pcrepattern


       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
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE's  regular  expressions  is
       intended as reference material.

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

SPECIAL START-OF-PATTERN ITEMS

       A number of options that can be passed to pcre_compile()  can  also  be
       set by special items at the start of a pattern. These are not Perl-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
       together right at the start of the pattern string, and the letters must
       be in upper case.

   UTF support

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there  is  now also support for UTF-8 strings in the original
       library, an extra library that supports  16-bit  and  UTF-16  character
       strings,  and a third library that supports 32-bit and UTF-32 character
       strings. To use these features, PCRE must be built to include 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:

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

       (*UTF)  is  a  generic  sequence  that  can  be  used  with  any of the
       libraries.  Starting a pattern with such a sequence  is  equivalent  to
       (*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
       sequence, the pattern

         (*CR)a.b

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

       The  newline  convention affects where the circumflex and dollar 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-
       by items at the start of the pattern of the form

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

       where d is any number of decimal digits. However, the value of the 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.

EBCDIC CHARACTER CODES

       PCRE can be compiled to run in an environment that uses EBCDIC  as  its
       character code rather than ASCII or Unicode (typically a mainframe 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.

CHARACTERS AND METACHARACTERS

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

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE_CASELESS option), letters  are
       matched  independently  of case. In a UTF mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is always possible. For characters with higher 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
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are  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

       class". In a character class the only metacharacters are:

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

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

BACKSLASH

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

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a 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
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

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

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.   An  isolated \E that is not preceded by \Q is ignored. If \Q
       is not followed by \E later in the 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.
         \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@
       encodes 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
       sequence \0\x\015 specifies two binary zeros followed by a CR character
       (code value 13). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

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

       inside and outside character classes. In addition, inside  a  character
       class, \b is interpreted as the backspace character (hex 08).

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

   Unsupported escape sequences

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

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  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.

       (11),  FF  (12),  CR  (13),  and space (32), which are defined as white
       space in the "C" locale. This list may vary if locale-specific matching
       is  taking place. For example, in some locales the "non-breaking space"
       character (\xA0) is recognized as white space, and  in  others  the  VT
       character is not.

       A  "word"  character is an underscore or any character that is a letter
       or digit.  By default, the definition of letters  and  digits  is  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+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

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

   Newline sequences

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

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

       This is an example of an "atomic group", details  of  which  are  given
       below.  This particular group matches either the two-character sequence
       CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
       U+000A),  VT  (vertical  tab, U+000B), FF (form feed, U+000C), CR (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 abbrevation for "backslash R".) This can be made the default
       when PCRE is built; if this is the case, the  other  behaviour  can  be
       requested  via  the  PCRE_BSR_UNICODE  option.   It is also possible to
       specify these settings by starting a pattern string  with  one  of  the
       following sequences:

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

       These override the default and the options given to the compiling 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:

         (*ANY)(*BSR_ANYCRLF)

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

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

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

         \p{Greek}
         \P{Han}

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

       Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak,  Bengali,
       Bopomofo,  Brahmi,  Braille, Buginese, Buhid, Canadian_Aboriginal, 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,
       Imperial_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_Hieroglyphs,  Miao,  Modi, Mongolian, Mro, Myanmar, Nabataean,
       New_Tai_Lue,  Nko,  Ogham,  Ol_Chiki,  Old_Italic,   Old_North_Arabian,
       Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya,
       Pahawh_Hmong,    Palmyrene,    Pau_Cin_Hau,    Phags_Pa,    Phoenician,
       Psalter_Pahlavi,  Rejang,  Runic,  Samaritan, Saurashtra, Sharada, 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,
       Yi.

       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
       \P{Lu}.

       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:

         \p{L}
         \pL

       The following general category property codes are supported:

         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.
       (see below).  Up to and including release 8.31, PCRE  matched  an  ear-
       lier, simpler definition that was equivalent to

         (?>\PM\pM*)

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

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

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

       1. End at the end of the subject string.

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

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

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

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

       As  well  as the standard Unicode properties described above, PCRE sup-
       ports four more that make it possible  to  convert  traditional  escape
       sequences  such as \w and \s to use Unicode properties. PCRE uses these
       non-standard, non-Perl properties internally when PCRE_UCP is set. 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,
       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:

         foo\Kbar

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

         (foo)\Kbar

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

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

   Simple assertions

       The  final use of backslash is for certain simple assertions. An 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_EXTRA  option is set, an "invalid escape sequence" error is gener-
       ated instead.

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       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.

CIRCUMFLEX AND DOLLAR

       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
       before a newline at the end of the string (by default). Note,  however,
       that  it  does  not  actually match the newline. Dollar need not be the
       last character of the pattern if a number of alternatives are involved,
       but  it should be the last item in any branch in which it appears. Dol-
       lar has no special meaning in a character class.

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

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

FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one 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
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

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

MATCHING A SINGLE DATA UNIT

       Outside  a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. In the 8-bit library, one  data
       unit  is  one  byte;  in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a  dot,  \C  always  matches
       line-ending  characters.  The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode, but it is unclear how it can 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
       PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or  PCRE_NO_UTF32_CHECK  option
         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

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

SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not 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.

       example, [b-d-z] matches letters in the range b to d, a hyphen  charac-
       ter, 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
       escape  sequence other than one that defines a single character appears
       at a point where a range ending character  is  expected.  For  example,
       [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.

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

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

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

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

       The only metacharacters that are recognized in  character  classes  are
       backslash,  hyphen  (only  where  it can be interpreted as specifying a
       range), circumflex (only at the start), opening  square  bracket  (only
       when  it can be interpreted as introducing a POSIX class name, or for a
       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

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

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

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

         [12[:^digit:]]

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

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

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

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

       [: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.

COMPATIBILITY FEATURE FOR WORD BOUNDARIES

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

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

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

VERTICAL BAR

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

         gilbert|sullivan

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

INTERNAL OPTION SETTING

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

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

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       below for a description of subpatterns) affects only that part  of  the
       subpattern that follows it, so

         (a(?i)b)c

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

         (a(?i)b|c)

       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
       first branch is abandoned before the option setting.  This  is  because
       the  effects  of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

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

SUBPATTERNS

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

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

         cat(aract|erpillar|)

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

       2.  It  sets  up  the  subpattern as a capturing subpattern. This means
       that, when the whole pattern  matches,  that  portion  of  the  subject
       string that matched the subpattern is passed back to the caller via the
       ovector argument of the matching function. (This applies  only  to  the
       traditional  matching functions; the DFA matching functions do not 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 "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

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

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

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

DUPLICATE SUBPATTERN NUMBERS

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

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

       Because  the two alternatives are inside a (?| group, both sets of 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":

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

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

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

       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
       allocated  numbers  as  well as names, exactly as if the names were not
       present. The PCRE API provides function calls for extracting the  name-
       to-number  translation  table  from a compiled pattern. There is also a
       convenience function for extracting a captured substring by name.

       By default, a name must be unique within a pattern, but it is  possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time. (Duplicate names are also always permitted for  subpatterns  with
       the  same  number, set up as described in the previous section.) 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:

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

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

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

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

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


       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

       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:

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

       matches  exactly  8  digits. An opening curly bracket that appears in a
       position where a quantifier is not allowed, or one that does not  match
       the  syntax of a quantifier, is taken as a literal character. For 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
       lengths).
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

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

         (a?)*

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

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

         /\*.*\*/

       to the string

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

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

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

         /\*.*?\*/

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

         \d??\d

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

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


       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:

         (.*)abc\1

       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:

         (?>.*?a)b

       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

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

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

         /(a|(b))+/

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

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

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

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       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
       digits.

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

         \d++foo

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

         (abc|xyz){2,3}+

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

       The  possessive  quantifier syntax is an extension to the Perl 5.8 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

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

       when a single character is used. They remember the last single  charac-
       ter  that  is required for a match, and fail early if it is not present
       in the string.) If the pattern is changed so that  it  uses  an  atomic
       group, like this:

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

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

BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing 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-
       tion.

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

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

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

       An unsigned number specifies an absolute reference without the  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:

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

       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
       ple,

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

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

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

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

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

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

         (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back 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
       delimiter must  be  used  to  terminate  the  back  reference.  If  the
       PCRE_EXTENDED  option  is  set, this can be white space. Otherwise, the
       \g{ syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

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

         (a|b\1)+

       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.

       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
       assertions. (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,
       unfortunately,  a fundamental limitation of the current implementation,
       and as PCRE1 is now in maintenance-only status, it is unlikely ever  to
       change.

       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
       matching.

   Lookahead assertions

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

         \w+(?=;)

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

         foo(?!bar)

       matches  any  occurrence  of  "foo" that is not followed by "bar". Note
       that the apparently similar pattern
       is a synonym for (?!).

   Lookbehind assertions

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

         (?<!foo)bar

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

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

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

         (?<=ab(c|de))

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

         (?<=abc|abde)

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

       The implementation of lookbehind assertions is, for  each  alternative,
       to  temporarily  move the current position back by the fixed length and
       then try to match. If there are insufficient characters before the 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
       assertions to specify efficient matching of fixed-length strings at the
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so  on.  Once
       again  the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

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

   Using multiple assertions

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

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

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of  the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous  three
       characters  are  all  digits,  and  then there is a check that the same
       three characters are not "999".  This pattern does not match "foo" 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

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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

CONDITIONAL SUBPATTERNS

       It  is possible to cause the matching process to obey a subpattern 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:


       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,
       optionally enclosed in parentheses.

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

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

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

   Checking for a used subpattern by name

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

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

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )
         (?(R3)...) or (?(R&name)...)

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

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

   Defining subpatterns for use by reference only

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

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

       The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the  named  group
       to  match the four dot-separated components of an IPv4 address, 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
       assertion.   This may be a positive or negative lookahead or lookbehind
       assertion. Consider  this  pattern,  again  containing  non-significant
       white space, and with the two alternatives on the second line:

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

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

COMMENTS

       There are two ways of including comments in patterns that are processed
       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.

RECURSIVE PATTERNS

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

       For some time, Perl has provided a facility that allows regular 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
       recursive call of the entire regular expression.

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

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

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a  correctly  parenthe-
       sized substring).  Finally there is a closing parenthesis. Note the use
       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

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

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

         (ab(cd)ef)

       the  value  for  the  inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing  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-
       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"):

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

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

       At the top level, the first character is matched, but as it is  not  at
       the end of the string, the first alternative fails; the second 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-
       enter the recursion and try the second alternative.)  However,  if  the
       pattern is written with the alternatives in the other order, things are
       different:

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

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

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

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

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

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

       If  you  want  to match typical palindromic phrases, the pattern has to
       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
       pattern:

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

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

SUBPATTERNS AS SUBROUTINES

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

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

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

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

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

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

       All subroutine calls, whether recursive or not, are always  treated  as
       atomic  groups. That is, once a subroutine has matched some of the 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.
       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:

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

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

CALLOUTS

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

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

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

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to a compiling function,  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:

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

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

       During matching, when PCRE reaches a callout point, the external  func-

BACKTRACKING CONTROL

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

       The new verbs make use of what was previously invalid syntax: an  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
       pattern.

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

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

          (*ACCEPT)

       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-
       tern:

         a+(?C)(*FAIL)

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

   Recording which path was taken

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

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

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

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

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

       The (*MARK) name is tagged with "MK:" in this output, and in this 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
       assertions.

       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
       applies 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.

         (*COMMIT)

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

         a+(*COMMIT)b

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

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

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

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

       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
       (*COMMIT).

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

         (*SKIP)

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

         a+(*SKIP)b

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

         (*SKIP:NAME)

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

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

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost  alternative  when  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:

       remembered  for  passing  back  to  the  caller.  However, (*SKIP:NAME)
       searches only for names set with (*MARK).

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

         A (B(*THEN)C) | D

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

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

       The effect of (*THEN) is now confined to the inner subpattern. After  a
       failure in C, matching moves to (*FAIL), which causes the whole 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
       alternatives, because only one is ever used.  In  other  words,  the  |
       character in a conditional subpattern has a different meaning. Ignoring
       white space, consider:

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

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

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

   More than one backtracking verb

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

       causes  it to be triggered, and its action is taken. There can never be
       a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

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

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

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

   Backtracking verbs in assertions

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

       (*ACCEPT) in a positive assertion causes the assertion to succeed 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
       alternative  in  the  innermost  enclosing group that has alternations,
       whether or not this is within the assertion.

       Negative assertions are, however, different, in order  to  ensure  that
       changing  a  positive  assertion  into a negative assertion changes its
       result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg-
       ative 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.

REVISION

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

PCRE 8.40                       23 October 2016                 PCREPATTERN(3)
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