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NAME

       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS


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

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

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

SPECIAL START-OF-PATTERN ITEMS


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

   UTF support

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

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

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

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

   Unicode property support

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

   Disabling auto-possessification

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

   Disabling start-up optimizations

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

   Newline conventions

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

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

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

       These override the default and  the  options  given  to  the  compiling
       function. 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
b" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

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

   Setting match and recursion limits

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

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

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

EBCDIC CHARACTER CODES


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

CHARACTERS AND METACHARACTERS


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

         The quick brown fox

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

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

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

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

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

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

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

BACKSLASH


       The backslash character has several uses. Firstly, if it is followed by
       a 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
       backslash, you write \.

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

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

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

         Pattern            PCRE matches   Perl matches

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

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

   Non-printing characters

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

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

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

       The \c facility was designed for use with ASCII  characters,  but  with
       the  extension  to  Unicode it is even less useful than it once was. It
       is, however, recognized when PCRE is compiled  in  EBCDIC  mode,  where
       data  items  are always bytes. In this mode, all values are valid after
       \c. If the next character is a lower case letter, it  is  converted  to
       upper  case.  Then  the  0xc0  bits  of the byte are inverted. Thus \cA
       becomes hex 01, as in ASCII (A is C1), but because the  EBCDIC  letters
       are  disjoint,  \cZ becomes hex 29 (Z is E9), and other characters also
       generate different values.

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

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

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

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

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

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

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

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

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

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

   Constraints on character values

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

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

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

   Escape sequences in character classes

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

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

   Unsupported escape sequences

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

   Absolute and relative back references

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

   Absolute and relative subroutine calls

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

   Generic character types

       Another use of backslash is for specifying generic character types:

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

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

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

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

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

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

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

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

       The vertical space characters are:

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

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

   Newline sequences

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

         (?>
|
|||
|)

       This  is  an  example  of an "atomic group", details of which are given
       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
       (carriage  return,  U+000D),  or  NEL  (next  line,  U+0085).  The two-
       character sequence is treated as a single unit that cannot be split.

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

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

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

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

         (*ANY)(*BSR_ANYCRLF)

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

   Unicode character properties

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

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

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

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

         \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,
       Hiragana,    Imperial_Aramaic,    Inherited,     Inscriptional_Pahlavi,
       Inscriptional_Parthian,  Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
       Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B,  Lisu,  Lycian,
       Lydian,    Malayalam,    Mandaic,    Meetei_Mayek,    Meroitic_Cursive,
       Meroitic_Hieroglyphs,  Miao,  Mongolian,  Myanmar,  New_Tai_Lue,   Nko,
       Ogham,    Old_Italic,   Old_Persian,   Old_South_Arabian,   Old_Turkic,
       Ol_Chiki,  Oriya,  Osmanya,  Phags_Pa,   Phoenician,   Rejang,   Runic,
       Samaritan,   Saurashtra,   Sharada,   Shavian,  Sinhala,  Sora_Sompeng,
       Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,  Tai_Le,  Tai_Tham,
       Tai_Viet,  Takri,  Tamil,  Telugu,  Thaana,  Thai,  Tibetan,  Tifinagh,
       Ugaritic, Vai, Yi.

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

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

         \p{L}
         \pL

       The following general category property codes are supported:

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

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

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

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

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

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

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

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

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

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

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

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

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

   Extended grapheme clusters

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

         (?>\PM\pM*)

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

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

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

       1. End at the end of the subject string.

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

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

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

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

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

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

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

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

   Resetting the match start

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

         foo\Kbar

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

         (foo)\Kbar

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

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

   Simple assertions

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

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

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

       A  word  boundary is a position in the subject string where the current
       character and the previous character do not both match \w or  \W  (i.e.
       one  matches  \w  and the other matches \W), or the start or end of the
       string if the first or last character matches \w,  respectively.  In  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"
       metasequence. However, whatever follows \b normally determines which it
       is. For example, the fragment \ba matches "a" at the start of a word.

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

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

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

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

CIRCUMFLEX AND DOLLAR


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

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

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

       The  dollar  character is an assertion that is true only if the current
       matching point is at the end of  the  subject  string,  or  immediately
       before  a newline at the end of the string (by default). Note, however,
       that it does not actually match the newline. Dollar  need  not  be  the
       last character of the pattern if a number of alternatives are involved,
       but it should be the last item in  any  branch  in  which  it  appears.
       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
abc"
       (where  
  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
       PCRE_MULTILINE is set.

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

FULL STOP (PERIOD, DOT) AND \N


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

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

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

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

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

MATCHING A SINGLE DATA UNIT


       Outside a character class, the escape sequence \C matches any one  data
       unit,  whether or not a UTF mode is set. In the 8-bit library, one data
       unit is one byte; in the 16-bit library it is a  16-bit  unit;  in  the
       32-bit  library  it  is  a 32-bit unit. Unlike a dot, \C always matches
       line-ending characters. The feature is provided in  Perl  in  order  to
       match  individual  bytes  in  UTF-8  mode, but it is unclear how it can
       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 character. This has undefined
       results, because PCRE assumes that it is dealing with valid UTF strings
       (and by default it checks this at the start of  processing  unless  the
       PCRE_NO_UTF8_CHECK,  PCRE_NO_UTF16_CHECK  or PCRE_NO_UTF32_CHECK option
       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
       calculate 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
       pattern,  which  could  be used with a UTF-8 string (ignore white space
       and line breaks):

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

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

SQUARE BRACKETS AND CHARACTER CLASSES


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

       A  character  class matches a single character in the subject. In 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
       consumes a character from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

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

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

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

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

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

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

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

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

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

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

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

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

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

         [12[:^digit:]]

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

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

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

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

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

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

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

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

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

COMPATIBILITY FEATURE FOR WORD BOUNDARIES


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

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

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

VERTICAL BAR


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

         gilbert|sullivan

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

INTERNAL OPTION SETTING


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

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

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

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

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

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

         (a(?i)b)c

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

         (a(?i)b|c)

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

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

SUBPATTERNS


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

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

         cat(aract|erpillar|)

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

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

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

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

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

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

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

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

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

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

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

DUPLICATE SUBPATTERN NUMBERS


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

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

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

       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)/

       If a condition test for a subpattern's having matched refers to a  non-
       unique  number,  the  test  is  true  if any of the subpatterns of that
       number have matched.

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

NAMED SUBPATTERNS


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

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

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

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

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

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

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

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

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

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

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

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

REPETITION


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

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

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

         z{2,4}

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

         [aeiou]{3,}

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

         \d{8}

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

       In UTF 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.
       Similarly, \X{3} matches three Unicode extended grapheme clusters, 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
       useful  for  subpatterns  that  are  referenced  as  subroutines   from
       elsewhere  in  the pattern (but see also the section entitled "Defining
       subpatterns for  use  by  reference  only"  below).  Items  other  than
       subpatterns  that  have  a {0} quantifier are omitted from the compiled
       pattern.

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

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

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

         (a?)*

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

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

         /\*.*\*/

       to the string

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

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

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

         /\*.*?\*/

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

         \d??\d

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

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

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

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

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

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

         (.*)abc

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

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

         (?>.*?a)b

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

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

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

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

         /(a|(b))+/

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS


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

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

         123456bar

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

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

         (?>\d+)foo

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

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

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

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

         \d++foo

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

         (abc|xyz){2,3}+

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

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

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

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

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

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

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

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

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

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

BACK REFERENCES


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

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

       It is not possible to have a numerical "forward back  reference"  to  a
       subpattern  whose  number  is  10  or  more using this syntax because a
       sequence such as ( 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), 
         (ring), \g1
         (ring), \g{1}

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

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

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

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

         (sens|respons)e and ibility

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

         ((?i)rah)\s+

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

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

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

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

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

         (a|(bc))

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

       Because there may be many  capturing  parentheses  in  a  pattern,  all
       digits  following  a  backslash  are  taken as part of a potential back
       reference number.  If the pattern continues  with  a  digit  character,
       some  delimiter  must  be  used to terminate the back reference. If the
       PCRE_EXTENDED option is set, this can be 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)  never
       matches.   However,  such  references  can  be  useful  inside repeated
       subpatterns. For example, the pattern

         (a|b)+

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

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

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
       assertion contains capturing subpatterns within it, these  are  counted
       for  the  purposes  of numbering the capturing subpatterns in the whole
       pattern. However, substring capturing is carried out only for  positive
       assertions.  (Perl  sometimes,  but  not  always,  does do capturing in
       negative assertions.)

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

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

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

         foo(?!bar)

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

         (?!foo)bar

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

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.  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
       several top-level alternatives, they do not all have to have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

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

         (?<=ab(c|de))

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

         (?<=abc|abde)

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

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

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

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

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

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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

CONDITIONAL SUBPATTERNS


       It is possible to cause the  matching  process  to  obey  a  subpattern
       conditionally   or  to  choose  between  two  alternative  subpatterns,
       depending on  the  result  of  an  assertion,  or  whether  a  specific
       capturing  subpattern  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
       alternatives in the subpattern, a compile-time error  occurs.  Each  of
       the two alternatives may itself contain nested subpatterns of any form,
       including conditional subpatterns; the restriction to two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

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

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

   Checking for a used subpattern by number

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

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

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

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

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

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

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

   Checking for a used subpattern by name

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

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

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

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

   Checking for pattern recursion

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

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

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

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

   Defining subpatterns for use by reference only

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

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

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

   Assertion conditions

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

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

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

COMMENTS


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

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

         abc #comment 
 still comment

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

RECURSIVE PATTERNS


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

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

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

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

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

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive 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
       parenthesized 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
       referenced.  They  are  always  non-recursive  subroutine   calls,   as
       described in the next section.

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

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

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

       This particular example pattern that we have been looking  at  contains
       nested unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses  is  important  when  applying  the
       pattern 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
       documentation). 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
       subpattern 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
       brackets,  allowing  for  arbitrary nesting. Only digits are allowed in
       nested brackets (that is, when recursing), whereas any  characters  are
       permitted at the outer level.

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

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

   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
       palindromic  string  that  contains  an  odd  number of characters (for
       example, "a", "aba", "abcba", "abcdcba"):

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

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

       At  the  top level, the first character is matched, but as it is not at
       the end  of  the  string,  the  first  alternative  fails;  the  second
       alternative  is taken and the recursion kicks in. The recursive call to
       subpattern 1 successfully matches the next character ("b"). (Note  that
       the beginning and end of line tests are not part of the recursion).

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

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

       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)|.?)$

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

         ^(?:((.)(?1)|)|((.)(?3)|.))

       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*+|)|((.)\W*+(?3)\W*+|\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
       backtracking into sequences of non-word characters. Without this,  PCRE
       takes a great deal longer (ten times or more) to match typical phrases,
       and Perl takes so long that you think it has gone into a loop.

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

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

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

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

SUBPATTERNS AS SUBROUTINES


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

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

       An earlier example pointed out that the pattern

         (sens|respons)e and ibility

       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
       subject  string,  it  is  never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure. Any  capturing
       parentheses  that  are  set  during the subroutine call revert to their
       previous values afterwards.

       Processing  options  such  as  case-independence  are  fixed   when   a
       subpattern  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,
       rewritten using this syntax:

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

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

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

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

CALLOUTS


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

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

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

         (?C1)abc(?C2)def

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

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

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

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

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

BACKTRACKING CONTROL


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

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

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

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

   Optimizations that affect backtracking verbs

       PCRE contains some optimizations that are used to speed up matching  by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that  a  particular
       character must be present. When one of these optimizations bypasses the
       running of a match,  any  included  backtracking  verbs  will  not,  of
       course, be processed. You can suppress the start-of-match optimizations
       by   setting   the   PCRE_NO_START_OPTIMIZE   option    when    calling
       pcre_compile()   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)  in  triggered  in  a
       positive  assertion,  the  assertion succeeds; in a negative assertion,
       the assertion fails.

       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
       captured 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
       pattern:

         a+(?C)(*FAIL)

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

   Recording which path was taken

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

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

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

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

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

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

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

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

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

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

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

   Verbs that act after backtracking

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

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

         (*COMMIT)

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

         a+(*COMMIT)b

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

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

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

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

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

         (*PRUNE) or (*PRUNE:NAME)

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

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

         (*SKIP)

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

         a+(*SKIP)b

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

         (*SKIP:NAME)

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

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

         (*THEN) or (*THEN:NAME)

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

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

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

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

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

         A (B(*THEN)C) | D

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

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

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

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

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

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

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

   More than one backtracking verb

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

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

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

         ...(*COMMIT)(*PRUNE)...

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

   Backtracking verbs in repeated groups

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

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

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

   Backtracking verbs in assertions

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

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

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

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

   Backtracking verbs in subroutines

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

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

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

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

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

SEE ALSO


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

AUTHOR


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

REVISION


       Last updated: 08 January 2014
       Copyright (c) 1997-2014 University of Cambridge.



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