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X11R7.4 - man page for pcrepattern (x11r4 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, you must build PCRE to include
       UTF-8 support, and then call pcre_compile() 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.

       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(). 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 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 non-alphanumeric
       character, it takes away any special meaning that character may have. This  use	of  back-
       slash as an escape character applies both inside and outside character classes.

       For example, if you want to match a * character, you write \* in the pattern.  This 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 \\.

       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.

   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 usually 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 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 backreference
	 \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,
       but \c{ becomes hex 3B, while \c; becomes hex 7B.

       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), and the sequences \R and \X are  interpreted  as  the
       characters "R" and "X", respectively. Outside a character class, these sequences have dif-
       ferent meanings (see below).

   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. The following are
       always recognized:

	 \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

       Each pair of escape sequences partitions the complete set of characters into two  disjoint
       sets. Any given character matches one, and only one, of each pair.

       These  character type sequences can appear both inside and outside character classes. They
       each match one character of the appropriate type. If the current matching point is at  the
       end of the subject string, all of them fail, since 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.

       In UTF-8 mode, characters with values greater than 128 never match  \d,	\s,  or  \w,  and
       always  match \D, \S, and \W. This is true even when Unicode character property support is
       available. These sequences retain their original meanings from before  UTF-8  support  was
       available,  mainly  for	efficiency reasons. Note that this also affects \b, because it is
       defined in terms of \w and \W.

       The sequences \h, \H, \v, and \V  are  Perl  5.10  features.  In  contrast  to  the  other
       sequences,  these  do  match certain high-valued codepoints in UTF-8 mode.  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

       A "word" character is an underscore or any character less than 256 that	is  a  letter  or
       digit.  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 matched by \w. The use of locales with Unicode is discouraged.

   Newline sequences

       Outside	a character class, by default, the escape sequence \R matches any Unicode newline
       sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is equivalent to  the  follow-


       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(), but they can be  over-
       ridden  by  options  given to pcre_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 com-
       bined with a change of newline convention, for example, a pattern can start with:


       Inside a character class, \R matches the letter "R".

   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, and "Any", which matches any character  (including	newline).
       Other 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, Balinese, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aborigi-
       nal,   Cherokee,  Common,  Coptic,  Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,
       Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han,	Hangul,  Hanunoo,
       Hebrew, Hiragana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Lin-
       ear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,	Ogham,	Old_Italic,  Old_Persian,
       Oriya, Osmanya, Phags_Pa, Phoenician, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Taga-
       log, Tagbanwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.

       Each character has exactly one general category property, specified by a two-letter abbre-
       viation.  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

       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).

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

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

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

       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.

   Resetting the match start

       The  escape sequence \K, which is a Perl 5.10 feature, causes any previously matched char-
       acters 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".

   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

       These  assertions  may  not  appear in character classes (but note that \b has a different
       meaning, namely the backspace character, inside a character class).

       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-

       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.


       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, what remains in the  string  may  be  a	malformed
       UTF-8 string. 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. 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 occupy more than one byte. 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 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 types \d, \D, \p, \P, \s, \S, \w, and \W may  also  appear  in  a	character
       class,  and  add  the  characters  that	they  match to the class. For example, [\dABCDEF]
       matches any hexadecimal digit. 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.

       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
	 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.

       In UTF-8 mode, characters with values greater than 128 do not match any of the POSIX char-
       acter classes.


       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 current pattern 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 is
       also the (*UTF8) leading sequence that can be used to set UTF-8 mode; this  is  equivalent
       to setting the PCRE_UTF8 option.


       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 one of the words "cat", "cataract", or "caterpillar". 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-

       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  buffers  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  backreference  or  a recursive call to a numbered subpattern always refers to the first
       one in the pattern with the given number.

       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.

       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 backreferences, 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. This can be useful	for  pat-
       terns  where  only  one	instance  of the named parentheses can match. Suppose you want to
       match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in
       both  cases  you want to extract the abbreviation. This pattern (ignoring the line breaks)
       does the job:


       There are five capturing substrings, but only one is ever set after a match.  (An 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  reference  to  a  non-
       unique  named  subpattern  from	elsewhere in the pattern, the one that corresponds to the
       lowest number is used. For further details of the interfaces for  handling  named  subpat-
       terns, see the pcreapi documentation.

       Warning:  You  cannot  use different names to distinguish between two subpatterns with the
       same number (see the previous section) because PCRE uses only the numbers when matching.


       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 that matches a single character
	 a character class
	 a back reference (see next section)
	 a parenthesized subpattern (unless it is an assertion)

       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. 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 backreference 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, which is a feature introduced in Perl 5.10. 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. Similarly, \g{-2} would be equivalent  to  \1.
       The  use of relative references can be helpful in long patterns, and also in patterns that
       are created by joining together fragments that contain references within themselves.

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

	 (sens|respons)e and \1ibility

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


       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. For exam-
       ple, the pattern


       always fails if it starts to match "a" rather than "bc". Because there may be many captur-
       ing  parentheses  in  a pattern, all digits following the backslash are taken as part of a
       potential back reference number. If the pattern continues with  a  digit  character,  some
       delimiter  must	be  used  to terminate the back reference. If the PCRE_EXTENDED option is
       set, this can be whitespace.  Otherwise an empty comment (see  "Comments"  below)  can  be

       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.


       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.

   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 (at least for 5.8), 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 if rewritten to use two top-level branches:


       In some cases, the Perl 5.10 escape sequence \K (see above) can be used instead of a look-
       behind assertion; this is not restricted to a fixed-length.

       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

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


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


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


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

   Using multiple assertions

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


       matches	"foo" preceded by three digits that are not "999". Notice that each of the 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 previous capturing subpattern matched or not. The two possible forms  of  condi-
       tional 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.

       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  the  capturing  subpattern of that number has previously matched. An alternative
       notation is to precede the digits with a plus or minus sign. In this case, the  subpattern
       number is relative rather than absolute.  The most recently opened parentheses can be ref-
       erenced by (?(-1), the next most recent by (?(-2), and so on. In looping constructs it can
       also make sense to refer to subsequent groups with constructs such as (?(+2).

       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
       the first set of parentheses matched or not. If they did, that is, if subject started with
       an  opening  parenthesis,  the condition is true, and so the yes-pattern is executed and a
       closing parenthesis is required. Otherwise, since no-pattern is not present,  the  subpat-
       tern  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> \( )?    [^()]+    (?(<OPEN>) \) )

   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 the subpattern whose number  or
       name is given. This condition does not check the entire recursion stack.

       At  "top  level",  all  these  recursion test conditions are false. Recursive patterns are
       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 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.


       The sequence (?# marks the start of a comment that continues up to the next closing paren-
       thesis. Nested parentheses are not permitted. The characters that make up a  comment  play
       no part in the pattern matching at all.

       If  the	PCRE_EXTENDED  option  is set, an unescaped # character outside a character class
       introduces a comment that continues to immediately after the next newline in the pattern.


       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 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

       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-

       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.

       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. (A Perl 5.10 feature.)  Instead of (?1) in the pat-
       tern 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 atomic grouping 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 atomic grouping is not used, the match runs  for
       a  very	long time indeed because there are so many different ways the + and * repeats can
       carve up the subject, and all have to be tested before failure can be reported.

       At the end of a match, the values set for any capturing subpatterns  are  those	from  the
       outermost  level  of  the  recursion at which the subpattern value is set.  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 capturing parentheses is "ef", which is the last value taken on at the
       top level. If additional parentheses are added, giving

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

       the string they capture is "ab(cd)ef", the contents of the top level parentheses. 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


       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.

       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().

       The new verbs make use of what was previously invalid syntax: an opening parenthesis  fol-
       lowed  by  an  asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does
       not support the use of arguments, so its general form is just (*VERB). Any number of these
       verbs may occur in a pattern. There are two kinds:

   Verbs that act immediately

       The following verbs act as soon as they are encountered:


       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.  PCRE  differs
       from  Perl  in what happens if the (*ACCEPT) is inside capturing parentheses. In Perl, the
       data so far is captured: in PCRE no data is captured. For example:


       This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is captured.

	 (*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).

   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, a failure is forced.  The verbs differ in
       exactly what kind of failure occurs.


       This verb 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 start 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."


       This verb causes the match to fail at the current position if the rest of the pattern does
       not match. If the pattern is unanchored, the normal "bumpalong" advance to the next start-
       ing character then happens. Backtracking can occur as usual to the left	of  (*PRUNE),  or
       when  matching to the right of (*PRUNE), but if there is no match to the right, backtrack-
       ing 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.


       This verb 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 in 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".


       This verb causes a skip to the next alternation 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

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

       If the COND1 pattern matches, FOO is tried (and possibly further items after  the  end  of
       the  group  if  FOO  succeeds); on failure the matcher skips to the second alternative and
       tries COND2, without backtracking into COND1. If (*THEN) is used outside of  any  alterna-
       tion, it acts exactly like (*PRUNE).


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


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


       Last updated: 11 April 2009
       Copyright (c) 1997-2009 University of Cambridge.


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