       Psalter_Pahlavi,  Rejang,  Runic,  Samaritan, Saurashtra, Sharada, Sha-
       vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri,  Syriac,
       Tagalog,  Tagbanwa,  Tai_Le,  Tai_Tham, Tai_Viet, Takri, Tamil, Telugu,
       Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic,  Vai,  Warang_Citi,

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

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


       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

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

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

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

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

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

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

   Extended grapheme clusters

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


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

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

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

       1. End at the end of the subject string.

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

       3.  Do  not  break  Hangul (a Korean script) syllable sequences. Hangul
       characters are of five types: L, V, T, LV, and LVT. An L character  may
       be  followed by an L, V, LV, or LVT character; an LV or V character may
       be followed by a V or T character; an LVT or T character  may  be  fol-
       lowed only by a T character.

       4.  Do not end before extending characters or spacing marks. Characters
       with the "mark" property always have  the  "extend"  grapheme  breaking

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

       As  well  as the standard Unicode properties described above, PCRE sup-
       ports four more that make it possible to convert traditional escape se-
       quences  such  as  \w and \s to use Unicode properties. PCRE uses these
       non-standard, non-Perl properties internally when PCRE_UCP is set. How-
       ever, they may also be used explicitly. These properties are:

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

       Xan  matches  characters that have either the L (letter) or the N (num-
       ber) property. Xps matches the characters tab, linefeed, vertical  tab,
       form  feed,  or carriage return, and any other character that has the Z
       (separator) property.  Xsp is the same as Xps; it used to exclude  ver-
       tical  tab,  for Perl compatibility, but Perl changed, and so PCRE fol-
       lowed at release 8.34. Xwd matches the same characters as Xan, plus un-

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

   Resetting the match start

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


       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 de-
       fined". In PCRE, \K is acted upon when it occurs inside positive asser-
       tions,  but is ignored in negative assertions. Note that when a pattern
       such as (?=ab\K) matches, the  reported  start  of  the  match  can  be
       greater than the end of the match.

   Simple assertions

       The  final use of backslash is for certain simple assertions. An asser-
       tion 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 char-
       acter (for example, \B matches the letter B). However, if the  PCRE_EX-
       TRA  option is set, an "invalid escape sequence" error is generated in-

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string  if  the  first or last character matches \w, respectively. In a
       UTF mode, the meanings of \w and \W  can  be  changed  by  setting  the
       PCRE_UCP  option. When this is done, it also affects \b and \B. Neither
       PCRE nor Perl has a separate "start of word" or "end of  word"  metase-
       quence.  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 asser-
       tions 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,  indi-
       cating 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 argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

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

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


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

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment 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  sub-
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

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

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

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

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

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


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

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code 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 ex-
       ception.  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  circum-
       flex  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  af-
       fected  by the PCRE_DOTALL option. In other words, it matches any char-
       acter except one that signifies the end of a line. Perl also uses \N to
       match characters by name; PCRE does not support this.


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

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

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

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

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


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

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

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

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

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

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when matching character classes, whatever line-ending se-
       quence is in use, and whatever setting of the PCRE_DOTALL and PCRE_MUL-
       TILINE  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  charac-
       ters  in  a  character class. For example, [d-m] matches any letter be-
       tween d and m, inclusive. If a minus character is required in a  class,
       it  must  be  escaped with a backslash or appear in a position where it
       cannot be interpreted as indicating a range, typically as the first  or
       last character in the class, or immediately after a range. For example,
       [b-d-z] matches letters in the range b to d, a hyphen character, or z.

       It is not possible to have the literal character "]" as the end charac-
       ter  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  inter-
       preted  as a class containing a range followed by two other characters.
       The octal or hexadecimal representation of "]" can also be used to  end
       a range.

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

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

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

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

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

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name, or for  a
       special  compatibility  feature  -  see the next two sections), and the
       terminating closing square bracket.  However,  escaping  other  non-al-
       phanumeric 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 (the same as \s from PCRE 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

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

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


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

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

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

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

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

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

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

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

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


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

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

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


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


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


       The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
       PCRE_EXTENDED  options  (which are Perl-compatible) can be changed from
       within the pattern by a sequence of Perl option  letters  enclosed  be-
       tween "(?" 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 possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-
       LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,
       is  also  permitted.  If a letter appears both before and after the hy-
       phen, the option is unset.

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

       When one of these option changes occurs at top level (that is, not  in-
       side  subpattern  parentheses),  the change applies to the remainder of
       the pattern that follows. An option change within a subpattern (see be-
       low  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


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


       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  the matching function. (This applies only to the
       traditional matching functions; the DFA matching functions do not  sup-
       port capturing.)

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

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

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil  two  functions  is  not  always
       helpful.   There are often times when a grouping subpattern is required
       without a capturing requirement. If an opening parenthesis is  followed
       by  a question mark and a colon, the subpattern does not do any captur-
       ing, 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  be-
       tween the "?" and the ":". Thus the two patterns


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


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


       Because the two alternatives are inside a (?| group, both sets of  cap-
       turing  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, paren-
       theses  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 fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

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

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


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


       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  num-
       ber 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 expres-
       sions. Furthermore, if an  expression  is  modified,  the  numbers  may
       change.  To help with this difficulty, PCRE supports the naming of sub-
       patterns. 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  syn-
       tax.  Perl  allows  identically  numbered subpatterns to have different
       names, but PCRE does not.

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

       Names consist of up to 32 alphanumeric characters and underscores,  but
       must  start with a non-digit. Named capturing parentheses are still al-
       located 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.)  Dupli-
       cate  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 subpatterns to which the name refers are
       checked in the order in which they appear in the overall  pattern.  The
       first one that is set is used for the reference. For example, this pat-
       tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":


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

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

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


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

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

       The  general repetition quantifier specifies a minimum and maximum num-
       ber 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  exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

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

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful 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-charac-
       ter 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 bro-

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


       to the string

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

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

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


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


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

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

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

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  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  new-
       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

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


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

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


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

       When a capturing subpattern is repeated, the value captured is the sub-
       string 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  itera-
       tions. For example, after


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


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

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


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

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


       This  kind  of  parenthesis "locks up" the  part of the pattern it con-
       tains 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  pre-
       pared  to  adjust  the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of

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


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


       Possessive  quantifiers  are always greedy; the setting of the PCRE_UN-
       GREEDY option is ignored. They are a convenient notation for  the  sim-
       pler  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  syn-
       tax.   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 sim-
       ple  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  ex-
       ample 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 sub-
       pattern earlier (that is, to its left) in the pattern,  provided  there
       have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it is always taken as a back reference, and causes  an  error  only  if
       there  are  not that many capturing left parentheses in the entire pat-
       tern. 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 itera-

       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 se-
       quence such as \50 is interpreted as a character defined in octal.  See
       the subsection entitled "Non-printing characters" above for further de-
       tails of the handling of digits following a backslash. There is no such
       problem  when  named parentheses are used. A back reference to any sub-
       pattern 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 ambigu-
       ity 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 captur-
       ing subpattern before \g, that is, is it equivalent to \2 in this exam-
       ple.  Similarly, \g{-2} would be equivalent to \1. The use of  relative
       references  can  be helpful in long patterns, and also in patterns that
       are created by  joining  together  fragments  that  contain  references
       within themselves.

       A  back  reference matches whatever actually matched the capturing sub-
       pattern 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  exam-


       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 ex-
       ample in any of the following ways:


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

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


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

       Because there may be many capturing parentheses in a pattern, all  dig-
       its  following a backslash are taken as part of a potential back refer-
       ence number.  If the pattern continues with a digit character, some de-
       limiter  must  be used to terminate the back reference. If the PCRE_EX-
       TENDED option is set, this can be white space. Otherwise, the \g{  syn-
       tax 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 sub-
       patterns. For example, the pattern


       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation  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  de-
       scribed above.

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

       Assertion  subpatterns are not capturing subpatterns. If such an asser-
       tion contains capturing subpatterns within it, these  are  counted  for
       the  purposes  of numbering the capturing subpatterns in the whole pat-
       tern. However, substring capturing is carried out only for positive as-
       sertions.  (Perl  sometimes, but not always, does do capturing in nega-
       tive assertions.)

       WARNING: If a positive assertion containing one or more capturing  sub-
       patterns  succeeds,  but  failure  to match later in the pattern causes
       backtracking over this assertion, the captures within the assertion are
       reset only if no higher numbered captures are already set. This is, un-
       fortunately, a fundamental limitation of  the  current  implementation,
       and  as PCRE1 is now in maintenance-only status, it is unlikely ever to

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

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

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

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

   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 semi-
       colon in the match, and


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


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

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way to do it is with (?!) because an empty string al-
       ways 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 sev-
       eral 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 cur-
       rent position, the assertion fails.

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

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

       Possessive quantifiers can be used in conjunction with  lookbehind  as-
       sertions  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" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". 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  con-
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a specific capturing  subpat-
       tern  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 alterna-
       tives in the subpattern, a compile-time error occurs. Each of  the  two
       alternatives may itself contain nested subpatterns of any form, includ-
       ing conditional subpatterns; the restriction to  two  alternatives  ap-
       plies  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,  refer-
       ences 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 pre-
       viously  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 alter-
       native notation is to precede the digits with a plus or minus sign.  In
       this  case, the subpattern number is relative rather than absolute. The
       most recently opened parentheses can be referenced by (?(-1), the  next
       most  recent  by (?(-2), and so on. Inside loops it can also make sense
       to refer to subsequent groups. The next parentheses to be opened can be
       referenced  as (?(+1), and so on. (The value zero in any of these forms
       is not used; it provokes a compile-time error.)

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

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

       The first part matches an optional opening  parenthesis,  and  if  that
       character is present, sets it as the first captured substring. The sec-
       ond 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.  Other-
       wise,  since no-pattern is not present, the subpattern matches nothing.
       In other words, this pattern matches a sequence of non-parentheses, op-
       tionally enclosed in parentheses.

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

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

       This makes the fragment independent of the parentheses  in  the  larger

   Checking for a used subpattern by name

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

       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 amper-
       sand 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 du-
       plicate, 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 DE-
       FINE is that it can be used to define subroutines that  can  be  refer-
       enced  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 white space and line breaks):

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

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

   Assertion conditions

       If  the condition is not in any of the above formats, it must be an as-
       sertion.  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  op-
       tional 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 let-
       ter  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 char-
       acter class, nor in the middle of any other sequence of related charac-
       ters  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  charac-
       ters are interpreted as newlines is controlled by the options passed to
       a compiling function or by a special sequence at the start of the  pat-
       tern, 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

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

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

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

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

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

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

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

       First  it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a sequence of non-parentheses, or a  re-
       cursive 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 refer-
       enced. They are always non-recursive subroutine calls, as described  in
       the next section.

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

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

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

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


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

       At  the  end  of a match, the values of capturing parentheses are those
       from the outermost level. If you want to obtain intermediate values,  a
       callout  function can be used (see below and the pcrecallout documenta-
       tion). 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 sub-
       pattern is not matched at the top level, its final  captured  value  is
       unset,  even  if  it was (temporarily) set at a deeper level during the
       matching process.

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

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

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

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

   Differences in recursion processing between PCRE and Perl

       Recursion  processing  in PCRE differs from Perl in two important ways.
       In PCRE (like Python, but unlike Perl), a recursive subpattern call  is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives  and  there  is a subsequent matching failure. This can be
       illustrated by the following pattern, which purports to match a  palin-
       dromic  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 alterna-
       tive is taken and the recursion kicks in. The recursive call to subpat-
       tern 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-en-
       ter the recursion and try the second alternative.) However, if the pat-
       tern is written with the alternatives in the other  order,  things  are


       This  time,  the recursing alternative is tried first, and continues to
       recurse until it runs out of characters, at which point  the  recursion
       fails.  But  this  time  we  do  have another alternative to try at the
       higher level. That is the big difference: in the previous case the  re-
       maining  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 alter-
       natives 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 backtrack-
       ing 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  sub-
       ject  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 alter-
       natives, so the entire match fails.

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


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


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


       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

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

       Processing options such as case-independence are fixed when  a  subpat-
       tern  is defined, so if it is used as a subroutine, such options 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,  rewrit-
       ten using this syntax:

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

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


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


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

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

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


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


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

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

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


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

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

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

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

   Optimizations that affect backtracking verbs

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

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

   Verbs that act immediately

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


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

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


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

         (*FAIL) or (*F)

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


       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 ar-
       rived at, though it also has a secondary use in  conjunction  with  ad-
       vancing the match starting point (see (*SKIP) below).

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

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

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

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

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

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

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

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

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

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

   Verbs that act after backtracking

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

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


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

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

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

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

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

         (*PRUNE) or (*PRUNE:NAME)

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

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


       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 encoun-
       tered. (*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 quan-
       tifier does not have the same effect as this example; although it would
       suppress  backtracking  during  the first match attempt, the second at-
       tempt would start at the second character instead  of  skipping  on  to


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

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

         (*THEN) or (*THEN:NAME)

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

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

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

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

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

         A (B(*THEN)C) | D

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

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

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

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

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

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

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

   More than one backtracking verb

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


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


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

   Backtracking verbs in repeated groups

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


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

   Backtracking verbs in assertions

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

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

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

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

   Backtracking verbs in subroutines

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

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

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

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

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


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


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


       Last updated: 23 October 2016
       Copyright (c) 1997-2016 University of Cambridge.

PCRE 8.40                       23 October 2016                 PCREPATTERN(3)
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