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

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-revision 155 by ph10,
Tue Apr 24 13:36:11 2007 UTC
+revision 406 by ph10,
Mon Mar 23 12:05:43 2009 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,
-Line 63 
 The remainder of this document discusses
+Line 75 
 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>. 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 180 
 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 174 
 represents:
+Line 229 
 represents:
    \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
-Line 190 
 Thus \cz becomes hex 1A, but \c{ becomes
+Line 245 
 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 311 
 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 subroutine call.
+ </P>
+ <br><b>
  Generic character types
  </b><br>
  <P>
-Line 268 
 following are always recognized:
+Line 338 
 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 359 
 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 299 
 in the
+Line 417 
 in the
  <a href="pcreapi.html"><b>pcreapi</b></a>
  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
  or "french" in Windows, some character codes greater than 128 are used for
- accented letters, and these are matched by \w.
+ accented letters, and these are matched by \w. The use of locales with Unicode
- </P>
+ is discouraged.
- <P>
+ <a name="newlineseq"></a></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. 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 332 
 Unicode character property support is no
+Line 445 
 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>, but
+ they can be overridden by options given to <b>pcre_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 339 
 Unicode character properties
+Line 472 
 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 497 
 the Lu, Ll, or Lt property, in other wor
+Line 632 
 the Lu, Ll, or Lt property, in other wor
  a modifier or "other".
  </P>
  <P>
+ 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).
+ </P>
+ <P>
  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".
-Line 521 
 or more characters with the "mark" prope
+Line 664 
 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".
  <a name="smallassertions"></a></P>
  <br><b>
  Simple assertions
-Line 589 
 If all the alternatives of a pattern beg
+Line 756 
 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 643 
 Note that the sequences \A, \Z, and \z c
+Line 810 
 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 668 
 The handling of dot is entirely independ
+Line 835 
 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 683 
 PCRE does not allow \C to appear in look
+Line 850 
 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
-Line 778 
 introducing a POSIX class name - see the
+Line 945 
 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 824 
 supported, and an error is given if they
+Line 991 
 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 839 
 that succeeds is used. If the alternativ
+Line 1006 
 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 858 
 permitted. If a letter appears both befo
+Line 1026 
 permitted. If a letter appears both befo
  unset.
  </P>
  <P>
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
+ changed in the same way as the Perl-compatible options by using the characters
+ J, U and X respectively.
+ </P>
+ <P>
  When an option change 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
-Line 883 
 option settings happen at compile time.
+Line 1056 
 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 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.
  <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 941 
 from left to right, and options are not
+Line 1117 
 from left to right, and options are not
  is reached, an option setting in one branch does affect subsequent branches, so
  the above patterns match "SUNDAY" as well as "Saturday".
  </P>
- <br><a name="SEC12" href="#TOC1">NAMED SUBPATTERNS</a><br>
+ <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 backreference or a recursive call to a numbered subpattern always refers to
+ the first one in the pattern with the given number.
+ </P>
+ <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 983 
 abbreviation. This pattern (ignoring the
+Line 1190 
 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
-Line 992 
 details of the interfaces for handling n
+Line 1203 
 details of the interfaces for handling n
  <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 (see the previous section) because PCRE uses
+ only the numbers when matching.
+ </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 1039 
 which may be several bytes long (and the
+Line 1255 
 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 1143 
 example, after
+Line 1363 
 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 1197 
 previous example can be rewritten as
+Line 1417 
 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 1242 
 an atomic group, like this:
+Line 1467 
 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 1271 
 subpattern is possible using named paren
+Line 1496 
 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 1309 
 matches "rah rah" and "RAH RAH", but not
+Line 1535 
 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 1350 
 that the first iteration does not need t
+Line 1581 
 that the first iteration does not need t
  done using alternation, as in the example above, or by a quantifier with a
  minimum of zero.
  <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 1432 
 lengths, but it is acceptable if rewritt
+Line 1663 
 lengths, but it is acceptable if rewritt
  <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; this is not restricted to a
+ fixed-length.
+ </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 1504 
 preceded by "foo", while
+Line 1741 
 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
-Line 1528 
 Checking for a used subpattern by number
+Line 1765 
 Checking for a used subpattern by number
  <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
- matched.
+ 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 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 1547 
 parenthesis is required. Otherwise, sinc
+Line 1788 
 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 1630 
 subject is matched against the first alt
+Line 1879 
 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 1641 
 If the PCRE_EXTENDED option is set, an u
+Line 1890 
 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 1697 
 pattern, so instead you could use this:
+Line 1946 
 pattern, so instead you could use this:
    ( \( ( (?&#62;[^()]+) | (?1) )* \) )
  </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
+ "subroutine" 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; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
  </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 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
  <pre>
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
  </pre>
-Line 1753 
 In this pattern, (?(R) is the start of a
+Line 2020 
 In this pattern, (?(R) is the start of a
  different alternatives for the recursive and non-recursive cases. The (?R) item
  is the actual recursive call.
  <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 1781 
 When a subpattern is used as a subroutin
+Line 2055 
 When a subpattern is used as a subroutin
  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 1821 
 description of the interface to the call
+Line 2113 
 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>
+ 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. 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:
+ <pre>
+   A(A|B(*ACCEPT)|C)D
+ </pre>
+ This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
+ captured.
+ <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 start 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 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".
+ <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).
  </P>
- <br><a name="SEC23" href="#TOC1">AUTHOR</a><br>
+ <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
  <P>
  Philip Hazel
  <br>
-Line 1834 
 University Computing Service
+Line 2242 
 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: 18 March 2009
  <br>
- Copyright &copy; 1997-2007 University of Cambridge.
+ Copyright &copy; 1997-2009 University of Cambridge.
  <br>
  <p>
  Return to the <a href="index.html">PCRE index page</a>.

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