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PCREPATTERN(3)							PCREPATTERN(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS

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

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

       The original operation of PCRE was on strings of	 one-byte  characters.
       However,	 there is now also support for UTF-8 character strings. To use
       this, PCRE must be built to include UTF-8 support, and  you  must  call
       pcre_compile()  or  pcre_compile2() with the PCRE_UTF8 option. There is
       also a special sequence that can be given at the start of a pattern:

	 (*UTF8)

       Starting a pattern with this sequence  is  equivalent  to  setting  the
       PCRE_UTF8  option.  This	 feature  is  not Perl-compatible. How setting
       UTF-8 mode affects pattern matching  is	mentioned  in  several	places
       below.  There  is  also	a  summary of UTF-8 features in the section on
       UTF-8 support in the main pcre page.

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

	 (*UCP)

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

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

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

NEWLINE CONVENTIONS

       PCRE supports five different conventions for indicating line breaks  in
       strings:	 a  single  CR (carriage return) character, a single LF (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  pcre_compile()  or
       pcre_compile2().	 For example, on a Unix system where LF is the default
       newline sequence, the pattern

	 (*CR)a.b

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

       The newline convention affects the interpretation of the dot  metachar‐
       acter  when  PCRE_DOTALL is not set, and also the behaviour of \N. How‐
       ever, it does not affect	 what  the  \R	escape	sequence  matches.  By
       default,	 this is any Unicode newline sequence, for Perl compatibility.
       However, this can be changed; see the description of \R in the  section
       entitled	 "Newline sequences" below. A change of \R setting can be com‐
       bined with a change of newline convention.

CHARACTERS AND METACHARACTERS

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

	 The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE_CASELESS option), letters  are
       matched	independently  of case. In UTF-8 mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless	 matching  is always possible. For characters with higher 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-8 support.

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

       There are two different sets of metacharacters: those that  are	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
		  syntax)
	 ]	terminates the character class

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

BACKSLASH

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

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

       In UTF-8 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,	whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap‐
       ing  backslash  can  be	used to include a whitespace or # character as
       part of the pattern.

       If you want to remove the special meaning from a	 sequence  of  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.

   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	   formfeed (hex 0C)
	 \n	   linefeed (hex 0A)
	 \r	   carriage return (hex 0D)
	 \t	   tab (hex 09)
	 \ddd	   character with octal code ddd, or back reference
	 \xhh	   character with hex code hh
	 \x{hhh..} character with hex code hhh..

       The precise effect of \cx is as follows: if x is a lower	 case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
       is  7B),	 while	\c; becomes hex 7B (; is 3B). If the byte following \c
       has a value greater than 127, a compile-time error occurs.  This	 locks
       out  non-ASCII  characters in both byte mode and UTF-8 mode. (When PCRE
       is compiled in EBCDIC mode, all byte values are	valid.	A  lower  case
       letter is converted to upper case, and then the 0xc0 bits are flipped.)

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

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

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

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

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

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

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

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

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class,  the  sequence \b is interpreted as the backspace character (hex
       08). The sequences \B, \N, \R, and \X are not special inside a  charac‐
       ter  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. Outside a character
       class, these sequences have different meanings.

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

       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.

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

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

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

       By  default,  in	 UTF-8	mode,  characters with values greater than 128
       never match \d, \s, or \w, and always  match  \D,  \S,  and  \W.	 These
       sequences  retain their original meanings from before UTF-8 support was
       available, mainly for efficiency reasons. However, if PCRE is  compiled
       with  Unicode property support, and the PCRE_UCP option is set, the be‐
       haviour is changed so that Unicode properties  are  used	 to  determine
       character types, as follows:

	 \d  any character that \p{Nd} matches (decimal digit)
	 \s  any character that \p{Z} matches, plus HT, LF, FF, CR
	 \w  any character that \p{L} or \p{N} matches, 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
       codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The  horizon‐
       tal space characters are:

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

       The vertical space characters are:

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

   Newline sequences

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

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

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

       In UTF-8 mode, two additional characters whose codepoints  are  greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph 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  pcre_compile()  or
       pcre_compile2(),	 but  they  can	 be  overridden	 by  options  given to
       pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
       are  not	 Perl-compatible,  are	recognized only at the very start of a
       pattern, and that they must be in upper case. If more than one of  them
       is present, the last one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

	 (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8) 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 not in UTF-8 mode, these sequences are of course
       limited to testing characters whose codepoints are less than  256,  but
       they do work in this mode.  The extra escape sequences are:

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

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

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

	 \p{Greek}
	 \P{Han}

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

       Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
       Buginese, Buhid, Canadian_Aboriginal, Carian, Cham,  Cherokee,  Common,
       Coptic,	 Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,	 Egyp‐
       tian_Hieroglyphs,  Ethiopic,  Georgian,	Glagolitic,   Gothic,	Greek,
       Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana, Impe‐
       rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
       Javanese,  Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
       Latin,  Lepcha,	Limbu,	Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
       Meetei_Mayek,  Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
       Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki,  Oriya,  Osmanya,
       Phags_Pa,  Phoenician,  Rejang,	Runic, Samaritan, Saurashtra, Shavian,
       Sinhala, Sundanese, Syloti_Nagri, Syriac,  Tagalog,  Tagbanwa,  Tai_Le,
       Tai_Tham,  Tai_Viet,  Tamil,  Telugu,  Thaana, Thai, Tibetan, Tifinagh,
       Ugaritic, Vai, Yi.

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

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

	 \p{L}
	 \pL

       The following general category property codes are supported:

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

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

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

	 N     Number
	 Nd    Decimal number
	 Nl    Letter number
	 No    Other number

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

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

	 Z     Separator
	 Zl    Line separator
	 Zp    Paragraph separator
	 Zs    Space separator

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

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

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

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

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

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

	 (?>\PM\pM*)

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

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

   PCRE's additional properties

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

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

       Xan  matches  characters that have either the L (letter) or the N (num‐
       ber) property. Xps matches the characters tab, linefeed, vertical  tab,
       formfeed,  or  carriage	return, and any other character that has the Z
       (separator) property.  Xsp is the same as Xps, except that vertical tab
       is excluded. Xwd matches the same characters as Xan, plus underscore.

   Resetting the match start

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

	 foo\Kbar

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

	 (foo)\Kbar

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

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

   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
       UTF-8 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.

CIRCUMFLEX AND DOLLAR

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

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

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

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

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

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

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

FULL STOP (PERIOD, DOT) AND \N

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

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

MATCHING A SINGLE BYTE

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

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

SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not 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	 UTF-8
       mode, the character may be more than one byte 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 mode, characters with values greater than 255 can be  included
       in  a  class as a literal string of bytes, or by using the \x{ escaping
       mechanism.

       When caseless matching is set, any letters in a	class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported	if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching in UTF8-mode for characters 128 and above,  you  must
       ensure  that  PCRE is compiled with Unicode property support as well as
       with UTF-8 support.

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

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

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

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

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

       The  character 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-8 mode, the PCRE_UCP option affects the  meanings  of
       \d,  \s,	 \w  and  their upper case partners, just as it does when they
       appear outside a character class, as described in the section  entitled
       "Generic character types" above. The escape sequence \b has a different
       meaning inside a character class; it matches the	 backspace  character.
       The  sequences  \B,  \N,	 \R, and \X are not special inside a character
       class. Like any other unrecognized escape sequences, they  are  treated
       as  the literal characters "B", "N", "R", and "X" by default, but cause
       an error if the PCRE_EXTRA option is set.

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

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

POSIX CHARACTER CLASSES

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

	 [01[:alpha:]%]

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

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

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

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

	 [12[:^digit:]]

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

       By default, in UTF-8 mode, characters with values greater than  128  do
       not  match any of the POSIX character classes. However, if the PCRE_UCP
       option is passed to pcre_compile(), some of the classes are changed  so
       that Unicode character properties are used. This is achieved by replac‐
       ing the POSIX classes by other sequences, as follows:

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

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

VERTICAL BAR

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

	 gilbert|sullivan

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

INTERNAL OPTION SETTING

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

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

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

	 (a(?i)b)c

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

	 (a(?i)b|c)

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

       Note:  There  are  other	 PCRE-specific	options that can be set by the
       application when the compile or match functions	are  called.  In  some
       cases the pattern can contain special leading sequences 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) and (*UCP) leading  sequences	 that  can  be
       used  to	 set  UTF-8 and Unicode property modes; they are equivalent to
       setting the PCRE_UTF8 and the PCRE_UCP options, respectively.

SUBPATTERNS

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

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

	 cat(aract|erpillar|)

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

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

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

       the captured substrings are "red king", "red", and "king", and are 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

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

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

DUPLICATE SUBPATTERN NUMBERS

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

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

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

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

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

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

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

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

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

NAMED SUBPATTERNS

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

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

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

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

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

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

       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  give  the same name to subpatterns with the same number, even when
       PCRE_DUPNAMES is not set.

REPETITION

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

	 a literal data character
	 the dot metacharacter
	 the \C escape sequence
	 the \X escape sequence (in UTF-8 mode with Unicode properties)
	 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 (unless it is an assertion)
	 a recursive or "subroutine" call to a subpattern

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

	 z{2,4}

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

	 [aeiou]{3,}

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

	 \d{8}

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

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

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be 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:

	 (a?)*

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

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

	 /\*.*\*/

       to the string

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

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

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

	 /\*.*?\*/

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

	 \d??\d

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

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

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

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv‐
       alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
       the pattern is implicitly anchored, because whatever  follows  will  be
       tried  against every character position in the subject string, so there
       is no point in retrying the overall match at  any  position  after  the
       first.  PCRE  normally treats such a pattern as though it were preceded
       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 is one situation where the optimization cannot be	 used.
       When .*	is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where
       a later one succeeds. Consider, for example:

	 (.*)abc\1

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

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

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

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

	 /(a|(b))+/

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

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

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

	 123456bar

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

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

	 (?>\d+)foo

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

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

       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
       digits.

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

	 \d++foo

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

	 (abc|xyz){2,3}+

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

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

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

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

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

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

	 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

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

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

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

BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing 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‐
       tion.

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

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

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

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

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

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

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

	 (sens|respons)e and \1ibility

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

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

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

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

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

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

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

	 (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back 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 whitespace. Otherwise, the \g{
       syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

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

	 (a|b\1)+

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

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

ASSERTIONS

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

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

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

   Lookahead assertions

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

	 \w+(?=;)

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

	 foo(?!bar)

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

	 (?!foo)bar

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

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

   Lookbehind assertions

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

	 (?<!foo)bar

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

	 (?<=bullock|donkey)

       is permitted, but

	 (?<!dogs?|cats?)

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

	 (?<=ab(c|de))

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

	 (?<=abc|abde)

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

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

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

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

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

	 abcd$

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

	 ^.*abcd$

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

	 ^.*+(?<=abcd)

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

   Using multiple assertions

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

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

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of	 the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous	 three
       characters  are	all  digits,  and  then there is a check that the same
       three characters are not "999".	This pattern does not match "foo" 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

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

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

       Assertions can be nested in any combination. For example,

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

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

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

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

CONDITIONAL SUBPATTERNS

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

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

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

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

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

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence  of  digits,
       the condition is true if a capturing subpattern of that number has pre‐
       viously matched. If there is more than one  capturing  subpattern  with
       the  same  number  (see	the earlier section about duplicate subpattern
       numbers), the condition is true if any of them have matched. An	alter‐
       native  notation is to precede the digits with a plus or minus sign. In
       this case, the subpattern number is relative rather than absolute.  The
       most  recently opened parentheses can be referenced by (?(-1), the next
       most recent by (?(-2), and so on. Inside loops it can also  make	 sense
       to refer to subsequent groups. The next parentheses to be opened can be
       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
       pattern.

   Checking for a used subpattern by name

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

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

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

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

   Checking for pattern recursion

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

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

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

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

   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  ref‐
       erenced	from elsewhere. (The use of "subroutines" is described below.)
       For  example,  a	 pattern  to   match   an   IPv4   address   such   as
       "192.168.23.245" could be written like this (ignore whitespace and line
       breaks):

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

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

   Assertion conditions

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

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

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

COMMENTS

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

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

	 abc #comment \n still comment

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

RECURSIVE PATTERNS

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

       For some time, Perl has provided a facility that allows regular 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 call of the subpattern of
       the given number, provided that it occurs inside that  subpattern.  (If
       not,  it	 is  a	"subroutine" call, which is described in the next sec‐
       tion.) The special item (?R) or (?0) is a recursive call of the	entire
       regular expression.

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

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

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

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

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

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

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

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

	 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

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

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

	 (ab(cd)ef)

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

       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.

   Recursion difference from Perl

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

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

       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
       different:

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

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

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

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

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

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

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

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

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

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

SUBPATTERNS AS SUBROUTINES

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

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

       An earlier example pointed out that the pattern

	 (sens|respons)e and \1ibility

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

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

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

       Like recursive subpatterns, a subroutine call is always treated	as  an
       atomic  group. That is, once it has matched some of the subject string,
       it is never re-entered, even if it contains  untried  alternatives  and
       there  is a subsequent matching failure. Any capturing parentheses that
       are set during the subroutine call  revert  to  their  previous	values
       afterwards.

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

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

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

ONIGURUMA SUBROUTINE SYNTAX

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

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

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

CALLOUTS

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

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

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

	 (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

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

BACKTRACKING CONTROL

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

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

       If any of these verbs are used in an assertion or subroutine subpattern
       (including  recursive  subpatterns),  their  effect is confined to that
       subpattern; it does not extend to the surrounding  pattern.  Note  that
       such  subpatterns are processed as anchored at the point where they are
       tested.

       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, with differing  be‐
       haviour, depending on whether or not an argument is present. An name is
       a sequence of letters, digits, and underscores. If the name  is	empty,
       that  is, if the closing parenthesis immediately follows the colon, the
       effect is as if the colon were not there. Any number of these verbs may
       occur in a pattern.

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

   Verbs that act immediately

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

	  (*ACCEPT)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. When inside a recursion, only the innermost pattern  is
       ended  immediately.  If	(*ACCEPT) is inside capturing parentheses, the
       data so far is captured. (This feature was added	 to  PCRE  at  release
       8.00.) For example:

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

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

	 (*FAIL) or (*F)

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

	 a+(?C)(*FAIL)

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

   Recording which path was taken

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

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

       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)  is
       passed  back  to	 the  caller  via  the	pcre_extra  data structure, as
       described in the section on pcre_extra in the pcreapi documentation. No
       data  is	 returned  for a partial match. Here is an example of pcretest
       output, where the /K modifier requests the retrieval and outputting  of
       (*MARK) data:

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

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

       A name may also be returned after a failed  match  if  the  final  path
       through	the  pattern involves (*MARK). However, unless (*MARK) used in
       conjunction with (*COMMIT), this is unlikely to	happen	for  an	 unan‐
       chored pattern because, as the starting point for matching is advanced,
       the final check is often with an empty string, causing a failure before
       (*MARK) is reached. For example:

	 /X(*MARK:A)Y|X(*MARK:B)Z/K
	 XP
	 No match

       There are three potential starting points for this match (starting with
       X, starting with P, and with  an	 empty	string).  If  the  pattern  is
       anchored, the result is different:

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

       PCRE's  start-of-match  optimizations can also interfere with this. For
       example, if, as a result of a call to pcre_study(), it knows the	 mini‐
       mum  subject  length for a match, a shorter subject will not be scanned
       at all.

       Note that similar anomalies (though different in detail) exist in Perl,
       no  doubt  for the same reasons. The use of (*MARK) data after a failed
       match of an unanchored pattern is not recommended, unless (*COMMIT)  is
       involved.

   Verbs that act after backtracking

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

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

	 (*COMMIT)

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

	 a+(*COMMIT)b

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

       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:

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

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

	 (*PRUNE) or (*PRUNE:NAME)

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

	 (*SKIP)

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

	 a+(*SKIP)b

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

	 (*SKIP:NAME)

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

	 (*THEN) or (*THEN:NAME)

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

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

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds);  on	 failure  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. The behaviour  of  (*THEN:NAME)  is	exactly	 the  same  as
       (*MARK:NAME)(*THEN)  if	the  overall  match  fails.  If (*THEN) is not
       directly inside an alternation, it acts like (*PRUNE).

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

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

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

       Once A has matched, PCRE is committed to this  match,  at  the  current
       starting	 position. If subsequently B matches, but C does not, the nor‐
       mal (*THEN) action of trying the next alternation (that is, D) does not
       happen because (*COMMIT) overrides.

SEE ALSO

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

AUTHOR

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

REVISION

       Last updated: 21 November 2010
       Copyright (c) 1997-2010 University of Cambridge.

								PCREPATTERN(3)
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