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NAME

       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS


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

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

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, you must build PCRE to  include  UTF-8  support,  and  then  call
       pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
       matching is mentioned in several places below. There is also a  summary
       of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
       page.

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

NEWLINE CONVENTIONS


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

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

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

       These override the default and the options given to pcre_compile(). 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. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start  of  a  pattern,
       and  that  they  must  be  in  upper  case. If more than one of them is
       present, the last one is used.

       The newline convention does not affect  what  the  \R  escape  sequence
       matches.  By  default,  this  is any Unicode newline sequence, for Perl
       compatibility. However, this can be changed; see the description of  \R
       in  the  section  entitled  "Newline  sequences"  below. A change of \R
       setting can be combined with a change of newline convention.

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 UTF-8 mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is  always  possible.  For  characters  with higher
       values, the concept of case is  supported  if  PCRE  is  compiled  with
       Unicode  property  support,  but  not  otherwise.   If  you want to use
       caseless matching for characters 128 and above, you  must  ensure  that
       PCRE  is  compiled  with Unicode property support as well as with UTF-8
       support.

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

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

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

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

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

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

BACKSLASH


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

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

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

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

         Pattern            PCRE matches   Perl matches

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

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

   Non-printing characters

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

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

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

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

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

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

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

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

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

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

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

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

   Absolute and relative back references

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

   Absolute and relative subroutine calls

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

   Generic character types

       Another use of backslash is for specifying generic character types. The
       following are always recognized:

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

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

       These character type sequences  can  appear  both  inside  and  outside
       character  classes.  They  each  match one character of the appropriate
       type. If the current matching point  is  at  the  end  of  the  subject
       string, all of them fail, since there is no character to match.

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

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

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

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

       The vertical space characters are:

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

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

   Newline sequences

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

         (?>\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 (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

       In  UTF-8  mode, two additional characters whose codepoints are greater
       than 255 are added: LS  (line  separator,  U+2028)  and  PS  (paragraph
       separator,  U+2029).   Unicode character property support is not needed
       for these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode  line  endings)  by  setting the option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when PCRE is built; if this is the case, the  other  behaviour  can  be
       requested  via  the  PCRE_BSR_UNICODE  option.   It is also possible to
       specify these settings by starting a pattern string  with  one  of  the
       following sequences:

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

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

         (*ANY)(*BSR_ANYCRLF)

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

   Unicode character properties

       When  PCRE  is  built  with  Unicode  character property support, three
       additional  escape  sequences  that  match  characters  with   specific
       properties  are available.  When not in UTF-8 mode, these sequences are
       of course limited to testing characters whose codepoints are less  than
       256, but they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

       The  property  names represented by xx above are limited to the Unicode
       script names, the general category properties, and "Any", which matches
       any   character   (including   newline).   Other   properties  such  as
       "InMusicalSymbols" are not  currently  supported  by  PCRE.  Note  that
       \P{Any}  does  not  match  any  characters,  so  always  causes a match
       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,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
       Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
       Gothic,  Greek,  Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,
       Hiragana,  Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
       Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.

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

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

         \p{L}
         \pL

       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

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

       The  Cs  (Surrogate)  property  applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see
       RFC  3629)  and  so  cannot  be  tested  by PCRE, unless UTF-8 validity
       checking has been turned off (see the discussion of  PCRE_NO_UTF8_CHECK
       in the pcreapi page).

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

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

       Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper case letters.

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

         (?>\PM\pM*)

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

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

   Resetting the match start

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

         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".

   Simple assertions

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

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

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

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

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

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

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

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

CIRCUMFLEX AND DOLLAR


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

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

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

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

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

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

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

FULL STOP (PERIOD, DOT)


       Outside  a  character  class,  a  dot  in  the  pattern matches any one
       character in the subject string except (by default)  a  character  that
       signifies  the  end of a line. In UTF-8 mode, the matched character may
       be more than one byte long.

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

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

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

MATCHING A SINGLE BYTE


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

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

SQUARE BRACKETS AND CHARACTER CLASSES


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

       A character class matches a single character in the subject.  In  UTF-8
       mode,  the character may occupy more than one byte. A matched character
       must be in the set of characters defined by the class, unless the first
       character  in  the  class definition is a circumflex, in which case the
       subject character must not be in the set defined by  the  class.  If  a
       circumflex  is actually required as a member of the class, ensure it is
       not the first character, or escape it with a backslash.

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

       In UTF-8 mode, characters with values greater than 255 can be  included
       in  a  class as a literal string of bytes, or by using the \x{ escaping
       mechanism.

       When caseless matching is set, any letters in a  class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported  if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching for characters 128 and above, you  must  ensure  that
       PCRE  is  compiled  with Unicode property support as well as with UTF-8
       support.

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

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

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

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

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

       The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
       in a character class, and add the characters that  they  match  to  the
       class.  For  example,  [\dABCDEF]  matches  any  hexadecimal  digit.  A
       circumflex can conveniently be used with the upper case character types
       to  specify a more restricted set of characters than the matching lower
       case type. For example, the class [^\W_] matches any letter  or  digit,
       but not underscore.

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

POSIX CHARACTER CLASSES


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

         [01[:alpha:]%]

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

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

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

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

         [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.

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

VERTICAL BAR


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

         gilbert|sullivan

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

INTERNAL OPTION SETTING


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

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

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

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

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

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

         (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  compile  or match functions are called. In some
       cases the pattern can contain special  leading  sequences  to  override
       what  the  application  has set or what has been defaulted. Details are
       given in the section entitled "Newline sequences" above.

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

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

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

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

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

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

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

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

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

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

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

DUPLICATE SUBPATTERN NUMBERS


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

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

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

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

       A  backreference  or  a  recursive call to a numbered subpattern always
       refers to the first one in the pattern with the given number.

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

NAMED SUBPATTERNS


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

       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
       backreferences,  recursion, and conditions, can be made by name as well
       as by number.

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

       By  default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time.  This  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  reference  to  a  non-unique  named
       subpattern  from  elsewhere in the pattern, the one that corresponds to
       the lowest number is used. For further details of  the  interfaces  for
       handling named subpatterns, see the pcreapi documentation.

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

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

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

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

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

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

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

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

         (a?)*

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

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

         /\*.*\*/

       to the string

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

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

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

         /\*.*?\*/

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

         \d??\d

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

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

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

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

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

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

         (.*)abc\1

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

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

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

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

         /(a|(b))+/

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS


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

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

         123456bar

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

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

         (?>\d+)foo

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

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

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

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

         \d++foo

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

         (abc|xyz){2,3}+

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

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

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

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

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

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

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

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

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

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

BACK REFERENCES


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

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

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

       Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g  escape  sequence,  which  is  a
       feature  introduced  in  Perl  5.10. This escape must be followed by an
       unsigned number or a negative number, optionally  enclosed  in  braces.
       These examples are all identical:

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

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

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

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

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

         (sens|respons)e and \1ibility

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

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

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

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

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

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

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

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". Because  there
       may  be  many  capturing parentheses in a pattern, all digits following
       the backslash are taken as part of a potential back  reference  number.
       If the pattern continues with a digit character, some delimiter must be
       used to terminate the back reference. If the  PCRE_EXTENDED  option  is
       set,   this  can  be  whitespace.   Otherwise  an  empty  comment  (see
       "Comments" below) can be used.

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

         (a|b\1)+

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

ASSERTIONS


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

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

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

   Lookahead assertions

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

         \w+(?=;)

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

         foo(?!bar)

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

         (?!foo)bar

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

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

   Lookbehind assertions

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

         (?<!foo)bar

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

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This  is  an  extension  compared  with  Perl (at least for 5.8), 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 if rewritten to  use  two  top-
       level branches:

         (?<=abc|abde)

       In some cases, the Perl 5.10 escape sequence \K (see above) can be used
       instead of a lookbehind assertion; this is not restricted to  a  fixed-
       length.

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

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

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

         abcd$

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

         ^.*abcd$

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

         ^.*+(?<=abcd)

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

   Using multiple assertions

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

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

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

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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

CONDITIONAL SUBPATTERNS


       It is possible to cause the  matching  process  to  obey  a  subpattern
       conditionally   or  to  choose  between  two  alternative  subpatterns,
       depending on  the  result  of  an  assertion,  or  whether  a  previous
       capturing  subpattern  matched  or  not.  The  two  possible  forms  of
       conditional subpattern are

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

       If the condition is satisfied, the yes-pattern is used;  otherwise  the
       no-pattern   (if   present)  is  used.  If  there  are  more  than  two
       alternatives in the subpattern, a compile-time error occurs.

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

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence  of  digits,
       the  condition  is  true if the capturing subpattern of that number has
       previously matched. An alternative notation is to  precede  the  digits
       with  a  plus  or  minus  sign.  In this case, the subpattern number is
       relative rather than absolute.  The most  recently  opened  parentheses
       can be referenced by (?(-1), the next most recent by (?(-2), and so on.
       In looping constructs it can also make sense  to  refer  to  subsequent
       groups with constructs such as (?(+2).

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

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

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

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

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

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

   Checking for a used subpattern by name

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

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

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

   Checking for pattern recursion

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

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

       the  condition  is  true  if  the  most  recent  recursion  is into the
       subpattern whose number or name is given. This condition does not check
       the entire recursion stack.

       At  "top  level",  all  these  recursion  test  conditions  are  false.
       Recursive patterns are described below.

   Defining subpatterns for use by reference only

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

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

       The  first part of the pattern is a DEFINE group inside which a another
       group named "byte" is defined. This matches an individual component  of
       an  IPv4  address  (a number less than 256). When matching takes place,
       this part of the pattern is skipped because DEFINE acts  like  a  false
       condition.

       The rest of the pattern uses references to the named group to match the
       four dot-separated components of an IPv4 address, insisting on  a  word
       boundary at each end.

   Assertion conditions

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

         (?(?=[^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


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

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

RECURSIVE PATTERNS


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if atomic grouping 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 set for any capturing subpatterns are
       those from the outermost level of the recursion at which the subpattern
       value  is  set.   If  you want to obtain intermediate values, a callout
       function can be used (see below and the pcrecallout documentation).  If
       the pattern above is matched against

         (ab(cd)ef)

       the  value  for  the  capturing  parentheses is "ef", which is the last
       value taken on at the top level. If additional parentheses  are  added,
       giving

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

       the  string  they  capture is "ab(cd)ef", the contents of the top level
       parentheses. If there are more  than  15  capturing  parentheses  in  a
       pattern,  PCRE  has  to  obtain  extra  memory  to  store data during a
       recursion, which it does by using pcre_malloc, freeing it via pcre_free
       afterwards.  If  no  memory  can  be obtained, the match fails with the
       PCRE_ERROR_NOMEMORY error.

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

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

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

SUBPATTERNS AS SUBROUTINES


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

         (...(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.

       Like recursive subpatterns, a "subroutine" call is always treated as an
       atomic group. That is, once it has matched some of the subject  string,
       it  is  never  re-entered, even if it contains untried alternatives and
       there is a subsequent matching failure.

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

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

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

ONIGURUMA SUBROUTINE SYNTAX


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

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

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

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

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

CALLOUTS


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

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an external function by putting its entry point in the global  variable
       pcre_callout.   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 pcre_compile(), callouts are
       automatically  installed  before each item in the pattern. They are all
       numbered 255.

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

BACKTRACKING CONTROL


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

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

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

   Verbs that act immediately

       The following verbs act as soon as they are encountered:

          (*ACCEPT)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. When inside a recursion, only the innermost pattern  is
       ended  immediately.  PCRE  differs  from  Perl  in  what happens if the
       (*ACCEPT) is inside capturing parentheses. In Perl, the data so far  is
       captured: in PCRE no data is captured. For example:

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

       This  matches  "AB", "AAD", or "ACD", but when it matches "AB", no data
       is captured.

         (*FAIL) or (*F)

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

         a+(?C)(*FAIL)

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

   Verbs that act after backtracking

       The following verbs do nothing  when  they  are  encountered.  Matching
       continues  with  what  follows,  but if there is no subsequent match, a
       failure is forced.  The verbs differ in exactly what  kind  of  failure
       occurs.

         (*COMMIT)

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

         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."

         (*PRUNE)

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

         (*SKIP)

       This verb is like (*PRUNE), except that if the pattern  is  unanchored,
       the  "bumpalong"  advance  is  not  to  the  next character, but to the
       position  in  the  subject  where  (*SKIP)  was  encountered.   (*SKIP)
       signifies  that  whatever  text  was matched leading up to it cannot be
       part of a successful match. Consider:

         a+(*SKIP)b

       If the subject is "aaaac...",  after  the  first  match  attempt  fails
       (starting  at  the  first  character in the string), the starting point
       skips on to start the next attempt  at  "c".  Note  that  a  possessive
       quantifer  does  not  have the same effect in 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".

         (*THEN)

       This verb causes a skip to the next alternation  if  the  rest  of  the
       pattern  does  not match. That is, it cancels pending backtracking, but
       only  within  the  current  alternation.  Its  name  comes   from   the
       observation that it can be used for a pattern-based if-then-else block:

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

       If the COND1 pattern matches, FOO is tried (and possibly further  items
       after  the  end  of  the group if FOO succeeds); on failure the matcher
       skips to the second alternative and tries COND2,  without  backtracking
       into  COND1.  If  (*THEN)  is  used outside of any alternation, it acts
       exactly like (*PRUNE).

SEE ALSO


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

AUTHOR


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

REVISION


       Last updated: 19 April 2008
       Copyright (c) 1997-2008 University of Cambridge.

                                                                PCREPATTERN(3)