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Linux 2.6 - man page for pcrepattern (linux section 3)

PCREPATTERN(3)			     Library Functions Manual			   PCREPATTERN(3)

       PCRE - Perl-compatible regular expressions


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

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


       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  men-
       tioned  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:


       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 concerned with the handling of newlines; they are
       described below.

       The remainder of this document discusses the patterns that are supported 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.


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

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

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

       These override the default and the options given to pcre_compile() or pcre_compile2(). For
       example, on a Unix system where LF is the default newline sequence, the pattern


       changes	the  convention to CR. That pattern matches "a\nb" because LF is no longer a new-
       line. 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  metacharacter   when
       PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not affect what the
       \R escape sequence matches. By default, this is any Unicode  newline  sequence,	for  Perl
       compatibility.  However,  this  can  be	changed; see the description of \R in the section
       entitled "Newline sequences" below. A change of \R setting can be combined with	a  change
       of newline convention.


       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 values,
       the  concept  of  case is supported if PCRE is compiled with Unicode property support, but
       not otherwise.  If you want to use caseless matching for characters  128  and  above,  you
       must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 sup-

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

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

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

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

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

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


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

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

       In 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 escaping 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 characters, you can do so by
       putting them between \Q and \E. This is different from Perl in that $ and @ are handled as
       literals  in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable 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 characters in patterns
       in a visible manner. There is no restriction on the appearance of non-printing characters,
       apart from the binary zero that terminates a pattern, but when a pattern is being prepared
       by text editing, it is often easier to use one of the following escape sequences than  the
       binary character it represents:

	 \a	   alarm, that is, the BEL character (hex 07)
	 \cx	   "control-x", where x is any ASCII character
	 \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 handled. 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 fol-
       lowed 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 complicated.	Outside a
       character class, PCRE reads it and any following digits as a decimal number. If the number
       is  less  than 10, or if there have been at least that many previous capturing left paren-
       theses 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 generate 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  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. Outside a character class, these sequences have different mean-

   Absolute and relative back references

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

   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 referenc-
       ing  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 character.  This is the
       same as the "." metacharacter when PCRE_DOTALL is not set.

       Each pair of lower and upper case escape sequences partitions the complete set of  charac-
       ters  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 sub-
       ject 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 character. 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 controlled by PCRE's low-valued character
       tables,	and may vary if locale-specific matching is taking place (see "Locale support" in
       the pcreapi page). For example, in a French locale such as "fr_FR" in  Unix-like  systems,
       or  "french"  in Windows, some character codes greater than 128 are used for accented let-
       ters, 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 behaviour 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 horizontal 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:


       This  is an example of an "atomic group", details of which are given below.  This particu-
       lar group matches either the two-character sequence CR followed by LF, or one of the  sin-
       gle 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 separator, U+2029).  Unicode character prop-
       erty 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 start-
       ing 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:


       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 additional 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  "InMusicalSymbols"  are  not  currently  supported by PCRE. Note that \P{Any} does not
       match any characters, so always causes a match failure.

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


       Those  that are not part of an identified script are lumped together as "Common". The cur-
       rent 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, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic,  Greek,
       Gujarati,  Gurmukhi,  Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited,
       Inscriptional_Pahlavi,  Inscriptional_Parthian,	Javanese,  Kaithi,   Kannada,	Katakana,
       Kayah_Li,  Kharoshthi,  Khmer,  Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian,
       Malayalam, Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, Old_Per-
       sian,  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, Tifi-
       nagh, Ugaritic, Vai, Yi.

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

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


       The following general category property codes are supported:

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

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

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

	 N     Number
	 Nd    Decimal number
	 Nl    Letter number
	 No    Other number

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

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

	 Z     Separator
	 Zl    Line separator
	 Zp    Paragraph separator
	 Zs    Space separator

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

       The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such
       characters  are not valid in UTF-8 strings (see RFC 3629) and so cannot be tested by PCRE,
       unless  UTF-8  validity	checking  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 sup-
       ported by PCRE, nor is it permitted to prefix any of these properties with "Is".

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

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

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


       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 preced-
       ing 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 struc-
       ture 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 sup-
       ports four more that make it possible to convert traditional escape sequences such  as  \w
       and  \s	and  POSIX character classes to use Unicode properties. PCRE uses these non-stan-
       dard, non-Perl properties 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  (number)  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:


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


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

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

   Simple assertions

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

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

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

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

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

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


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

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

       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 alterna-
       tives are involved, but it should be the last item in any branch in which it appears. Dol-
       lar has no special meaning in a character class.

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

       The meanings of the circumflex and dollar characters are  changed  if  the  PCRE_MULTILINE
       option is set. When this is the case, a circumflex matches immediately after internal new-
       lines 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 repre-
       sents 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 multi-
       line 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

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


       Outside a character class, a dot in the pattern matches any one character in  the  subject
       string  except  (by  default) a character that signifies the end of a line. 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 Unicode line endings are being recognized, dot does not match CR or LF or any of
       the other line ending characters.

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

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

       The escape sequence \N behaves like  a  dot,  except  that  it  is  not	affected  by  the
       PCRE_DOTALL option. In other words, it matches any character except one that signifies the
       end of a line.


       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  characters 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 calculate the length of the lookbehind.


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

       A character class matches a single character in the subject. In 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 cir-
       cumflex is actually required as a member of the class, ensure it is not the first  charac-
       ter, or escape it with a backslash.

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

       In UTF-8 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 characters in a character
       class. For example, [d-m] matches any letter between d and m, inclusive. If a minus  char-
       acter  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 char-
       acter in the class.

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

       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  hexadecimal	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 char-
       acter 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 spe-
       cial 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 under-
       score.  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.


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


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

	 alnum	  letters and digits
	 alpha	  letters
	 ascii	  character codes 0 - 127
	 blank	  space or tab only
	 cntrl	  control characters
	 digit	  decimal digits (same as \d)
	 graph	  printing characters, excluding space
	 lower	  lower case letters
	 print	  printing characters, including space
	 punct	  printing characters, excluding letters and digits and space
	 space	  white space (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 exam-


       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 replacing 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  characters are used to separate alternative patterns. For example, the pat-


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


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

	 s  for PCRE_DOTALL

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

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

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

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


       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,


       matches	"ab", "aB", "c", and "C", even though when matching "C" the first branch is aban-
       doned 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 lead-
       ing  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,


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

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


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

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

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

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

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

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

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

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


       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 "SUN-
       DAY" as well as "Saturday".


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


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

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

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


       In contrast, a 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":


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

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


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

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

       Names  consist of up to 32 alphanumeric characters and underscores. Named capturing paren-
       theses 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 transla-
       tion 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 con-
       straint by setting the PCRE_DUPNAMES option at compile time.  (Duplicate  names	are  also
       always permitted for subpatterns with the same number, set up as described in the previous
       section.) Duplicate names can be useful for patterns where only one instance of the  named
       parentheses can match. Suppose you want to match the name of a weekday, either as a 3-let-
       ter abbreviation or as the full name, and in both cases you want to extract the	abbrevia-
       tion. This pattern (ignoring the line breaks) does the job:


       There are five capturing substrings, but only one is ever set after a match.  (An alterna-
       tive 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 pat-
       tern, the one that corresponds to the first occurrence of the name is used. In the absence
       of duplicate numbers (see the previous section) this is the one with the lowest number. If
       you use a named reference in a condition test (see the section  about  conditions  below),
       either  to  check whether a subpattern has matched, or to check for recursion, all subpat-
       terns with the same name are tested. If the condition is true for any  one  of  them,  the
       overall	condition  is  true. This is the same behaviour as testing by number. For further
       details of the interfaces for handling named subpatterns, see the pcreapi documentation.

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


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

	 a literal data character
	 the dot metacharacter
	 the \C escape sequence
	 the \X escape sequence (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  number  of	permitted
       matches,  by  giving the two numbers in curly brackets (braces), separated by a comma. The
       numbers must be less than 65536, and the first must be less than or equal to  the  second.
       For example:


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


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


       matches exactly 8 digits. An opening curly bracket that appears	in  a  position  where	a
       quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken
       as a literal character. For example, {,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 characters, 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 useful for subpatterns	that  are  refer-
       enced  as  subroutines  from  elsewhere	in the pattern (but see also the section entitled
       "Defining subpatterns for use by reference only" below). Items other than subpatterns that
       have a {0} quantifier are omitted from the compiled pattern.

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

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

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


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

       By  default,  the quantifiers are "greedy", that is, they match as much as possible (up to
       the maximum number of permitted times), without causing the rest of the pattern	to  fail.
       The  classic example of where this gives problems is in trying to match comments in C pro-
       grams. These appear between /* and */ and within the comment, individual * and  /  charac-
       ters 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 ques-
       tion mark with its use as a quantifier in its own right. Because it has two uses,  it  can
       sometimes appear doubled, as in


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

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

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

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

       In cases where it is known that the subject string contains no newlines, it is worth  set-
       ting  PCRE_DOTALL  in order to obtain this optimization, or alternatively using ^ to indi-
       cate 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:


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

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


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


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


       With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition,  failure
       of what follows normally causes the repeated item to be re-evaluated to see if a different
       number of repeats allows the rest of the pattern to match. Sometimes it is useful to  pre-
       vent  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


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

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


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

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

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

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


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


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

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

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

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


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


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


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


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

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

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

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

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

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


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

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

	 (sens|respons)e and \1ibility

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


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

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


       A subpattern that is referenced by name may appear in the pattern before or after the ref-

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


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

       Because there may be many capturing parentheses in a pattern, all digits following a back-
       slash are taken as part of a potential back reference number.  If  the  pattern	continues
       with  a	digit  character, some delimiter must be used to terminate the back reference. If
       the PCRE_EXTENDED option is set, this can be 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 sub-
       pattern is first used, so, for example, (a\1) never matches.  However, such references can
       be useful inside repeated subpatterns. For example, the pattern


       matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of the subpat-
       tern, the back reference matches the character string corresponding to the previous itera-
       tion.  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  exam-
       ple above, or by a quantifier with a minimum of zero.

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


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

       More complicated assertions are coded as subpatterns. There are two kinds: those that look
       ahead of the current position in the subject string, and those that  look  behind  it.  An
       assertion  subpattern is matched in the normal way, except that it does not cause the cur-
       rent 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  cap-
       turing subpatterns 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  asser-
       tions. For example,


       matches	a  word followed by a semicolon, but does not include the semicolon in the match,


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


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

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

   Lookbehind assertions

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


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


       is permitted, but


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


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


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

       The  implementation of lookbehind assertions is, for each alternative, to temporarily move
       the current position back by the fixed length and then try to match. If there are insuffi-
       cient characters before the current 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 impossible to calculate the length of the look-
       behind.	The  \X  and \R escapes, which can match different numbers of bytes, are also not

       "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  sim-
       ple pattern such as


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


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


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

   Using multiple assertions

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


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


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

       Assertions can be nested in any combination. For example,


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


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


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


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

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

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

   Checking for a used subpattern by number

       If  the	text  between  the parentheses consists of a sequence of digits, the condition is
       true if a capturing subpattern of that number has previously matched.  If  there  is  more
       than  one  capturing subpattern with the same number (see the earlier section about dupli-
       cate subpattern numbers), the condition is true if any of them have matched.  An  alterna-
       tive  notation  is to precede the digits with a plus or minus sign. In this case, the sub-
       pattern 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

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

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

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

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

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

   Checking for a used subpattern by name

       Perl  uses  the	syntax	(?(<name>)...) or (?('name')...) to test for a used subpattern by
       name. For compatibility with earlier versions of PCRE,  which  had  this  facility  before
       Perl,  the  syntax (?(name)...) is also recognized. However, there is a possible ambiguity
       with this syntax, 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 dig-
       its, PCRE looks for a subpattern of that number, which must be greater  than  zero.  Using
       subpattern names that consist 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 condi-
       tion  is true if a recursive call to the whole pattern or any subpattern has been made. If
       digits or a name preceded by ampersand follow the letter R, for example:

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

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

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

   Defining subpatterns for use by reference only

       If the condition is the string (DEFINE), and there is no subpattern with the name  DEFINE,
       the condition is always false. In this case, there may be only one alternative in the sub-
       pattern. It is always skipped if control reaches this point in the pattern;  the  idea  of
       DEFINE  is  that  it can be used to define "subroutines" that can be referenced from else-
       where. (The use of "subroutines" is described below.) For example, a pattern to	match  an
       IPv4  address  such  as "" 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, insisting on a  word  boundary
       at each end.

   Assertion conditions

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

	 \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

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


       There  are  two ways of including comments in patterns that are processed by PCRE. In both
       cases, the start of the comment must not be in a character class, nor in the middle of any
       other sequence of related characters such as (?: or a subpattern name or number. The char-
       acters 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 paren-
       thesis.	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  immedi-
       ately 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  new-
       line  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  new-
       line 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 com-
       ment. Only an actual character with the code value 0x0a (the default newline) does so.


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

       For some time, Perl has provided a facility that allows	regular  expressions  to  recurse
       (amongst  other	things). It does this by interpolating Perl code in the expression at run
       time, and the code can refer to the expression itself. A Perl pattern using code  interpo-
       lation 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  spe-
       cial syntax for recursion of the entire pattern, and also for individual subpattern recur-
       sion. 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 section.) The special item (?R) or (?0) is a recursive call of the entire regular

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

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

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

       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 eas-
       ier  by the use of relative references. Instead of (?1) in the pattern above you can write
       (?-2) to refer to the second most recently opened parentheses preceding the recursion.  In
       other  words,  a  negative number counts capturing parentheses leftwards from the point at
       which it is encountered.

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

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

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

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

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


       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

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


       the  value  for	the  inner  capturing parentheses (numbered 2) is "ef", which is the last
       value taken on at the top level. If a capturing subpattern  is  not  matched  at  the  top
       level, its final 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

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

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

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

   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 fail-
       ure. 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",


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

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

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


       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 previ-
       ous 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:


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


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


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

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


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


       An earlier example pointed out that the pattern

	 (sens|respons)e and \1ibility

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

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

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

       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:


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


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

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

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


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


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

       PCRE provides a similar feature, but of course it cannot obey  arbitrary  Perl  code.  The
       feature	is  called "callout". The caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout.  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:


       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
       altogether.  A  complete  description of the interface to the callout function is given in
       the pcrecallout documentation.


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

       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 recur-
       sive 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 opening parenthesis  fol-
       lowed  by  an  asterisk.  They are generally of the form (*VERB) or (*VERB:NAME). Some may
       take either form, with differing behaviour, 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  match-
       ing  subject,  or that a particular character must be present. When one of these optimiza-
       tions 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_compile() 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


       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:


       This  matches  "AB",  "AAD",  or "ACD"; when it matches "AB", "B" is captured by the outer

	 (*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 pattern:


       A  match  with  the string "aaaa" always fails, but the callout is taken before each back-
       track 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)

	  0: XY
	 MK: A
	  0: XZ
	 MK: B

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

       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 unanchored 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:

	 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:

	 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 minimum 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  continues  with  what
       follows,  but  if there is no subsequent match, causing a backtrack to the verb, a failure
       is forced. That is, backtracking cannot pass to the left of the verb. However, when one of
       these  verbs appears inside an atomic group, its effect is confined to that group, because
       once the group has been matched, there is never any backtracking into it. In  this  situa-
       tion, 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 backtracking reaches them.


       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:


       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:

	  0: abc
	 No match

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


       This verb causes the match to fail at the current starting position in the subject if  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 alternative 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 succeeds. In an anchored pattern (*PRUNE) has the same effect as (*COM-


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


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


       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 corresponds 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 subsequent 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 character (for an unanchored  pattern).  (*SKIP)  is  similar,	except	that  the
       advance	may  be  more  than one character. (*COMMIT) is the strongest, causing the entire
       match to fail.

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


       Once  A has matched, PCRE is committed to this match, at the current starting position. If
       subsequently B matches, but C does not, the normal  (*THEN)  action  of	trying	the  next
       alternation (that is, D) does not happen because (*COMMIT) overrides.


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


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


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


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