pcrepattern


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  syn-
       tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular  expression  syntax (which does not conflict with the Perl syn-
       tax) in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

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

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

         (*UTF8)
         (*UTF16)

       Starting a pattern with such a sequence is equivalent  to  setting  the
       relevant option. This feature is not Perl-compatible. How setting a UTF
       mode affects pattern matching is mentioned  in  several  places  below.
       There is also a summary of features in the pcreunicode page.

       Another  special  sequence that may appear at the start of a pattern or
       in combination with (*UTF8) or (*UTF16) is:

         (*UCP)

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

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

       The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE  when  one  its  main  matching functions, pcre_exec()
       (8-bit) or pcre16_exec() (16-bit), is used. PCRE also  has  alternative
       matching  functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
       using a different algorithm that is not Perl-compatible.  Some  of  the
       in the options arguments for the compiling and matching functions.

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

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

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

         (*CR)a.b

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

       The newline convention affects the interpretation of the dot  metachar-
       acter  when  PCRE_DOTALL is not set, and also the behaviour of \N. How-
       ever, it does not affect  what  the  \R  escape  sequence  matches.  By
       default,  this is any Unicode newline sequence, for Perl compatibility.
       However, this can be changed; see the description of \R in the  section
       entitled  "Newline sequences" below. A change of \R setting can be com-
       bined 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 a UTF mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is always possible. For characters with higher val-
       ues, the concept of case is supported if PCRE is compiled with  Unicode
       property  support,  but  not  otherwise.   If  you want to use caseless
       matching for characters 128 and above, you must  ensure  that  PCRE  is
       compiled with Unicode property support as well as with UTF support.

       The  power  of  regular  expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.
         (      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 character that is not a number or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

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

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

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

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

         Pattern            PCRE matches   Perl matches

   Non-printing characters

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

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

       The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
       is  7B),  while  \c; becomes hex 7B (; is 3B). If the byte following \c
       has a value greater than 127, a compile-time error occurs.  This  locks
       out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
       mode, all byte values are valid. A lower case letter  is  converted  to
       upper case, and then the 0xc0 bits are flipped.)

       By  default,  after  \x,  from  zero to two hexadecimal digits are read
       (letters can be in upper or lower case). Any number of hexadecimal dig-
       its may appear between \x{ and }, but the character code is constrained
       as follows:

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

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

       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.

       If  the  PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
       is as just described only when it is followed by two  hexadecimal  dig-
       its.   Otherwise,  it  matches  a  literal "x" character. In JavaScript
       mode, support for code points greater than 256 is provided by \u, which
       must  be  followed  by  four hexadecimal digits; otherwise it matches a
       (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 compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its  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 gen-
       erate a data character. Any subsequent digits stand for themselves. The
       value  of  the  character  is constrained in the same way as characters
       specified in hexadecimal.  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 value 255 (decimal)
         \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, \b is interpreted as the backspace character (hex 08).

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

   Unsupported escape sequences

       In  Perl, the sequences \l, \L, \u, and \U are recognized by its string
       handler and used  to  modify  the  case  of  following  characters.  By
       default,  PCRE does not support these escape sequences. However, if the
       PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U"  character,  and
       \u can be used to define a character by code point, as described in the
       previous section.
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

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

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

       Each  pair of lower and upper case escape sequences partitions the com-
       plete set of characters into two disjoint  sets.  Any  given  character
       matches  one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character  of
       the  appropriate  type.  If the current matching point is at the end of
       the subject string, all of them fail, because there is no character  to
       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 charac-
       ter. In PCRE, it never does.

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

       By default, in a UTF mode, characters  with  values  greater  than  128
       never  match  \d,  \s,  or  \w,  and always match \D, \S, and \W. These
       sequences retain their original meanings from before  UTF  support  was
       available,  mainly for efficiency reasons. However, if PCRE is compiled
       with Unicode property support, and the PCRE_UCP option is set, the  be-
       haviour  is  changed  so  that Unicode properties are used to determine
       The sequences \h, \H, \v, and \V are features that were added  to  Perl
       at  release  5.10. In contrast to the other sequences, which match only
       ASCII characters by default, these  always  match  certain  high-valued
       codepoints,  whether or not PCRE_UCP is set. The horizontal space char-
       acters 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     Form feed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

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

   Newline sequences

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

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

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

       following sequences:

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

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

         (*ANY)(*BSR_ANYCRLF)

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

   Unicode character properties

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

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

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

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

         \p{Greek}
         \P{Han}

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

       Arabic,  Armenian,  Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
       Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian,  Chakma,
       Cham,  Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
       Devanagari,  Egyptian_Hieroglyphs,  Ethiopic,   Georgian,   Glagolitic,
       Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

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

         \p{L}
         \pL

       The following general category property codes are supported:

         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

       so  cannot  be  tested  by  PCRE, unless UTF validity checking has been
       turned   off   (see   the   discussion   of   PCRE_NO_UTF8_CHECK    and
       PCRE_NO_UTF16_CHECK  in the pcreapi page). Perl does not support the Cs
       property.

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

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

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

       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 8-bit non-UTF-8  mode
       \X matches any one character.

       Note that recent versions of Perl have changed \X to match what Unicode
       calls an "extended grapheme cluster", which has a more complicated def-
       inition.

       Matching  characters  by Unicode property is not fast, because PCRE has
       to search a structure that contains  data  for  over  fifteen  thousand
       characters. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE by  default,  though  you  can
       make  them do so by setting the PCRE_UCP option or by starting the pat-
       tern with (*UCP).

   PCRE's additional properties

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

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

       Xan matches characters that have either the L (letter) or the  N  (num-
       ber)  property. Xps matches the characters tab, linefeed, vertical tab,
       is similar to a lookbehind assertion (described  below).   However,  in
       this  case, the part of the subject before the real match does not have
       to be of fixed length, as lookbehind assertions do. The use of \K  does
       not  interfere  with  the setting of captured substrings.  For example,
       when the pattern

         (foo)\Kbar

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

       Perl documents that the use  of  \K  within  assertions  is  "not  well
       defined".  In  PCRE,  \K  is  acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions.

   Simple assertions

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

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

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

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

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect only the behaviour of the circumflex and dollar  metacharacters.
       However,  if the startoffset argument of pcre_exec() is non-zero, indi-
       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  argu-
       ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
       PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
       has an entirely different meaning (see below).

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

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

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

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

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

MATCHING A SINGLE DATA UNIT

       Outside  a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. In the 8-bit library, one  data
       unit  is  one byte; in the 16-bit library it is a 16-bit unit. Unlike a
       dot, \C always matches line-ending characters. The feature is  provided
       in  Perl  in  order  to match individual bytes in UTF-8 mode, but it is
       unclear how it can usefully be used. Because \C  breaks  up  characters
       into  individual  data  units,  matching one unit with \C in a UTF mode
       means that the rest of the string may start with a malformed UTF  char-
       acter.  This  has  undefined  results,  because PCRE assumes that it is
       dealing with valid UTF strings (and by default it checks  this  at  the
       start     of    processing    unless    the    PCRE_NO_UTF8_CHECK    or
       PCRE_NO_UTF16_CHECK option is used).

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

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

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A group that starts with (?| resets the capturing  parentheses  numbers
       in  each  alternative  (see  "Duplicate Subpattern Numbers" below). The
       assertions at the start of each branch check the next  UTF-8  character
       present) or escaped with a backslash.

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

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

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

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

       Characters  that  might  indicate  line breaks are never treated in any
       special way  when  matching  character  classes,  whatever  line-ending
       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 charac-
       ters 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 charac-
       ter  of a range. A pattern such as [W-]46] is interpreted as a class of
       two characters ("W" and "-") followed by a literal string "46]", so  it
       would  match  "W46]"  or  "-46]". However, if the "]" is escaped with a
       backslash it is interpreted as the end of range, so [W-\]46] is  inter-
       preted  as a class containing a range followed by two other characters.
       The octal or hexadecimal representation of "]" can also be used to  end
       concept  of  case for characters with values greater than 128 only when
       it is compiled with Unicode property support.

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

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

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

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

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

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

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

VERTICAL BAR

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

         gilbert|sullivan

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

INTERNAL OPTION SETTING

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

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

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
       LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,

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

         (a(?i)b)c

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

         (a(?i)b|c)

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

       Note: There are other PCRE-specific options that  can  be  set  by  the
       application  when  the  compiling  or matching functions are called. In
       some cases the pattern can contain special leading  sequences  such  as
       (*CRLF)  to  override  what  the  application  has set or what has been
       defaulted.  Details  are  given  in  the  section   entitled   "Newline
       sequences"  above.  There  are  also  the (*UTF8), (*UTF16), and (*UCP)
       leading sequences that can be used to  set  UTF  and  Unicode  property
       modes;  they  are  equivalent to setting the PCRE_UTF8, PCRE_UTF16, and
       the PCRE_UCP options, respectively.

SUBPATTERNS

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

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

         cat(aract|erpillar|)

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

       2. It sets up the subpattern as  a  capturing  subpattern.  This  means
       that,  when  the  whole  pattern  matches,  that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       ovector  argument  of  the matching function. (This applies only to the
       traditional matching functions; the DFA matching functions do not  sup-
       port capturing.)

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

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

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

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

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

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

DUPLICATE SUBPATTERN NUMBERS

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

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

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

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

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

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

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

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

       change.  To help with this difficulty, PCRE supports the naming of sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had  the  feature earlier, and PCRE introduced it at release 4.0, using
       the Python syntax. PCRE now supports both the Perl and the Python  syn-
       tax.  Perl  allows  identically  numbered subpatterns to have different
       names, but PCRE does not.

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

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

       By  default, a name must be unique within a pattern, but it is possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time.  (Duplicate  names are also always permitted for subpatterns with
       the same number, set up as described in the previous  section.)  Dupli-
       cate  names  can  be useful for patterns where only one instance of the
       named parentheses can match. Suppose you want to match the  name  of  a
       weekday,  either as a 3-letter abbreviation or as the full name, and in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

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

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

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

       If you make a back reference to  a  non-unique  named  subpattern  from
       elsewhere  in the pattern, the one that corresponds to the first occur-
       rence of the name is used. In the absence of duplicate numbers (see the
       previous  section) this is the one with the lowest number. If you use a
       named reference in a condition test (see the section  about  conditions
       below),  either  to check whether a subpattern has matched, or to check
       for recursion, all subpatterns with the same name are  tested.  If  the
       condition  is  true for any one of them, the overall condition is true.
       This is the same behaviour as testing by number. For further details of
       Repetition  is  specified  by  quantifiers, which can follow any of the
       following items:

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

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

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

       In UTF modes, quantifiers apply to characters rather than to individual
       data  units. Thus, for example, \x{100}{2} matches two characters, each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly,  \X{3}  matches  three Unicode extended sequences, each of which
       may be several data units 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 use-
       ful for subpatterns that are referenced as subroutines  from  elsewhere
       in the pattern (but see also the section entitled "Defining subpatterns
       for use by reference only" below). Items other  than  subpatterns  that
       have a {0} quantifier are omitted from the compiled pattern.

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

       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

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

         /\*.*\*/

       to the string

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

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

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

         /\*.*?\*/

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

         \d??\d

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

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

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

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

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

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

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

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

         /(a|(b))+/

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

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

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

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       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  con-
       tains  once  it  has matched, and a failure further into the pattern is
       prevented from backtracking into it. Backtracking past it  to  previous
       items, however, works as normal.

       An  alternative  description  is that a subpattern of this type matches
       the string of characters that an  identical  standalone  pattern  would
       match, if anchored at the current point in the subject string.
       consists of an additional + character  following  a  quantifier.  Using
       this notation, the previous example can be rewritten as

         \d++foo

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

         (abc|xyz){2,3}+

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

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

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

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

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

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

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

       sequences of non-digits cannot be broken, and failure happens quickly.
       to the left of the reference for numbers less than 10. A "forward  back
       reference"  of  this  type can make sense when a repetition is involved
       and the subpattern to the right has participated in an  earlier  itera-
       tion.

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

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

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

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

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

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

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

         (sens|respons)e and \1ibility

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

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

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

       There  are  several  different ways of writing back references to named
       before or after the reference.

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

         (a|(bc))\2

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

       Because  there may be many capturing parentheses in a pattern, all dig-
       its following a backslash are taken as part of a potential back  refer-
       ence  number.   If  the  pattern continues with a digit character, some
       delimiter must  be  used  to  terminate  the  back  reference.  If  the
       PCRE_EXTENDED  option  is  set, this can be white space. Otherwise, the
       \g{ syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

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

         (a|b\1)+

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

       Back references of this type cause the group that they reference to  be
       treated  as  an atomic group.  Once the whole group has been matched, a
       subsequent matching failure cannot cause backtracking into  the  middle
       of the group.

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. If such an  asser-
       groups that are called from elsewhere via the subroutine mechanism.

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

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

   Lookahead assertions

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

         \w+(?=;)

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

         foo(?!bar)

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

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

   Lookbehind assertions

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

         (?<!foo)bar

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

         (?<=bullock|donkey)

       different lengths, but it is acceptable to PCRE if rewritten to use two
       top-level branches:

         (?<=abc|abde)

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

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

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

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

       Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. 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,


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

       Assertions can be nested in any combination. For example,

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

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

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

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

CONDITIONAL SUBPATTERNS

       It is possible to cause the matching process to obey a subpattern  con-
       ditionally  or to choose between two alternative subpatterns, depending
       on the result of an assertion, or whether a specific capturing  subpat-
       tern  has  already  been matched. The two possible forms of conditional
       subpattern are:

         (?(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 alterna-
       tives in the subpattern, a compile-time error occurs. Each of  the  two
       alternatives may itself contain nested subpatterns of any form, includ-
       ing  conditional  subpatterns;  the  restriction  to  two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

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


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

   Checking for a used subpattern by number

       If  the  text between the parentheses consists of a sequence of digits,
       the condition is true if a capturing subpattern of that number has pre-
       viously  matched.  If  there is more than one capturing subpattern with
       the same number (see the earlier  section  about  duplicate  subpattern
       numbers),  the condition is true if any of them have matched. An alter-
       native notation is to precede the digits with a plus or minus sign.  In
       this  case, the subpattern number is relative rather than absolute. The
       most recently opened parentheses can be referenced by (?(-1), the  next
       most  recent  by (?(-2), and so on. Inside loops it can also make sense
       to refer to subsequent groups. The next parentheses to be opened can be
       third  part  is  a conditional subpattern that tests whether or not the
       first set of parentheses matched. If they  did,  that  is,  if  subject
       started  with an opening parenthesis, the condition is true, and so the
       yes-pattern is executed and a closing parenthesis is  required.  Other-
       wise,  since no-pattern is not present, the subpattern matches nothing.
       In other words, this pattern matches  a  sequence  of  non-parentheses,
       optionally enclosed in parentheses.

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

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

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

   Checking for a used subpattern by name

       Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a
       used subpattern by name. For compatibility  with  earlier  versions  of
       PCRE,  which  had this facility before Perl, the syntax (?(name)...) is
       also recognized. However, there is a possible ambiguity with this  syn-
       tax,  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 num-
       ber, which must be greater than zero. Using subpattern names that  con-
       sist entirely of digits is not recommended.

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

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

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

   Checking for pattern recursion

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

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

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

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

         \b (?&byte) (\.(?&byte)){3} \b

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

   Assertion conditions

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

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

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

COMMENTS

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

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

         abc #comment \n still comment

       On encountering the # character, pcre_compile()  skips  along,  looking
       for  a newline in the pattern. The sequence \n is still literal at this
       stage, so it does not terminate the comment. Only an  actual  character
       Perl  code in the expression at run time, and the code can refer to the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

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

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

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

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

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

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

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

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

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

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

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

       It is also possible to refer to  subsequently  opened  parentheses,  by
       writing  references  such  as (?+2). However, these cannot be recursive
       because the reference is not inside the  parentheses  that  are  refer-
       enced.  They are always non-recursive subroutine calls, as described in
       the next section.
       matching strings of non-parentheses is important when applying the pat-
       tern to strings that do not match. For example, when  this  pattern  is
       applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

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

         (ab(cd)ef)

       the  value  for  the  inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing  sub-
       pattern  is  not  matched at the top level, its final captured value is
       unset, even if it was (temporarily) set at a deeper  level  during  the
       matching process.

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

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

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

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

   Differences in recursion processing between PCRE and Perl

       Recursion processing in PCRE differs from Perl in two  important  ways.
       In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.  This  can  be
       illustrated  by the following pattern, which purports to match a palin-
       dromic string that contains an odd number of characters  (for  example,
       "a", "aba", "abcba", "abcdcba"):

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

       subpattern  2 matched, which was "a". This fails. Because the recursion
       is treated as an atomic group, there are now  no  backtracking  points,
       and  so  the  entire  match fails. (Perl is able, at this point, to re-
       enter the recursion and try the second alternative.)  However,  if  the
       pattern is written with the alternatives in the other order, things are
       different:

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

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

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

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

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

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

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

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases such
       as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
       Perl. Note the use of the possessive quantifier *+ to avoid  backtrack-
       ing  into  sequences of non-word characters. Without this, PCRE takes a
       great deal longer (ten times or more) to  match  typical  phrases,  and
       Perl takes so long that you think it has gone into a loop.

       WARNING:  The  palindrome-matching patterns above work only if the sub-
       ject string does not start with a palindrome that is shorter  than  the
       entire  string.  For example, although "abcba" is correctly matched, if
       the subject is "ababa", PCRE finds the palindrome "aba" at  the  start,
       then  fails at top level because the end of the string does not follow.
       Once again, it cannot jump back into the recursion to try other  alter-
       natives, so the entire match fails.

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

SUBPATTERNS AS SUBROUTINES

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

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

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

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

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

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

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

       Processing  options  such as case-independence are fixed when a subpat-
       tern is defined, so if it is used as a subroutine, such options  cannot
       be changed for different calls. For example, consider this pattern:

         (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, rewrit-
       ten using this syntax:

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

       PCRE supports an extension to Oniguruma: if a number is preceded  by  a
       plus or a minus sign it is taken as a relative reference. For example:
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout (8-bit library) or  pcre16_callout  (16-bit  library).  By
       default, this variable contains NULL, which disables all calling out.

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

         (?C1)abc(?C2)def

       If  the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call-
       outs are automatically installed before each item in the pattern.  They
       are all numbered 255.

       During  matching, when PCRE reaches a callout point, the external func-
       tion 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 the matching function. 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 sub-
       ject  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 one of
       the traditional matching functions, which use a backtracking algorithm.
       With  the  exception  of (*FAIL), which behaves like a failing negative
       assertion, they cause an error if encountered by a DFA  matching  func-
       tion.

       If  any of these verbs are used in an assertion or in a subpattern that
       is called as a subroutine (whether or not recursively), their effect is
       confined to that subpattern; it does not extend to the surrounding pat-
       tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
       that  is  encountered in a successful positive assertion is passed back
       when a match succeeds (compare capturing  parentheses  in  assertions).
       Note that such subpatterns are processed as anchored at the point where
       they are tested. Note also that Perl's  treatment  of  subroutines  and
       assertions is different in some cases.

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

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

   Verbs that act immediately

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

          (*ACCEPT)

       This verb causes the match to end successfully, skipping the  remainder
       of  the pattern. However, when it is inside a subpattern that is called
       as a subroutine, only that subpattern is ended  successfully.  Matching
       then  continues  at  the  outer level. If (*ACCEPT) is inside capturing
       parentheses, the data so far is captured. For example:

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

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

         (*FAIL) or (*F)

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

         a+(?C)(*FAIL)

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

   Recording which path was taken

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

         (*MARK:NAME) or (*:NAME)
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

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

       If (*MARK) is encountered in a positive assertion, its name is recorded
       and passed back if it is the last-encountered. This does not happen for
       negative assertions.

       After a partial match or a failed match, the name of the  last  encoun-
       tered (*MARK) in the entire match process is returned. For example:

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

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

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

   Verbs that act after backtracking

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

       These verbs differ in exactly what kind of failure  occurs  when  back-
       tracking reaches them.

         (*COMMIT)

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

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

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

         (*PRUNE) or (*PRUNE:NAME)

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

         (*SKIP)

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

         a+(*SKIP)b

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

         (*SKIP:NAME)

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

       into COND1. The behaviour  of  (*THEN:NAME)  is  exactly  the  same  as
       (*MARK:NAME)(*THEN).   If (*THEN) is not inside an alternation, it acts
       like (*PRUNE).

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

         A (B(*THEN)C) | D

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

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

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

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

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

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

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

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

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

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

REVISION

       Last updated: 17 June 2012
       Copyright (c) 1997-2012 University of Cambridge.



PCRE 8.31                         04 May 2012                   PCREPATTERN(3)
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