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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\nb" because LF is no longer a newline. If more than
       one of these settings is present, the last one is used.

       The  newline  convention affects where the circumflex and dollar 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.   In  an  ASCII  or
       Unicode environment, these escapes are as follows:

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

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

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

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

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

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

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

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

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

       By default, after \x that is not followed by {, from zero to two 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, \xdc  is  exactly  the
       same as \x{dc} (or \u00dc in JavaScript mode).

   Constraints on character values

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

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

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

   Escape sequences in character classes

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

       \N is not allowed in a character class. \B, \R, and \X are not special inside  a  character  class.  Like
       other  unrecognized  escape  sequences,  they  are treated as the literal characters "B", "R", and "X" by
       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
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline 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 (\xA0) is recognized as
       white space, and in others the VT character is not.

       A "word" character is an underscore or any character  that  is  a  letter  or  digit.   By  default,  the
       definition  of  letters  and  digits is 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 \v
         \w  any character that matches \p{L} or \p{N}, plus underscore

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

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

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

       This is an example of an "atomic group", details of which are given below.  This particular group matches
       either  the  two-character  sequence  CR  followed  by  LF, or one of the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (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,  Bassa_Vah,  Batak,  Bengali,  Bopomofo, Brahmi, Braille,
       Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic,
       Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic,
       Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,
       Imperial_Aramaic,  Inherited,  Inscriptional_Pahlavi,  Inscriptional_Parthian, Javanese, Kaithi, Kannada,
       Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A,  Linear_B,
       Lisu,   Lycian,   Lydian,   Mahajani,   Malayalam,   Mandaic,  Manichaean,  Meetei_Mayek,  Mende_Kikakui,
       Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko,
       Ogham,  Ol_Chiki,  Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic,
       Oriya, Osmanya, Pahawh_Hmong, Palmyrene,  Pau_Cin_Hau,  Phags_Pa,  Phoenician,  Psalter_Pahlavi,  Rejang,
       Runic,  Samaritan, Saurashtra, Sharada, Shavian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri,
       Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,  Takri,  Tamil,  Telugu,  Thaana,  Thai,  Tibetan,
       Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.

       Each character has exactly one Unicode general category property, 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\nabc" (where \n represents a newline) in
       multiline  mode,  but not otherwise. Consequently, patterns that are anchored in single line mode because
       all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when
       the  startoffset  argument  of  pcre_exec()  is  non-zero.  The  PCRE_DOLLAR_ENDONLY option is ignored if
       PCRE_MULTILINE is set.

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

FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one 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):

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

       A group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate
       Subpattern Numbers" below). The assertions at the start of each branch check the next UTF-8 character for
       values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The character's individual bytes  are  then
       captured by the appropriate 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-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.

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

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

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,  \V,  \w,  and  \W  may  appear  in  a
       character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any
       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))\1/

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

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

       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\1

       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 \50 is interpreted as a character defined in octal. See
       the  subsection  entitled  "Non-printing  characters" above for further details of the handling of digits
       following a backslash. There is no such problem when named parentheses are used. A back reference to  any
       subpattern is possible using named parentheses (see below).

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

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

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

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

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

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

         (sens|respons)e and \1ibility

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

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

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

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

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

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

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

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". However, if the PCRE_JAVASCRIPT_COMPAT option is
       set at compile time, a back 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\1) never matches.  However, such  references  can  be  useful  inside  repeated
       subpatterns. For example, the pattern

         (a|b\1)+

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

       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 \n still comment

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

RECURSIVE PATTERNS

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

       For  some  time,  Perl  has provided a facility that allows regular 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)\2)$

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

       At the top level, the first character is matched, but as it is not at the end of the  string,  the  first
       alternative  fails;  the  second  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)\2|.)$

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

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

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

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

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

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

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

       If run with the PCRE_CASELESS option, this pattern matches phrases such as  "A  man,  a  plan,  a  canal:
       Panama!"  and  it works well in both PCRE and Perl. Note the use of the possessive quantifier *+ to avoid
       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:

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

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

SUBPATTERNS AS SUBROUTINES

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

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

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

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

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

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

       All subroutine calls, whether recursive or not, are always treated as atomic  groups.  That  is,  once  a
       subroutine  has  matched  some of the 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: 14 June 2015
       Copyright (c) 1997-2015 University of Cambridge.