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       PCRE - Perl-compatible regular expressions


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

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

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


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

   UTF support

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


       (*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	deter‐
       mine character types, instead of recognizing only characters with codes
       less than 128 via a lookup table.

   Disabling auto-possessification

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

   Disabling start-up optimizations

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

   Newline conventions

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

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

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

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


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

       The  newline  convention affects where the circumflex and dollar asser‐
       tions are true. It also affects the interpretation of the dot metachar‐
       acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it
       does not affect what the \R escape sequence matches. By	default,  this
       is  any Unicode newline sequence, for Perl compatibility. However, this
       can be changed; see the description of \R in the section entitled "New‐
       line  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 recur‐
       sive 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


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


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


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

	 The quick brown fox

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

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

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

	 \	general escape character with several uses
	 ^	assert start of string (or line, in multiline mode)
	 $	assert end of string (or line, in multiline mode)
	 .	match any character except newline (by default)
	 [	start character class definition
	 |	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
	 ]	terminates the character class

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


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

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

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

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

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

	 Pattern	    PCRE matches   Perl matches

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

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

   Non-printing characters

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

	 \a	   alarm, that is, the BEL character (hex 07)
	 \cx	   "control-x", where x is any ASCII character
	 \e	   escape (hex 1B)
	 \f	   form feed (hex 0C)
	 \n	   linefeed (hex 0A)
	 \r	   carriage return (hex 0D)
	 \t	   tab (hex 09)
	 \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.

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

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

       The 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 charac‐
       ter numbers, and \g{} to specify back references. The  following	 para‐
       graphs describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is compli‐
       cated, and Perl has changed in recent releases, causing	PCRE  also  to
       change. Outside a character class, PCRE reads the digit and any follow‐
       ing 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  oth‐
       erwise re-reads up to three octal digits following the backslash, using
       them to generate a data character.  Any	subsequent  digits  stand  for
       themselves. For example:

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

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

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

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

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

   Constraints on character values

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

	 8-bit non-UTF mode    less than 0x100
	 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,  option‐
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis‐
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

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

   Generic character types

       Another use of backslash is for specifying generic character types:

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

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

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

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

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

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

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

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

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

       The vertical space characters are:

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

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

   Newline sequences

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


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

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

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

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

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


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

	 \p{xx}	  a character with the xx property
	 \P{xx}	  a character without the xx property
	 \X	  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 "InMu‐
       sicalSymbols" are not currently supported by PCRE.  Note	 that  \P{Any}
       does not match any characters, so always causes a match failure.

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


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

       Arabic, Armenian, Avestan, Balinese, Bamum, Batak,  Bengali,  Bopomofo,
       Brahmi,	Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
       Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic,  Deseret,
       Devanagari,   Egyptian_Hieroglyphs,   Ethiopic,	Georgian,  Glagolitic,
       Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,	 Hira‐
       gana,   Imperial_Aramaic,  Inherited,  Inscriptional_Pahlavi,  Inscrip‐
       tional_Parthian,	 Javanese,  Kaithi,   Kannada,	 Katakana,   Kayah_Li,
       Kharoshthi,  Khmer,  Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
       Lydian,	  Malayalam,	Mandaic,    Meetei_Mayek,    Meroitic_Cursive,
       Meroitic_Hieroglyphs,   Miao,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham,	Old_Italic,   Old_Persian,   Old_South_Arabian,	   Old_Turkic,
       Ol_Chiki,  Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari‐
       tan, Saurashtra, Sharada, Shavian,  Sinhala,  Sora_Sompeng,  Sundanese,
       Syloti_Nagri,  Syriac,  Tagalog,	 Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
       Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,  Ugaritic,  Vai,

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

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


       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) prop‐
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

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

       Matching	 characters  by Unicode property is not fast, because PCRE has
       to do a multistage table lookup in order to find	 a  character's	 prop‐
       erty. 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

   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  ear‐
       lier, simpler definition that was equivalent to


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

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

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

       1. End at the end of the subject string.

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

       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

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

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

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

       Xan  matches  characters that have either the L (letter) or the N (num‐
       ber) property. Xps matches the characters tab, linefeed, vertical  tab,
       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  ver‐
       tical  tab,  for Perl compatibility, but Perl changed, and so PCRE fol‐
       lowed at release 8.34. Xwd matches the same  characters	as  Xan,  plus

       There  is another non-standard property, Xuc, which matches any charac‐
       ter 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 char‐
       acters that they represent.)

   Resetting the match start

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


       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


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

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

   Simple assertions

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

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

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

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

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

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

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

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


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

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

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

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

       The meaning of dollar can be changed so that it	matches	 only  at  the
       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


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

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

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

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

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


       Outside	a character class, the escape sequence \C matches any one data
       unit, whether or not a UTF mode is set. In the 8-bit library, one  data
       unit  is	 one  byte;  in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a  dot,  \C  always  matches
       line-ending  characters.	 The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode, but it is unclear how it can use‐
       fully  be  used.	 Because  \C breaks up characters into individual data
       units, matching one unit with \C in a UTF mode means that the  rest  of
       the string may start with a malformed UTF character. This has undefined
       results, because PCRE assumes that it is dealing with valid UTF strings
       (and  by	 default  it checks this at the start of processing unless the
       is used).

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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


       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,


       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,


       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 char‐
		 acters 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 (punctua‐
		 tion)	property,  plus those characters whose code points are
		 less than 128 that have the S (Symbol) property.

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


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

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

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


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


       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.


       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

	 s  for PCRE_DOTALL

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


       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


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

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


       Subpatterns 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


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

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

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

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

       the captured substrings are "red king", "red", and "king", and are num‐
       bered 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  captur‐
       ing,  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


       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


       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:


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

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

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


       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":


       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 num‐
       ber have matched.

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


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

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

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

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


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

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

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


       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 sub‐
       patterns	 with  the same number because PCRE uses only the numbers when
       matching. For this reason, an error is given at compile time if differ‐
       ent  names  are given to subpatterns with the same number. However, you
       can always give the same name to subpatterns with the same number, even
       when PCRE_DUPNAMES is not set.


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

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

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


       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


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


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

       In UTF modes, quantifiers apply to characters rather than to individual
       data  units. Thus, for example, \x{100}{2} matches two characters, each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi‐
       larly,  \X{3} matches three Unicode extended grapheme clusters, each of
       which may be several data units long (and  they	may  be	 of  different

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use‐
       ful  for	 subpatterns that are referenced as subroutines from elsewhere
       in the pattern (but see also the section entitled "Defining subpatterns
       for  use	 by  reference only" below). Items other than subpatterns that
       have a {0} quantifier are omitted from the compiled pattern.

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

	 *    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:


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

       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


       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


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

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

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

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

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

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


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

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


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

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


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


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


       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


       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:


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

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

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

       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


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


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

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

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

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


       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


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


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


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

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

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

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

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

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


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

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

	 (sens|respons)e and \1ibility

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


       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:


       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


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

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

   Recursive back references

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


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

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


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

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

       Assertion subpatterns are not capturing subpatterns. If such an	asser‐
       tion  contains  capturing  subpatterns within it, these are counted for
       the purposes of numbering the capturing subpatterns in the  whole  pat‐
       tern.  However,	substring  capturing  is carried out only for positive
       assertions. (Perl sometimes, but not always, does do capturing in nega‐
       tive assertions.)

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

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

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

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

   Lookahead assertions

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


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


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


       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,


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


       is permitted, but


       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


       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:


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

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

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

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

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


       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


       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


       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,


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


       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,


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


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


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


       If  the	condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. If there are more  than	 two  alterna‐
       tives  in  the subpattern, a compile-time error occurs. Each of the two
       alternatives may itself contain nested subpatterns of any form, includ‐
       ing  conditional	 subpatterns;  the  restriction	 to  two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

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

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

   Checking for a used subpattern by number

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

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

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

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

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

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

       This  makes  the	 fragment independent of the parentheses in the larger

   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 amper‐
       sand follow the letter R, for example:

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

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

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

   Defining subpatterns for use by reference only

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

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

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

   Assertion conditions

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

	 \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.


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

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

	 abc #comment \n still comment

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


       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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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


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

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

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

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

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

   Differences in recursion processing between PCRE and Perl

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


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

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

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


       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

       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:


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


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


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

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

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


       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.


       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:


       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

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

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


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


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

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

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


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


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

       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


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


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

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

       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.


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

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

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

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

   Optimizations that affect backtracking verbs

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

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

   Verbs that act immediately

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


       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 posi‐
       tive  assertion,	 the  assertion succeeds; in a negative assertion, the
       assertion fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far  is  cap‐
       tured. For example:


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

	 (*FAIL) or (*F)

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


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

   Recording which path was taken

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

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

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

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

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

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

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

       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 con‐
       tinues with what follows, but if there is no subsequent match,  causing
       a  backtrack  to	 the  verb, a failure is forced. That is, backtracking
       cannot pass to the left of the verb. However, when one of  these	 verbs
       appears inside an atomic group or an assertion that is true, its effect
       is confined to that group, because once the  group  has	been  matched,
       there  is never any backtracking into it. In this situation, backtrack‐
       ing can "jump back" to the left of the entire atomic  group  or	asser‐
       tion.  (Remember	 also,	as  stated  above, that this localization also
       applies in subroutine calls.)

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


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


       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 pcretest example:

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

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


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

       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).


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


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


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

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

	 (*THEN) or (*THEN:NAME)

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

	 ( 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	subse‐
       quently	BAZ fails, there are no more alternatives, so there is a back‐
       track 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

	 A (B(*THEN)C) | D

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

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

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

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

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

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

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

   More than one backtracking verb

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


       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:


       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:


       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

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

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

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

   Backtracking verbs in subroutines

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

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

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

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

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


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


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


       Last updated: 03 December 2013
       Copyright (c) 1997-2013 University of Cambridge.

PCRE 8.34		       03 December 2013			PCREPATTERN(3)

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