/[pcre]/code/trunk/doc/html/pcrepattern.html

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-revision 111 by ph10,
Thu Mar  8 16:53:09 2007 UTC
+revision 507 by ph10,
Wed Mar 10 16:08:01 2010 UTC
 Line 14 
 man page, in case the conversion went wr
  <br>
  <ul>
  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
- <li><a name="TOC2" href="#SEC2">CHARACTERS AND METACHARACTERS</a>
+ <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
- <li><a name="TOC3" href="#SEC3">BACKSLASH</a>
+ <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
- <li><a name="TOC4" href="#SEC4">CIRCUMFLEX AND DOLLAR</a>
+ <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
- <li><a name="TOC5" href="#SEC5">FULL STOP (PERIOD, DOT)</a>
+ <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
- <li><a name="TOC6" href="#SEC6">MATCHING A SINGLE BYTE</a>
+ <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
- <li><a name="TOC7" href="#SEC7">SQUARE BRACKETS AND CHARACTER CLASSES</a>
+ <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
- <li><a name="TOC8" href="#SEC8">POSIX CHARACTER CLASSES</a>
+ <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
- <li><a name="TOC9" href="#SEC9">VERTICAL BAR</a>
+ <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
- <li><a name="TOC10" href="#SEC10">INTERNAL OPTION SETTING</a>
+ <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
- <li><a name="TOC11" href="#SEC11">SUBPATTERNS</a>
+ <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
- <li><a name="TOC12" href="#SEC12">NAMED SUBPATTERNS</a>
+ <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
- <li><a name="TOC13" href="#SEC13">REPETITION</a>
+ <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
- <li><a name="TOC14" href="#SEC14">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
+ <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
- <li><a name="TOC15" href="#SEC15">BACK REFERENCES</a>
+ <li><a name="TOC15" href="#SEC15">REPETITION</a>
- <li><a name="TOC16" href="#SEC16">ASSERTIONS</a>
+ <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
- <li><a name="TOC17" href="#SEC17">CONDITIONAL SUBPATTERNS</a>
+ <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
- <li><a name="TOC18" href="#SEC18">COMMENTS</a>
+ <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
- <li><a name="TOC19" href="#SEC19">RECURSIVE PATTERNS</a>
+ <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
- <li><a name="TOC20" href="#SEC20">SUBPATTERNS AS SUBROUTINES</a>
+ <li><a name="TOC20" href="#SEC20">COMMENTS</a>
- <li><a name="TOC21" href="#SEC21">CALLOUTS</a>
+ <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
- <li><a name="TOC22" href="#SEC22">SEE ALSO</a>
+ <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
- <li><a name="TOC23" href="#SEC23">AUTHOR</a>
+ <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
- <li><a name="TOC24" href="#SEC24">REVISION</a>
+ <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
+ <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
+ <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
+ <li><a name="TOC27" href="#SEC27">AUTHOR</a>
+ <li><a name="TOC28" href="#SEC28">REVISION</a>
  </ul>
  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
  <P>
- The syntax and semantics of the regular expressions supported by PCRE are
+ The syntax and semantics of the regular expressions that are supported by PCRE
- described below. Regular expressions are also described in the Perl
+ are described in detail below. There is a quick-reference syntax summary in the
- documentation and in a number of books, some of which have copious examples.
+ <a href="pcresyntax.html"><b>pcresyntax</b></a>
- Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
+ page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
- regular expressions in great detail. This description of PCRE's regular
+ also supports some alternative regular expression syntax (which does not
- expressions is intended as reference material.
+ conflict with the Perl syntax) in order to provide some compatibility with
+ regular expressions in Python, .NET, and Oniguruma.
+ </P>
+ <P>
+ Perl's regular expressions are described in its own documentation, and
+ regular expressions in general are covered in a number of books, some of which
+ have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
+ published by O'Reilly, covers regular expressions in great detail. This
+ description of PCRE's regular expressions is intended as reference material.
  </P>
  <P>
  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
+ there is now also support for UTF-8 character strings. To use this,
- build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
+ PCRE must be built to include UTF-8 support, and you must call
- the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
+ <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
- places below. There is also a summary of UTF-8 features in the
+ is also a special sequence that can be given at the start of a pattern:
+ <pre>
+   (*UTF8)
+ </pre>
+ Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
+ option. This feature is not Perl-compatible. How setting UTF-8 mode affects
+ pattern matching is mentioned in several places below. There is also a summary
+ of UTF-8 features in the
  <a href="pcre.html#utf8support">section on UTF-8 support</a>
  in the main
  <a href="pcre.html"><b>pcre</b></a>
-Line 63 
 The remainder of this document discusses
+Line 82 
 The remainder of this document discusses
  PCRE when its main matching function, <b>pcre_exec()</b>, is used.
  From release 6.0, PCRE offers a second matching function,
  <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
- Perl-compatible. The advantages and disadvantages of the alternative function,
+ Perl-compatible. Some of the features discussed below are not available when
- and how it differs from the normal function, are discussed in the
+ <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
+ alternative function, and how it differs from the normal function, are
+ discussed in the
  <a href="pcrematching.html"><b>pcrematching</b></a>
  page.
  </P>
- <br><a name="SEC2" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
+ <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
+ <P>
+ 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
+ <a href="pcreapi.html"><b>pcreapi</b></a>
+ page has
+ <a href="pcreapi.html#newlines">further discussion</a>
+ about newlines, and shows how to set the newline convention in the
+ <i>options</i> arguments for the compiling and matching functions.
+ </P>
+ <P>
+ It is also possible to specify a newline convention by starting a pattern
+ string with one of the following five sequences:
+ <pre>
+   (*CR)        carriage return
+   (*LF)        linefeed
+   (*CRLF)      carriage return, followed by linefeed
+   (*ANYCRLF)   any of the three above
+   (*ANY)       all Unicode newline sequences
+ </pre>
+ These override the default and the options given to <b>pcre_compile()</b> or
+ <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
+ newline sequence, the pattern
+ <pre>
+   (*CR)a.b
+ </pre>
+ changes the convention to CR. That pattern matches "a\nb" because LF is no
+ longer a newline. Note that these special settings, which are not
+ Perl-compatible, are recognized only at the very start of a pattern, and that
+ they must be in upper case. If more than one of them is present, the last one
+ is used.
+ </P>
+ <P>
+ 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
+ <a href="#newlineseq">"Newline sequences"</a>
+ below. A change of \R setting can be combined with a change of newline
+ convention.
+ </P>
+ <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
  <P>
  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
-Line 125 
 a character class the only metacharacter
+Line 188 
 a character class the only metacharacter
  </pre>
  The following sections describe the use of each of the metacharacters.
  </P>
- <br><a name="SEC3" href="#TOC1">BACKSLASH</a><br>
+ <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
  <P>
  The backslash character has several uses. Firstly, if it is followed by a
  non-alphanumeric character, it takes away any special meaning that character
-Line 166 
 Non-printing characters
+Line 229 
 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
+ but when a pattern is being prepared by text editing, it is often easier to use
- use one of the following escape sequences than the binary character it
+ one of the following escape sequences than the binary character it represents:
- represents:
  <pre>
    \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        newline (hex 0A)
+   \n        linefeed (hex 0A)
    \r        carriage return (hex 0D)
    \t        tab (hex 09)
-   \ddd      character with octal code ddd, or backreference
+   \ddd      character with octal code ddd, or back reference
    \xhh      character with hex code hh
    \x{hhh..} character with hex code hhh..
  </pre>
-Line 190 
 Thus \cz becomes hex 1A, but \c{ becomes
+Line 252 
 Thus \cz becomes hex 1A, but \c{ becomes
  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 hexadecimal value
+ mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
- is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{
+ hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
- and }, or if there is no terminating }, this form of escape is not recognized.
+ point, which is 10FFFF.
- Instead, the initial \x will be interpreted as a basic hexadecimal escape,
+ </P>
- with no following digits, giving a character whose value is zero.
+ <P>
+ 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.
  </P>
  <P>
  Characters whose value is less than 256 can be defined by either of the two
-Line 252 
 meanings
+Line 318 
 meanings
  Absolute and relative back references
  </b><br>
  <P>
- The sequence \g followed by a positive or negative number, optionally enclosed
+ The sequence \g followed by an unsigned or a negative number, optionally
- in braces, is an absolute or relative back reference. Back references are
+ enclosed in braces, is an absolute or relative back reference. A named back
- discussed
+ reference can be coded as \g{name}. Back references are discussed
  <a href="#backreferences">later,</a>
  following the discussion of
  <a href="#subpattern">parenthesized subpatterns.</a>
  </P>
  <br><b>
+ Absolute and relative subroutine calls
+ </b><br>
+ <P>
+ For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
+ a number enclosed either in angle brackets or single quotes, is an alternative
+ syntax for referencing a subpattern as a "subroutine". Details are discussed
+ <a href="#onigurumasubroutines">later.</a>
+ Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
+ synonymous. The former is a back reference; the latter is a
+ <a href="#subpatternsassubroutines">subroutine</a>
+ call.
+ </P>
+ <br><b>
  Generic character types
  </b><br>
  <P>
-Line 268 
 following are always recognized:
+Line 347 
 following are always recognized:
  <pre>
    \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
  </pre>
-Line 285 
 there is no character to match.
+Line 368 
 there is no character to match.
  <P>
  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
+ 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.)
+ does.
+ </P>
+ <P>
+ 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.
+ </P>
+ <P>
+ 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:
+ <pre>
+   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
+ </pre>
+ The vertical space characters are:
+ <pre>
+   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
+ </PRE>
  </P>
  <P>
  A "word" character is an underscore or any character less than 256 that is a
-Line 297 
 place (see
+Line 424 
 place (see
  <a href="pcreapi.html#localesupport">"Locale support"</a>
  in the
  <a href="pcreapi.html"><b>pcreapi</b></a>
- page). For example, in the "fr_FR" (French) locale, some character codes
+ page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- greater than 128 are used for accented letters, and these are matched by \w.
+ or "french" in Windows, some character codes greater than 128 are used for
- </P>
+ accented letters, and these are matched by \w. The use of locales with Unicode
- <P>
+ is discouraged.
- In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
+ <a name="newlineseq"></a></P>
- \w, and always match \D, \S, and \W. This is true even when Unicode
- character property support is available. The use of locales with Unicode is
- discouraged.
- </P>
  <br><b>
  Newline sequences
  </b><br>
  <P>
- Outside a character class, the escape sequence \R matches any Unicode newline
+ Outside a character class, by default, the escape sequence \R matches any
- sequence. This is an extension to Perl. In non-UTF-8 mode \R is equivalent to
+ Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
- the following:
+ equivalent to the following:
  <pre>
    (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
  </pre>
-Line 331 
 Unicode character property support is no
+Line 454 
 Unicode character property support is no
  recognized.
  </P>
  <P>
+ It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
+ complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
+ either at compile time or when the pattern is matched. (BSR is an abbrevation
+ for "backslash R".) This can be made the default when PCRE is built; if this is
+ the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
+ It is also possible to specify these settings by starting a pattern string with
+ one of the following sequences:
+ <pre>
+   (*BSR_ANYCRLF)   CR, LF, or CRLF only
+   (*BSR_UNICODE)   any Unicode newline sequence
+ </pre>
+ These override the default and the options given to <b>pcre_compile()</b> or
+ <b>pcre_compile2()</b>, but they can be overridden by options given to
+ <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
+ which are not Perl-compatible, are recognized only at the very start of a
+ pattern, and that they must be in upper case. If more than one of them is
+ present, the last one is used. They can be combined with a change of newline
+ convention, for example, a pattern can start with:
+ <pre>
+   (*ANY)(*BSR_ANYCRLF)
+ </pre>
  Inside a character class, \R matches the letter "R".
  <a name="uniextseq"></a></P>
  <br><b>
-Line 338 
 Unicode character properties
+Line 482 
 Unicode character properties
  </b><br>
  <P>
  When PCRE is built with Unicode character property support, three additional
- escape sequences to match character properties are available when UTF-8 mode
+ escape sequences that match characters with specific properties are available.
- is selected. They are:
+ 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:
  <pre>
    \p{<i>xx</i>}   a character with the <i>xx</i> property
    \P{<i>xx</i>}   a character without the <i>xx</i> property
-Line 365 
 Those that are not part of an identified
+Line 511 
 Those that are not part of an identified
  <P>
  Arabic,
  Armenian,
+ Avestan,
  Balinese,
+ Bamum,
  Bengali,
  Bopomofo,
  Braille,
  Buginese,
  Buhid,
  Canadian_Aboriginal,
+ Carian,
+ Cham,
  Cherokee,
  Common,
  Coptic,
-Line 380 
 Cypriot,
+Line 530 
 Cypriot,
  Cyrillic,
  Deseret,
  Devanagari,
+ Egyptian_Hieroglyphs,
  Ethiopic,
  Georgian,
  Glagolitic,
-Line 392 
 Hangul,
+Line 543 
 Hangul,
  Hanunoo,
  Hebrew,
  Hiragana,
+ Imperial_Aramaic,
  Inherited,
+ Inscriptional_Pahlavi,
+ Inscriptional_Parthian,
+ Javanese,
+ Kaithi,
  Kannada,
  Katakana,
+ Kayah_Li,
  Kharoshthi,
  Khmer,
  Lao,
  Latin,
+ Lepcha,
  Limbu,
  Linear_B,
+ Lisu,
+ Lycian,
+ Lydian,
  Malayalam,
+ Meetei_Mayek,
  Mongolian,
  Myanmar,
  New_Tai_Lue,
-Line 409 
 Nko,
+Line 571 
 Nko,
  Ogham,
  Old_Italic,
  Old_Persian,
+ Old_South_Arabian,
+ Old_Turkic,
+ Ol_Chiki,
  Oriya,
  Osmanya,
  Phags_Pa,
  Phoenician,
+ Rejang,
  Runic,
+ Samaritan,
+ Saurashtra,
  Shavian,
  Sinhala,
+ Sundanese,
  Syloti_Nagri,
  Syriac,
  Tagalog,
  Tagbanwa,
  Tai_Le,
+ Tai_Tham,
+ Tai_Viet,
  Tamil,
  Telugu,
  Thaana,
-Line 428 
 Thai,
+Line 599 
 Thai,
  Tibetan,
  Tifinagh,
  Ugaritic,
+ Vai,
  Yi.
  </P>
  <P>
-Line 496 
 the Lu, Ll, or Lt property, in other wor
+Line 668 
 the Lu, Ll, or Lt property, in other wor
  a modifier or "other".
  </P>
  <P>
- The long synonyms for these properties that Perl supports (such as \p{Letter})
+ 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
+ <a href="pcreapi.html"><b>pcreapi</b></a>
+ page). Perl does not support the Cs property.
+ </P>
+ <P>
+ The long synonyms for property names that Perl supports (such as \p{Letter})
  are not supported by PCRE, nor is it permitted to prefix any of these
  properties with "Is".
  </P>
-Line 520 
 or more characters with the "mark" prope
+Line 700 
 or more characters with the "mark" prope
  atomic group
  <a href="#atomicgroup">(see below).</a>
  Characters with the "mark" property are typically accents that affect the
- preceding character.
+ preceding character. None of them have codepoints less than 256, so in
+ non-UTF-8 mode \X matches any one character.
  </P>
  <P>
  Matching characters by Unicode property is not fast, because PCRE has to search
  a structure that contains data for over fifteen thousand characters. That is
  why the traditional escape sequences such as \d and \w do not use Unicode
  properties in PCRE.
+ <a name="resetmatchstart"></a></P>
+ <br><b>
+ Resetting the match start
+ </b><br>
+ <P>
+ The escape sequence \K, which is a Perl 5.10 feature, causes any previously
+ matched characters not to be included in the final matched sequence. For
+ example, the pattern:
+ <pre>
+   foo\Kbar
+ </pre>
+ matches "foobar", but reports that it has matched "bar". This feature is
+ similar to a lookbehind assertion
+ <a href="#lookbehind">(described below).</a>
+ 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
+ <a href="#subpattern">captured substrings.</a>
+ For example, when the pattern
+ <pre>
+   (foo)\Kbar
+ </pre>
+ matches "foobar", the first substring is still set to "foo".
+ </P>
+ <P>
+ Perl documents that the use of \K within assertions is "not well defined". In
+ PCRE, \K is acted upon when it occurs inside positive assertions, but is
+ ignored in negative assertions.
  <a name="smallassertions"></a></P>
  <br><b>
  Simple assertions
-Line 554 
 different meaning, namely the backspace
+Line 763 
 different meaning, namely the backspace
  A word boundary is a position in the subject string where the current character
  and the previous character do not both match \w or \W (i.e. one matches
  \w and the other matches \W), or the start or end of the string if the
- first or last character matches \w, respectively.
+ first or last character matches \w, respectively. Neither PCRE nor Perl has a
+ separte "start of word" or "end of word" metasequence. However, whatever
+ follows \b normally determines which it is. For example, the fragment
+ \ba matches "a" at the start of a word.
  </P>
  <P>
  The \A, \Z, and \z assertions differ from the traditional circumflex and
-Line 588 
 If all the alternatives of a pattern beg
+Line 800 
 If all the alternatives of a pattern beg
  to the starting match position, and the "anchored" flag is set in the compiled
  regular expression.
  </P>
- <br><a name="SEC4" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
+ <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
  <P>
  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
-Line 642 
 Note that the sequences \A, \Z, and \z c
+Line 854 
 Note that the sequences \A, \Z, and \z c
  end of the subject in both modes, and if all branches of a pattern start with
  \A it is always anchored, whether or not PCRE_MULTILINE is set.
  </P>
- <br><a name="SEC5" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
+ <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
  <P>
  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 667 
 The handling of dot is entirely independ
+Line 879 
 The handling of dot is entirely independ
  dollar, the only relationship being that they both involve newlines. Dot has no
  special meaning in a character class.
  </P>
- <br><a name="SEC6" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
+ <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
  <P>
  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
-Line 682 
 PCRE does not allow \C to appear in look
+Line 894 
 PCRE does not allow \C to appear in look
  because in UTF-8 mode this would make it impossible to calculate the length of
  the lookbehind.
  <a name="characterclass"></a></P>
- <br><a name="SEC7" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
+ <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
  <P>
  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
+ square bracket. A closing square bracket on its own is not special by default.
- closing square bracket is required as a member of the class, it should be the
+ However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
- first data character in the class (after an initial circumflex, if present) or
+ bracket causes a compile-time error. If a closing square bracket is required as
- escaped with a backslash.
+ 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.
  </P>
  <P>
  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
+ character may be more than one byte long. A matched character must be in the
- of characters defined by the class, unless the first character in the class
+ set of characters defined by the class, unless the first character in the class
  definition is a circumflex, in which case the subject character must not be in
  the set defined by the class. If a circumflex is actually required as a member
  of the class, ensure it is not the first character, or escape it with a
-Line 704 
 For example, the character class [aeiou]
+Line 917 
 For example, the character class [aeiou]
  [^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
+ 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.
  </P>
-Line 720 
 caseful version would. In UTF-8 mode, PC
+Line 933 
 caseful version would. In UTF-8 mode, PC
  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
+ If you want to use caseless matching in UTF8-mode for characters 128 and above,
- ensure that PCRE is compiled with Unicode property support as well as with
+ you must ensure that PCRE is compiled with Unicode property support as well as
- UTF-8 support.
+ with UTF-8 support.
  </P>
  <P>
  Characters that might indicate line breaks are never treated in any special way
-Line 756 
 example [\x{100}-\x{2ff}].
+Line 969 
 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 the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E
+ 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.
-Line 777 
 introducing a POSIX class name - see the
+Line 990 
 introducing a POSIX class name - see the
  closing square bracket. However, escaping other non-alphanumeric characters
  does no harm.
  </P>
- <br><a name="SEC8" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
+ <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
  <P>
  Perl supports the POSIX notation for character classes. This uses names
  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
-Line 823 
 supported, and an error is given if they
+Line 1036 
 supported, and an error is given if they
  In UTF-8 mode, characters with values greater than 128 do not match any of
  the POSIX character classes.
  </P>
- <br><a name="SEC9" href="#TOC1">VERTICAL BAR</a><br>
+ <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
  <P>
  Vertical bar characters are used to separate alternative patterns. For example,
  the pattern
-Line 838 
 that succeeds is used. If the alternativ
+Line 1051 
 that succeeds is used. If the alternativ
  "succeeds" means matching the rest of the main pattern as well as the
  alternative in the subpattern.
  </P>
- <br><a name="SEC10" href="#TOC1">INTERNAL OPTION SETTING</a><br>
+ <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
  <P>
  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
- PCRE_EXTENDED options can be changed from within the pattern by a sequence of
+ PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
- Perl option letters enclosed between "(?" and ")". The option letters are
+ the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
+ The option letters are
  <pre>
    i  for PCRE_CASELESS
    m  for PCRE_MULTILINE
-Line 857 
 permitted. If a letter appears both befo
+Line 1071 
 permitted. If a letter appears both befo
  unset.
  </P>
  <P>
- When an option change occurs at top level (that is, not inside subpattern
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- parentheses), the change applies to the remainder of the pattern that follows.
+ changed in the same way as the Perl-compatible options by using the characters
- If the change is placed right at the start of a pattern, PCRE extracts it into
+ J, U and X respectively.
- the global options (and it will therefore show up in data extracted by the
+ </P>
- <b>pcre_fullinfo()</b> function).
+ <P>
+ 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 <b>pcre_fullinfo()</b> function).
  </P>
  <P>
  An option change within a subpattern (see below for a description of
-Line 882 
 option settings happen at compile time.
+Line 1101 
 option settings happen at compile time.
  behaviour otherwise.
  </P>
  <P>
- The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
+ <b>Note:</b> There are other PCRE-specific options that can be set by the
- changed in the same way as the Perl-compatible options by using the characters
+ application when the compile or match functions are called. In some cases the
- J, U and X respectively.
+ pattern can contain special leading sequences such as (*CRLF) to override what
+ the application has set or what has been defaulted. Details are given in the
+ section entitled
+ <a href="#newlineseq">"Newline sequences"</a>
+ 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.
  <a name="subpattern"></a></P>
- <br><a name="SEC11" href="#TOC1">SUBPATTERNS</a><br>
+ <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
  <P>
  Subpatterns are delimited by parentheses (round brackets), which can be nested.
  Turning part of a pattern into a subpattern does two things:
-Line 939 
 match exactly the same set of strings. B
+Line 1163 
 match exactly the same set of strings. B
  from left to right, and options are not reset until the end of the subpattern
  is reached, an option setting in one branch does affect subsequent branches, so
  the above patterns match "SUNDAY" as well as "Saturday".
+ <a name="dupsubpatternnumber"></a></P>
+ <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
+ <P>
+ Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
+ the same numbers for its capturing parentheses. Such a subpattern starts with
+ (?| and is itself a non-capturing subpattern. For example, consider this
+ pattern:
+ <pre>
+   (?|(Sat)ur|(Sun))day
+ </pre>
+ Because the two alternatives are inside a (?| group, both sets of capturing
+ parentheses are numbered one. Thus, when the pattern matches, you can look
+ at captured substring number one, whichever alternative matched. This construct
+ is useful when you want to capture part, but not all, of one of a number of
+ alternatives. Inside a (?| group, parentheses are numbered as usual, but the
+ number is reset at the start of each branch. The numbers of any capturing
+ 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 underneath show in which buffer the captured content will be
+ stored.
+ <pre>
+   # before  ---------------branch-reset----------- after
+   / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
+   # 1            2         2  3        2     3     4
+ </pre>
+ A back reference to a numbered subpattern uses the most recent value that is
+ set for that number by any subpattern. The following pattern matches "abcabc"
+ or "defdef":
+ <pre>
+   /(?|(abc)|(def))\1/
+ </pre>
+ In contrast, a recursive or "subroutine" call to a numbered subpattern always
+ refers to the first one in the pattern with the given number. The following
+ pattern matches "abcabc" or "defabc":
+ <pre>
+   /(?|(abc)|(def))(?1)/
+ </pre>
+ If a
+ <a href="#conditions">condition test</a>
+ for a subpattern's having matched refers to a non-unique number, the test is
+ true if any of the subpatterns of that number have matched.
  </P>
- <br><a name="SEC12" href="#TOC1">NAMED SUBPATTERNS</a><br>
+ <P>
+ An alternative approach to using this "branch reset" feature is to use
+ duplicate named subpatterns, as described in the next section.
+ </P>
+ <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
  <P>
  Identifying capturing parentheses by number is simple, but it can be very hard
  to keep track of the numbers in complicated regular expressions. Furthermore,
-Line 948 
 if an expression is modified, the number
+Line 1217 
 if an expression is modified, the number
  difficulty, PCRE supports the naming of subpatterns. This feature was not
  added to Perl until release 5.10. Python had the feature earlier, and PCRE
  introduced it at release 4.0, using the Python syntax. PCRE now supports both
- the Perl and the Python syntax.
+ the Perl and the Python syntax. Perl allows identically numbered subpatterns to
+ have different names, but PCRE does not.
  </P>
  <P>
  In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
  (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
  parentheses from other parts of the pattern, such as
- <a href="#backreferences">backreferences,</a>
+ <a href="#backreferences">back references,</a>
  <a href="#recursion">recursion,</a>
  and
  <a href="#conditions">conditions,</a>
-Line 969 
 is also a convenience function for extra
+Line 1239 
 is also a convenience function for extra
  </P>
  <P>
  By default, a name must be unique within a pattern, but it is possible to relax
- this constraint by setting the PCRE_DUPNAMES option at compile time. This can
+ this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
- be useful for patterns where only one instance of the named parentheses can
+ names are also always permitted for subpatterns with the same number, set up as
- match. Suppose you want to match the name of a weekday, either as a 3-letter
+ described in the previous section.) Duplicate names can be useful for patterns
- abbreviation or as the full name, and in both cases you want to extract the
+ where only one instance of the named parentheses can match. Suppose you want to
- abbreviation. This pattern (ignoring the line breaks) does the job:
+ 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:
  <pre>
    (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
    (?&#60;DN&#62;Tue)(?:sday)?|
-Line 982 
 abbreviation. This pattern (ignoring the
+Line 1254 
 abbreviation. This pattern (ignoring the
    (?&#60;DN&#62;Sat)(?:urday)?
  </pre>
  There are five capturing substrings, but only one is ever set after a match.
+ (An alternative way of solving this problem is to use a "branch reset"
+ subpattern, as described in the previous section.)
+ </P>
+ <P>
  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
+ matched. This saves searching to find which numbered subpattern it was.
- make a reference to a non-unique named subpattern from elsewhere in the
+ </P>
- pattern, the one that corresponds to the lowest number is used. For further
+ <P>
- details of the interfaces for handling named subpatterns, see the
+ If you make a back reference to a non-unique named subpattern from elsewhere in
+ the pattern, the one that corresponds to the first occurrence of the name is
+ used. In the absence of duplicate numbers (see the previous section) this is
+ the one with the lowest number. If you use a named reference in a condition
+ test (see the
+ <a href="#conditions">section about conditions</a>
+ below), either to check whether a subpattern has matched, or to check for
+ recursion, all subpatterns with the same name are tested. If the condition is
+ true for any one of them, the overall condition is true. This is the same
+ behaviour as testing by number. For further details of the interfaces for
+ handling named subpatterns, see the
  <a href="pcreapi.html"><b>pcreapi</b></a>
  documentation.
  </P>
- <br><a name="SEC13" href="#TOC1">REPETITION</a><br>
+ <P>
+ <b>Warning:</b> You cannot use different names to distinguish between two
+ subpatterns with the same number because PCRE uses only the numbers when
+ matching. For this reason, an error is given at compile time if different names
+ are given to subpatterns with the same number. However, you can give the same
+ name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
+ </P>
+ <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
  <P>
  Repetition is specified by quantifiers, which can follow any of the following
  items:
-Line 1005 
 items:
+Line 1298 
 items:
    a character class
    a back reference (see next section)
    a parenthesized subpattern (unless it is an assertion)
+   a recursive or "subroutine" call to a subpattern
  </pre>
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
-Line 1038 
 which may be several bytes long (and the
+Line 1332 
 which may be several bytes long (and the
  </P>
  <P>
  The quantifier {0} is permitted, causing the expression to behave as if the
- previous item and the quantifier were not present.
+ previous item and the quantifier were not present. This may be useful for
+ subpatterns that are referenced as
+ <a href="#subpatternsassubroutines">subroutines</a>
+ from elsewhere in the pattern. Items other than subpatterns that have a {0}
+ quantifier are omitted from the compiled pattern.
  </P>
  <P>
  For convenience, the three most common quantifiers have single-character
-Line 1118 
 alternatively using ^ to indicate anchor
+Line 1416 
 alternatively using ^ to indicate anchor
  </P>
  <P>
  However, there is one situation where the optimization cannot be used. When .*
- is inside capturing parentheses that are the subject of a backreference
+ is inside capturing parentheses that are the subject of a back reference
  elsewhere in the pattern, a match at the start may fail where a later one
  succeeds. Consider, for example:
  <pre>
-Line 1142 
 example, after
+Line 1440 
 example, after
  </pre>
  matches "aba" the value of the second captured substring is "b".
  <a name="atomicgroup"></a></P>
- <br><a name="SEC14" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
+ <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
  <P>
  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
  repetition, failure of what follows normally causes the repeated item to be
-Line 1196 
 previous example can be rewritten as
+Line 1494 
 previous example can be rewritten as
  <pre>
    \d++foo
  </pre>
+ Note that a possessive quantifier can be used with an entire group, for
+ example:
+ <pre>
+   (abc|xyz){2,3}+
+ </pre>
  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
-Line 1241 
 an atomic group, like this:
+Line 1544 
 an atomic group, like this:
  </pre>
  sequences of non-digits cannot be broken, and failure happens quickly.
  <a name="backreferences"></a></P>
- <br><a name="SEC15" href="#TOC1">BACK REFERENCES</a><br>
+ <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
  <P>
  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
-Line 1270 
 subpattern is possible using named paren
+Line 1573 
 subpattern is possible using named paren
  <P>
  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 a positive or a negative number,
+ Perl 5.10. This escape must be followed by an unsigned number or a negative
- optionally enclosed in braces. These examples are all identical:
+ number, optionally enclosed in braces. These examples are all identical:
  <pre>
    (ring), \1
    (ring), \g1
    (ring), \g{1}
  </pre>
- A positive number specifies an absolute reference without the ambiguity that is
+ An unsigned number specifies an absolute reference without the ambiguity that
- present in the older syntax. It is also useful when literal digits follow the
+ is present in the older syntax. It is also useful when literal digits follow
- reference. A negative number is a relative reference. Consider this example:
+ the reference. A negative number is a relative reference. Consider this
+ example:
  <pre>
    (abc(def)ghi)\g{-1}
  </pre>
-Line 1308 
 matches "rah rah" and "RAH RAH", but not
+Line 1612 
 matches "rah rah" and "RAH RAH", but not
  capturing subpattern is matched caselessly.
  </P>
  <P>
- Back references to named subpatterns use the Perl syntax \k&#60;name&#62; or \k'name'
+ There are several different ways of writing back references to named
- or the Python syntax (?P=name). We could rewrite the above example in either of
+ subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; 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:
  <pre>
    (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
+   (?'p1'(?i)rah)\s+\k{p1}
    (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
+   (?&#60;p1&#62;(?i)rah)\s+\g{p1}
  </pre>
  A subpattern that is referenced by name may appear in the pattern before or
  after the reference.
-Line 1321 
 after the reference.
+Line 1630 
 after the reference.
  <P>
  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 example, the pattern
+ references to it always fail by default. For example, the pattern
  <pre>
    (a|(bc))\2
  </pre>
- always fails if it starts to match "a" rather than "bc". Because there may be
+ always fails if it starts to match "a" rather than "bc". However, if the
- many capturing parentheses in a pattern, all digits following the backslash are
+ PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
- taken as part of a potential back reference number. If the pattern continues
+ unset value matches an empty string.
- with a digit character, some delimiter must be used to terminate the back
+ </P>
- reference. If the PCRE_EXTENDED option is set, this can be whitespace.
+ <P>
- Otherwise an empty comment (see
+ Because there may be many capturing parentheses in a pattern, all digits
+ following a backslash are taken as part of a potential back reference number.
+ If the pattern continues with a digit character, some delimiter must be used to
+ terminate the back reference. If the PCRE_EXTENDED option is set, this can be
+ whitespace. Otherwise, the \g{ syntax or an empty comment (see
  <a href="#comments">"Comments"</a>
  below) can be used.
  </P>
+ <br><b>
+ Recursive back references
+ </b><br>
  <P>
  A back reference that occurs inside the parentheses to which it refers fails
  when the subpattern is first used, so, for example, (a\1) never matches.
-Line 1348 
 to the previous iteration. In order for
+Line 1664 
 to the previous iteration. In order for
  that the first iteration does not need to match the back reference. This can be
  done using alternation, as in the example above, or by a quantifier with a
  minimum of zero.
+ </P>
+ <P>
+ Back references of this type cause the group that they reference to be treated
+ as an
+ <a href="#atomicgroup">atomic group.</a>
+ Once the whole group has been matched, a subsequent matching failure cannot
+ cause backtracking into the middle of the group.
  <a name="bigassertions"></a></P>
- <br><a name="SEC16" href="#TOC1">ASSERTIONS</a><br>
+ <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
  <P>
  An assertion is a test on the characters following or preceding the current
  matching point that does not actually consume any characters. The simple
-Line 1398 
 lookbehind assertion is needed to achiev
+Line 1721 
 lookbehind assertion is needed to achiev
  If you want to force a matching failure at some point in a pattern, the most
  convenient way to do it is with (?!) because an empty string always matches, so
  an assertion that requires there not to be an empty string must always fail.
+ The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
+ synonym for (?!).
  <a name="lookbehind"></a></P>
  <br><b>
  Lookbehind assertions
-Line 1421 
 is permitted, but
+Line 1746 
 is permitted, but
  </pre>
  causes an error at compile time. Branches that match different length strings
  are permitted only at the top level of a lookbehind assertion. This is an
- extension compared with Perl (at least for 5.8), which requires all branches to
+ extension compared with Perl (5.8 and 5.10), which requires all branches to
  match the same length of string. An assertion such as
  <pre>
    (?&#60;=ab(c|de))
  </pre>
  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:
+ lengths, but it is acceptable to PCRE if rewritten to use two top-level
+ branches:
  <pre>
    (?&#60;=abc|abde)
  </pre>
+ In some cases, the Perl 5.10 escape sequence \K
+ <a href="#resetmatchstart">(see above)</a>
+ can be used instead of a lookbehind assertion to get round the fixed-length
+ restriction.
+ </P>
+ <P>
  The implementation of lookbehind assertions is, for each alternative, to
  temporarily move the current position back by the fixed length and then try to
  match. If there are insufficient characters before the current position, the
-Line 1443 
 the length of the lookbehind. The \X and
+Line 1775 
 the length of the lookbehind. The \X and
  different numbers of bytes, are also not permitted.
  </P>
  <P>
+ <a href="#subpatternsassubroutines">"Subroutine"</a>
+ calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
+ as the subpattern matches a fixed-length string.
+ <a href="#recursion">Recursion,</a>
+ however, is not supported.
+ </P>
+ <P>
  Possessive quantifiers can be used in conjunction with lookbehind assertions to
- specify efficient matching at the end of the subject string. Consider a simple
+ specify efficient matching of fixed-length strings at the end of subject
- pattern such as
+ strings. Consider a simple pattern such as
  <pre>
    abcd$
  </pre>
-Line 1503 
 preceded by "foo", while
+Line 1842 
 preceded by "foo", while
  is another pattern that matches "foo" preceded by three digits and any three
  characters that are not "999".
  <a name="conditions"></a></P>
- <br><a name="SEC17" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
+ <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
  <P>
  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
+ the result of an assertion, or whether a specific capturing subpattern has
- or not. The two possible forms of conditional subpattern are
+ already been matched. The two possible forms of conditional subpattern are:
  <pre>
    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)
-Line 1526 
 Checking for a used subpattern by number
+Line 1865 
 Checking for a used subpattern by number
  </b><br>
  <P>
  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
+ condition is true if a capturing subpattern of that number has previously
- matched.
+ matched. If there is more than one capturing subpattern with the same number
+ (see the earlier
+ <a href="#recursion">section about duplicate subpattern numbers),</a>
+ the condition is true if any of them have been set. An alternative notation is
+ to precede the digits with a plus or minus sign. In this case, the subpattern
+ number is relative rather than absolute. The most recently opened parentheses
+ can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
+ looping constructs it can also make sense to refer to subsequent groups with
+ constructs such as (?(+2).
  </P>
  <P>
  Consider the following pattern, which contains non-significant white space to
-Line 1546 
 parenthesis is required. Otherwise, sinc
+Line 1893 
 parenthesis is required. Otherwise, sinc
  subpattern matches nothing. In other words, this pattern matches a sequence of
  non-parentheses, optionally enclosed in parentheses.
  </P>
+ <P>
+ If you were embedding this pattern in a larger one, you could use a relative
+ reference:
+ <pre>
+   ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
+ </pre>
+ This makes the fragment independent of the parentheses in the larger pattern.
+ </P>
  <br><b>
  Checking for a used subpattern by name
  </b><br>
-Line 1563 
 names that consist entirely of digits is
+Line 1918 
 names that consist entirely of digits is
  Rewriting the above example to use a named subpattern gives this:
  <pre>
    (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
+ </pre>
- </PRE>
+ If the name used in a condition of this kind is a duplicate, the test is
+ applied to all subpatterns of the same name, and is true if any one of them has
+ matched.
  </P>
  <br><b>
  Checking for pattern recursion
-Line 1577 
 letter R, for example:
+Line 1934 
 letter R, for example:
  <pre>
    (?(R3)...) or (?(R&name)...)
  </pre>
- the condition is true if the most recent recursion is into the subpattern whose
+ the condition is true if the most recent recursion is into a subpattern whose
  number or name is given. This condition does not check the entire recursion
- stack.
+ stack. If the name used in a condition of this kind is a duplicate, the test is
+ applied to all subpatterns of the same name, and is true if any one of them is
+ the most recent recursion.
  </P>
  <P>
- At "top level", all these recursion test conditions are false. Recursive
+ At "top level", all these recursion test conditions are false.
- patterns are described below.
+ <a href="#recursion">The syntax for recursive patterns</a>
+ is described below.
  </P>
  <br><b>
  Defining subpatterns for use by reference only
-Line 1593 
 If the condition is the string (DEFINE),
+Line 1953 
 If the condition is the string (DEFINE),
  name DEFINE, the condition is always false. In this case, there may be only one
  alternative in the subpattern. It is always skipped if control reaches this
  point in the pattern; the idea of DEFINE is that it can be used to define
- "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
+ "subroutines" that can be referenced from elsewhere. (The use of
+ <a href="#subpatternsassubroutines">"subroutines"</a>
  is described below.) For example, a pattern to match an IPv4 address could be
  written like this (ignore whitespace and line breaks):
  <pre>
-Line 1603 
 written like this (ignore whitespace and
+Line 1964 
 written like this (ignore whitespace and
  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.
+ pattern is skipped because DEFINE acts like a false condition. The rest of the
- </P>
+ pattern uses references to the named group to match the four dot-separated
- <P>
+ components of an IPv4 address, insisting on a word boundary at each end.
- 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.
  </P>
  <br><b>
  Assertion conditions
-Line 1629 
 subject is matched against the first alt
+Line 1987 
 subject is matched against the first alt
  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.
  <a name="comments"></a></P>
- <br><a name="SEC18" href="#TOC1">COMMENTS</a><br>
+ <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
  <P>
  The sequence (?# marks the start of a comment that continues up to the next
  closing parenthesis. Nested parentheses are not permitted. The characters
-Line 1640 
 If the PCRE_EXTENDED option is set, an u
+Line 1998 
 If the PCRE_EXTENDED option is set, an u
  character class introduces a comment that continues to immediately after the
  next newline in the pattern.
  <a name="recursion"></a></P>
- <br><a name="SEC19" href="#TOC1">RECURSIVE PATTERNS</a><br>
+ <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
  <P>
  Consider the problem of matching a string in parentheses, allowing for
  unlimited nested parentheses. Without the use of recursion, the best that can
-Line 1663 
 recursively to the pattern in which it a
+Line 2021 
 recursively to the pattern in which it a
  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
  supports special syntax for recursion of the entire pattern, and also for
  individual subpattern recursion. After its introduction in PCRE and Python,
- this kind of recursion was introduced into Perl at release 5.10.
+ this kind of recursion was subsequently introduced into Perl at release 5.10.
  </P>
  <P>
  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"
+ provided that it occurs inside that subpattern. (If not, it is a
+ <a href="#subpatternsassubroutines">"subroutine"</a>
  call, which is described in the next section.) The special item (?R) or (?0) is
  a recursive call of the entire regular expression.
  </P>
  <P>
- In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
- treated as an atomic group. That is, once it has matched some of the subject
- string, it is never re-entered, even if it contains untried alternatives and
- there is a subsequent matching failure.
- </P>
- <P>
  This PCRE pattern solves the nested parentheses problem (assume the
  PCRE_EXTENDED option is set so that white space is ignored):
  <pre>
-   \( ( (?&#62;[^()]+) | (?R) )* \)
+   \( ( [^()]++ | (?R) )* \)
  </pre>
  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.
+ Finally there is a closing parenthesis. Note the use of a possessive quantifier
+ to avoid backtracking into sequences of non-parentheses.
  </P>
  <P>
  If this were part of a larger pattern, you would not want to recurse the entire
  pattern, so instead you could use this:
  <pre>
-   ( \( ( (?&#62;[^()]+) | (?1) )* \) )
+   ( \( ( [^()]++ | (?1) )* \) )
  </pre>
  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
+ them instead of the whole pattern.
- parenthesis numbers can be tricky. It may be more convenient to use named
+ </P>
- parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier
+ <P>
- syntax (?P&#62;name) is also supported. We could rewrite the above example as
+ In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- follows:
+ is made easier by the use of relative references (a Perl 5.10 feature).
+ Instead of (?1) in the pattern above you can write (?-2) to refer to the second
+ most recently opened parentheses preceding the recursion. In other words, a
+ negative number counts capturing parentheses leftwards from the point at which
+ it is encountered.
+ </P>
+ <P>
+ 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
+ <a href="#subpatternsassubroutines">"subroutine"</a>
+ calls, as described in the next section.
+ </P>
+ <P>
+ An alternative approach is to use named parentheses instead. The Perl syntax
+ for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
+ could rewrite the above example as follows:
  <pre>
-   (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
+   (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
  </pre>
  If there is more than one subpattern with the same name, the earliest one is
- used. This particular example pattern contains nested unlimited repeats, and so
+ used.
- the use of atomic grouping for matching strings of non-parentheses is important
+ </P>
- when applying the pattern to strings that do not match. For example, when this
+ <P>
- pattern is applied to
+ This particular example pattern that we have been looking at contains nested
+ unlimited repeats, and so the use of a possessive quantifier for matching
+ strings of non-parentheses is important when applying the pattern to strings
+ that do not match. For example, when this pattern is applied to
  <pre>
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
  </pre>
- it yields "no match" quickly. However, if atomic grouping is not used,
+ it yields "no match" quickly. However, if a possessive quantifier is not used,
  the match runs for a very long time indeed because there are so many different
  ways the + and * repeats can carve up the subject, and all have to be tested
  before failure can be reported.
  </P>
  <P>
- At the end of a match, the values set for any capturing subpatterns are those
+ At the end of a match, the values of capturing parentheses are those from
- from the outermost level of the recursion at which the subpattern value is set.
+ the outermost level. If you want to obtain intermediate values, a callout
- If you want to obtain intermediate values, a callout function can be used (see
+ function can be used (see below and the
- below and the
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation). If the pattern above is matched against
  <pre>
    (ab(cd)ef)
  </pre>
- the value for the capturing parentheses is "ef", which is the last value taken
+ the value for the inner capturing parentheses (numbered 2) is "ef", which is
- on at the top level. If additional parentheses are added, giving
+ the last value taken on at the top level. If a capturing subpattern is not
- <pre>
+ matched at the top level, its final value is unset, even if it is (temporarily)
-   \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
+ set at a deeper level.
-      ^                        ^
+ </P>
-      ^                        ^
+ <P>
- </pre>
+ If there are more than 15 capturing parentheses in a pattern, PCRE has to
- the string they capture is "ab(cd)ef", the contents of the top level
+ obtain extra memory to store data during a recursion, which it does by using
- parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
+ <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
- has to obtain extra memory to store data during a recursion, which it does by
+ be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
- using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
- memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
  </P>
  <P>
  Do not confuse the (?R) item with the condition (R), which tests for recursion.
-Line 1751 
 recursing), whereas any characters are p
+Line 2121 
 recursing), whereas any characters are p
  In this pattern, (?(R) is the start of a conditional subpattern, with two
  different alternatives for the recursive and non-recursive cases. The (?R) item
  is the actual recursive call.
+ <a name="recursiondifference"></a></P>
+ <br><b>
+ Recursion difference from Perl
+ </b><br>
+ <P>
+ In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
+ treated as an atomic group. That is, once it has matched some of the subject
+ string, it is never re-entered, even if it contains untried alternatives and
+ there is a subsequent matching failure. This can be illustrated by the
+ following pattern, which purports to match a palindromic string that contains
+ an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
+ <pre>
+   ^(.|(.)(?1)\2)$
+ </pre>
+ The idea is that it either matches a single character, or two identical
+ characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
+ it does not if the pattern is longer than three characters. Consider the
+ subject string "abcba":
+ </P>
+ <P>
+ At the top level, the first character is matched, but as it is not at the end
+ of the string, the first alternative fails; the second alternative is taken
+ and the recursion kicks in. The recursive call to subpattern 1 successfully
+ matches the next character ("b"). (Note that the beginning and end of line
+ tests are not part of the recursion).
+ </P>
+ <P>
+ Back at the top level, the next character ("c") is compared with what
+ subpattern 2 matched, which was "a". This fails. Because the recursion is
+ treated as an atomic group, there are now no backtracking points, and so the
+ entire match fails. (Perl is able, at this point, to re-enter the recursion and
+ try the second alternative.) However, if the pattern is written with the
+ alternatives in the other order, things are different:
+ <pre>
+   ^((.)(?1)\2|.)$
+ </pre>
+ This time, the recursing alternative is tried first, and continues to recurse
+ until it runs out of characters, at which point the recursion fails. But this
+ time we do have another alternative to try at the higher level. That is the big
+ difference: in the previous case the remaining alternative is at a deeper
+ recursion level, which PCRE cannot use.
+ </P>
+ <P>
+ To change the pattern so that matches all palindromic strings, not just those
+ with an odd number of characters, it is tempting to change the pattern to this:
+ <pre>
+   ^((.)(?1)\2|.?)$
+ </pre>
+ Again, this works in Perl, but not in PCRE, and for the same reason. When a
+ deeper recursion has matched a single character, it cannot be entered again in
+ order to match an empty string. The solution is to separate the two cases, and
+ write out the odd and even cases as alternatives at the higher level:
+ <pre>
+   ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
+ </pre>
+ If you want to match typical palindromic phrases, the pattern has to ignore all
+ non-word characters, which can be done like this:
+ <pre>
+   ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
+ </pre>
+ If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
+ man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
+ the use of the possessive quantifier *+ to avoid backtracking into sequences of
+ non-word characters. Without this, PCRE takes a great deal longer (ten times or
+ more) to match typical phrases, and Perl takes so long that you think it has
+ gone into a loop.
+ </P>
+ <P>
+ <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
+ string does not start with a palindrome that is shorter than the entire string.
+ For example, although "abcba" is correctly matched, if the subject is "ababa",
+ PCRE finds the palindrome "aba" at the start, then fails at top level because
+ the end of the string does not follow. Once again, it cannot jump back into the
+ recursion to try other alternatives, so the entire match fails.
  <a name="subpatternsassubroutines"></a></P>
- <br><a name="SEC20" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
+ <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
  <P>
  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. An earlier example pointed out that the pattern
+ before or after the reference. A numbered reference can be absolute or
+ relative, as in these examples:
+ <pre>
+   (...(absolute)...)...(?2)...
+   (...(relative)...)...(?-1)...
+   (...(?+1)...(relative)...
+ </pre>
+ An earlier example pointed out that the pattern
  <pre>
    (sens|respons)e and \1ibility
  </pre>
-Line 1770 
 is used, it does match "sense and respon
+Line 2221 
 is used, it does match "sense and respon
  strings. Another example is given in the discussion of DEFINE above.
  </P>
  <P>
- Like recursive subpatterns, a "subroutine" call is always treated as an atomic
+ 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.
+ matching failure. Any capturing parentheses that are set during the subroutine
+ call revert to their previous values afterwards.
  </P>
  <P>
  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:
  <pre>
-   (abc)(?i:(?1))
+   (abc)(?i:(?-1))
  </pre>
  It matches "abcabc". It does not match "abcABC" because the change of
  processing option does not affect the called subpattern.
+ <a name="onigurumasubroutines"></a></P>
+ <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
+ <P>
+ For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
+ a number enclosed either in angle brackets or single quotes, is an alternative
+ syntax for referencing a subpattern as a subroutine, possibly recursively. Here
+ are two of the examples used above, rewritten using this syntax:
+ <pre>
+   (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
+   (sens|respons)e and \g'1'ibility
+ </pre>
+ 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:
+ <pre>
+   (abc)(?i:\g&#60;-1&#62;)
+ </pre>
+ Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
+ synonymous. The former is a back reference; the latter is a subroutine call.
  </P>
- <br><a name="SEC21" href="#TOC1">CALLOUTS</a><br>
+ <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
  <P>
  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
-Line 1804 
 function is to be called. If you want to
+Line 2274 
 function is to be called. If you want to
  can put a number less than 256 after the letter C. The default value is zero.
  For example, this pattern has two callout points:
  <pre>
-   (?C1)\dabc(?C2)def
+   (?C1)abc(?C2)def
  </pre>
  If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
  automatically installed before each item in the pattern. They are all numbered
-Line 1820 
 description of the interface to the call
+Line 2290 
 description of the interface to the call
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation.
  </P>
- <br><a name="SEC22" href="#TOC1">SEE ALSO</a><br>
+ <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
+ <P>
+ 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.
+ </P>
+ <P>
+ Since these verbs are specifically related to backtracking, most of them can be
+ used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
+ a backtracking algorithm. With the exception of (*FAIL), which behaves like a
+ failing negative assertion, they cause an error if encountered by
+ <b>pcre_dfa_exec()</b>.
+ </P>
+ <P>
+ If any of these verbs are used in an assertion or subroutine subpattern
+ (including recursive subpatterns), their effect is confined to that subpattern;
+ it does not extend to the surrounding pattern. Note that such subpatterns are
+ processed as anchored at the point where they are tested.
+ </P>
+ <P>
+ The new verbs make use of what was previously invalid syntax: an opening
+ parenthesis followed 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:
+ </P>
+ <br><b>
+ Verbs that act immediately
+ </b><br>
+ <P>
+ The following verbs act as soon as they are encountered:
+ <pre>
+    (*ACCEPT)
+ </pre>
+ This verb causes the match to end successfully, skipping the remainder of the
+ pattern. When inside a recursion, only the innermost pattern is ended
+ immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
+ captured. (This feature was added to PCRE at release 8.00.) For example:
+ <pre>
+   A((?:A|B(*ACCEPT)|C)D)
+ </pre>
+ This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
+ the outer parentheses.
+ <pre>
+   (*FAIL) or (*F)
+ </pre>
+ 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:
+ <pre>
+   a+(?C)(*FAIL)
+ </pre>
+ A match with the string "aaaa" always fails, but the callout is taken before
+ each backtrack happens (in this example, 10 times).
+ </P>
+ <br><b>
+ Verbs that act after backtracking
+ </b><br>
+ <P>
+ 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.
+ <pre>
+   (*COMMIT)
+ </pre>
+ 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 starting point take place. Once (*COMMIT) has been
+ passed, <b>pcre_exec()</b> is committed to finding a match at the current
+ starting point, or not at all. For example:
+ <pre>
+   a+(*COMMIT)b
+ </pre>
+ 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."
+ <pre>
+   (*PRUNE)
+ </pre>
+ 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 starting 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, backtracking cannot cross (*PRUNE).
+ In simple cases, the use of (*PRUNE) is just an alternative to an atomic
+ group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
+ be expressed in any other way.
+ <pre>
+   (*SKIP)
+ </pre>
+ 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:
+ <pre>
+   a+(*SKIP)b
+ </pre>
+ If the subject is "aaaac...", after the first match attempt fails (starting at
+ the first character in the string), the starting point skips on to start the
+ next attempt at "c". Note that a possessive quantifer does not have the same
+ effect as this example; although it would suppress backtracking during the
+ first match attempt, the second attempt would start at the second character
+ instead of skipping on to "c".
+ <pre>
+   (*THEN)
+ </pre>
+ 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 block:
+ <pre>
+   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
+ </pre>
+ 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 alternation, it acts exactly like (*PRUNE).
+ </P>
+ <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
  <P>
- <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
+ <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
+ <b>pcresyntax</b>(3), <b>pcre</b>(3).
  </P>
- <br><a name="SEC23" href="#TOC1">AUTHOR</a><br>
+ <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
  <P>
  Philip Hazel
  <br>
-Line 1833 
 University Computing Service
+Line 2425 
 University Computing Service
  Cambridge CB2 3QH, England.
  <br>
  </P>
- <br><a name="SEC24" href="#TOC1">REVISION</a><br>
+ <br><a name="SEC28" href="#TOC1">REVISION</a><br>
  <P>
- Last updated: 06 March 2007
+ Last updated: 06 March 2010
  <br>
- Copyright &copy; 1997-2007 University of Cambridge.
+ Copyright &copy; 1997-2010 University of Cambridge.
  <br>
  <p>
  Return to the <a href="index.html">PCRE index page</a>.

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