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

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-revision 91 by nigel,
Sat Feb 24 21:41:34 2007 UTC
+revision 247 by ph10,
Mon Sep 17 09:38:32 2007 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">BACKSLASH</a>
+ <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
- <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>
+ <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
- <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>
+ <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
- <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>
+ <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
- <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a>
+ <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
- <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>
+ <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
- <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>
+ <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
- <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>
+ <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
- <li><a name="TOC10" href="#SEC10">SUBPATTERNS</a>
+ <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
- <li><a name="TOC11" href="#SEC11">NAMED SUBPATTERNS</a>
+ <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
- <li><a name="TOC12" href="#SEC12">REPETITION</a>
+ <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
- <li><a name="TOC13" href="#SEC13">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
+ <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
- <li><a name="TOC14" href="#SEC14">BACK REFERENCES</a>
+ <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
- <li><a name="TOC15" href="#SEC15">ASSERTIONS</a>
+ <li><a name="TOC15" href="#SEC15">REPETITION</a>
- <li><a name="TOC16" href="#SEC16">CONDITIONAL SUBPATTERNS</a>
+ <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
- <li><a name="TOC17" href="#SEC17">COMMENTS</a>
+ <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
- <li><a name="TOC18" href="#SEC18">RECURSIVE PATTERNS</a>
+ <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
- <li><a name="TOC19" href="#SEC19">SUBPATTERNS AS SUBROUTINES</a>
+ <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
- <li><a name="TOC20" href="#SEC20">CALLOUTS</a>
+ <li><a name="TOC20" href="#SEC20">COMMENTS</a>
+ <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
+ <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
+ <li><a name="TOC23" href="#SEC23">CALLOUTS</a>
+ <li><a name="TOC24" href="#SEC24">BACKTRACKING CONTROL</a>
+ <li><a name="TOC25" href="#SEC25">SEE ALSO</a>
+ <li><a name="TOC26" href="#SEC26">AUTHOR</a>
+ <li><a name="TOC27" href="#SEC27">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. Perl's regular expressions are described in its own documentation, and
- regular expressions in great detail. This description of PCRE's regular
+ regular expressions in general are covered in a number of books, some of which
- expressions is intended as reference material.
+ 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 59 
 The remainder of this document discusses
+Line 68 
 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">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 90 
 interpreted in some special way.
+Line 143 
 interpreted in some special way.
  <P>
  There are two different sets of metacharacters: those that are recognized
  anywhere in the pattern except within square brackets, and those that are
- recognized in square brackets. Outside square brackets, the metacharacters are
+ recognized within square brackets. Outside square brackets, the metacharacters
- as follows:
+ are as follows:
  <pre>
    \      general escape character with several uses
    ^      assert start of string (or line, in multiline mode)
-Line 120 
 a character class the only metacharacter
+Line 173 
 a character class the only metacharacter
  </pre>
  The following sections describe the use of each of the metacharacters.
  </P>
- <br><a name="SEC2" 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 169 
 represents:
+Line 222 
 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 185 
 Thus \cz becomes hex 1A, but \c{ becomes
+Line 238 
 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 216 
 following the discussion of
+Line 273 
 following the discussion of
  <P>
  Inside a character class, or if the decimal number is greater than 9 and there
  have not been that many capturing subpatterns, PCRE re-reads up to three octal
- digits following the backslash, ane uses them to generate a data character. Any
+ digits following the backslash, and uses them to generate a data character. Any
  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
  character specified in octal must be less than \400. In UTF-8 mode, values up
  to \777 are permitted. For example:
-Line 238 
 zero, because no more than three octal d
+Line 295 
 zero, because no more than three octal d
  All the sequences that define a single character value can be used both inside
  and outside character classes. In addition, inside a character class, the
  sequence \b is interpreted as the backspace character (hex 08), and the
- sequence \X is interpreted as the character "X". Outside a character class,
+ sequences \R and \X are interpreted as the characters "R" and "X",
- these sequences have different meanings
+ respectively. Outside a character class, these sequences have different
+ meanings
  <a href="#uniextseq">(see below).</a>
  </P>
  <br><b>
+ Absolute and relative back references
+ </b><br>
+ <P>
+ The sequence \g followed by an unsigned or a negative number, optionally
+ enclosed in braces, is an absolute or relative back reference. A named back
+ reference can be coded as \g{name}. Back references are discussed
+ <a href="#backreferences">later,</a>
+ following the discussion of
+ <a href="#subpattern">parenthesized subpatterns.</a>
+ </P>
+ <br><b>
  Generic character types
  </b><br>
  <P>
- The third use of backslash is for specifying generic character types. The
+ Another use of backslash is for specifying generic character types. The
  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 268 
 there is no character to match.
+Line 341 
 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.
+ </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 280 
 place (see
+Line 396 
 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
+ is discouraged.
+ <a name="newlineseq"></a></P>
+ <br><b>
+ Newline sequences
+ </b><br>
  <P>
- In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
+ Outside a character class, by default, the escape sequence \R matches any
- \w, and always match \D, \S, and \W. This is true even when Unicode
+ Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
- character property support is available. The use of locales with Unicode is
+ equivalent to the following:
- discouraged.
+ <pre>
+   (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
+ </pre>
+ This is an example of an "atomic group", details of which are given
+ <a href="#atomicgroup">below.</a>
+ This particular group matches either the two-character sequence CR followed by
+ LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
+ U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
+ line, U+0085). The two-character sequence is treated as a single unit that
+ cannot be split.
+ </P>
+ <P>
+ In UTF-8 mode, two additional characters whose codepoints are greater than 255
+ are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
+ Unicode character property support is not needed for these characters to be
+ 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>
  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 321 
 Those that are not part of an identified
+Line 482 
 Those that are not part of an identified
  <P>
  Arabic,
  Armenian,
+ Balinese,
  Bengali,
  Bopomofo,
  Braille,
-Line 330 
 Canadian_Aboriginal,
+Line 492 
 Canadian_Aboriginal,
  Cherokee,
  Common,
  Coptic,
+ Cuneiform,
  Cypriot,
  Cyrillic,
  Deseret,
-Line 359 
 Malayalam,
+Line 522 
 Malayalam,
  Mongolian,
  Myanmar,
  New_Tai_Lue,
+ Nko,
  Ogham,
  Old_Italic,
  Old_Persian,
  Oriya,
  Osmanya,
+ Phags_Pa,
+ Phoenician,
  Runic,
  Shavian,
  Sinhala,
-Line 447 
 the Lu, Ll, or Lt property, in other wor
+Line 613 
 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 471 
 or more characters with the "mark" prope
+Line 645 
 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
  </b><br>
  <P>
- The fourth use of backslash is for certain simple assertions. An assertion
+ The final use of backslash is for certain simple assertions. An assertion
  specifies a condition that has to be met at a particular point in a match,
  without consuming any characters from the subject string. The use of
  subpatterns for more complicated assertions is described
-Line 492 
 The backslashed assertions are:
+Line 690 
 The backslashed assertions are:
  <pre>
    \b     matches at a word boundary
    \B     matches when not at a word boundary
-   \A     matches at start of subject
+   \A     matches at the start of the subject
-   \Z     matches at end of subject or before newline at end
+   \Z     matches at the end of the subject
-   \z     matches at end of subject
+           also matches before a newline at the end of the subject
-   \G     matches at first matching position in subject
+   \z     matches only at the end of the subject
+   \G     matches at the first matching position in the subject
  </pre>
  These assertions may not appear in character classes (but note that \b has a
  different meaning, namely the backspace character, inside a character class).
-Line 538 
 If all the alternatives of a pattern beg
+Line 737 
 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="SEC3" 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 592 
 Note that the sequences \A, \Z, and \z c
+Line 791 
 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="SEC4" 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. In UTF-8 mode, the matched character may be more than one byte long. When
+ line. In UTF-8 mode, the matched character may be more than one byte long.
- a line ending is defined as a single character (CR or LF), dot never matches
+ </P>
- that character; when the two-character sequence CRLF is used, dot does not
+ <P>
- match CR if it is immediately followed by LF, but otherwise it matches all
+ When a line ending is defined as a single character, dot never matches that
- characters (including isolated CRs and LFs).
+ character; when the two-character sequence CRLF is used, dot does not match CR
+ if it is immediately followed by LF, but otherwise it matches all characters
+ (including isolated CRs and LFs). When any Unicode line endings are being
+ recognized, dot does not match CR or LF or any of the other line ending
+ characters.
  </P>
  <P>
  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
- option is set, a dot matches any one character, without exception. If newline
+ option is set, a dot matches any one character, without exception. If the
- is defined as the two-character sequence CRLF, it takes two dots to match it.
+ two-character sequence CRLF is present in the subject string, it takes two dots
+ to match it.
  </P>
  <P>
  The handling of dot is entirely independent of the handling of circumflex and
  dollar, the only relationship being that they both involve newlines. Dot has no
  special meaning in a character class.
  </P>
- <br><a name="SEC5" 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 CR and LF. The
+ in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
- feature is provided in Perl in order to match individual bytes in UTF-8 mode.
+ characters. The feature is provided in Perl in order to match individual bytes
- Because it breaks up UTF-8 characters into individual bytes, what remains in
+ in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
- the string may be a malformed UTF-8 string. For this reason, the \C escape
+ what remains in the string may be a malformed UTF-8 string. For this reason,
- sequence is best avoided.
+ the \C escape sequence is best avoided.
  </P>
  <P>
  PCRE does not allow \C to appear in lookbehind assertions
-Line 627 
 PCRE does not allow \C to appear in look
+Line 831 
 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="SEC6" 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 670 
 ensure that PCRE is compiled with Unicod
+Line 874 
 ensure that PCRE is compiled with Unicod
  UTF-8 support.
  </P>
  <P>
- Characters that might indicate line breaks (CR and LF) are never treated in any
+ Characters that might indicate line breaks are never treated in any special way
- special way when matching character classes, whatever line-ending sequence is
+ when matching character classes, whatever line-ending sequence is in use, and
- in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is
+ whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
- used. A class such as [^a] always matches one of these characters.
+ such as [^a] always matches one of these characters.
  </P>
  <P>
  The minus (hyphen) character can be used to specify a range of characters in a
-Line 701 
 example [\x{100}-\x{2ff}].
+Line 905 
 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 722 
 introducing a POSIX class name - see the
+Line 926 
 introducing a POSIX class name - see the
  closing square bracket. However, escaping other non-alphanumeric characters
  does no harm.
  </P>
- <br><a name="SEC7" 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 768 
 supported, and an error is given if they
+Line 972 
 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="SEC8" 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 783 
 that succeeds is used. If the alternativ
+Line 987 
 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="SEC9" 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 802 
 permitted. If a letter appears both befo
+Line 1007 
 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 809 
 the global options (and it will therefor
+Line 1019 
 the global options (and it will therefor
  <b>pcre_fullinfo()</b> function).
  </P>
  <P>
- An option change within a subpattern affects only that part of the current
+ An option change within a subpattern (see below for a description of
- pattern that follows it, so
+ subpatterns) affects only that part of the current pattern that follows it, so
  <pre>
    (a(?i)b)c
  </pre>
-Line 825 
 matches "ab", "aB", "c", and "C", even t
+Line 1035 
 matches "ab", "aB", "c", and "C", even t
  branch is abandoned before the option setting. This is because the effects of
  option settings happen at compile time. There would be some very weird
  behaviour otherwise.
- </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.
  <a name="subpattern"></a></P>
- <br><a name="SEC10" 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 842 
 Turning part of a pattern into a subpatt
+Line 1047 
 Turning part of a pattern into a subpatt
    cat(aract|erpillar|)
  </pre>
  matches one of the words "cat", "cataract", or "caterpillar". Without the
- parentheses, it would match "cataract", "erpillar" or the empty string.
+ parentheses, it would match "cataract", "erpillar" or an empty string.
  <br>
  <br>
 . It sets up the subpattern as a capturing subpattern. This means that, when
-Line 870 
 the string "the white queen" is matched
+Line 1075 
 the string "the white queen" is matched
    the ((?:red|white) (king|queen))
  </pre>
  the captured substrings are "white queen" and "queen", and are numbered 1 and
-. The maximum number of capturing subpatterns is 65535, and the maximum depth
+. The maximum number of capturing subpatterns is 65535.
- of nesting of all subpatterns, both capturing and non-capturing, is 200.
  </P>
  <P>
  As a convenient shorthand, if any option settings are required at the start of
-Line 886 
 from left to right, and options are not
+Line 1090 
 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="SEC11" 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,
  if an expression is modified, the numbers may change. To help with this
- difficulty, PCRE supports the naming of subpatterns, something that Perl does
+ difficulty, PCRE supports the naming of subpatterns. This feature was not
- not provide. The Python syntax (?P&#60;name&#62;...) is used. References to capturing
+ 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.
+ </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="#recursion">recursion,</a>
-Line 902 
 can be made by name as well as by number
+Line 1143 
 can be made by name as well as by number
  </P>
  <P>
  Names consist of up to 32 alphanumeric characters and underscores. Named
- capturing parentheses are still allocated numbers as well as names. The PCRE
+ capturing parentheses are still allocated numbers as well as names, exactly as
- API provides function calls for extracting the name-to-number translation table
+ if the names were not present. The PCRE API provides function calls for
- from a compiled pattern. There is also a convenience function for extracting a
+ extracting the name-to-number translation table from a compiled pattern. There
- captured substring by name.
+ is also a convenience function for extracting a captured substring by name.
  </P>
  <P>
  By default, a name must be unique within a pattern, but it is possible to relax
-Line 915 
 match. Suppose you want to match the nam
+Line 1156 
 match. Suppose you want to match the nam
  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>
-   (?P&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
+   (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
-   (?P&#60;DN&#62;Tue)(?:sday)?|
+   (?&#60;DN&#62;Tue)(?:sday)?|
-   (?P&#60;DN&#62;Wed)(?:nesday)?|
+   (?&#60;DN&#62;Wed)(?:nesday)?|
-   (?P&#60;DN&#62;Thu)(?:rsday)?|
+   (?&#60;DN&#62;Thu)(?:rsday)?|
-   (?P&#60;DN&#62;Sat)(?:urday)?
+   (?&#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
+ for the first (and in this example, the only) subpattern of that name that
  matched. This saves searching to find which numbered subpattern it was. If you
  make a reference to a non-unique named subpattern from elsewhere in the
  pattern, the one that corresponds to the lowest number is used. For further
-Line 931 
 details of the interfaces for handling n
+Line 1176 
 details of the interfaces for handling n
  <a href="pcreapi.html"><b>pcreapi</b></a>
  documentation.
  </P>
- <br><a name="SEC12" href="#TOC1">REPETITION</a><br>
+ <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
  <P>
  Repetition is specified by quantifiers, which can follow any of the following
  items:
  <pre>
    a literal data character
-   the . metacharacter
+   the dot metacharacter
    the \C escape sequence
    the \X escape sequence (in UTF-8 mode with Unicode properties)
+   the \R escape sequence
    an escape such as \d that matches a single character
    a character class
    a back reference (see next section)
-Line 980 
 The quantifier {0} is permitted, causing
+Line 1226 
 The quantifier {0} is permitted, causing
  previous item and the quantifier were not present.
  </P>
  <P>
- For convenience (and historical compatibility) the three most common
+ For convenience, the three most common quantifiers have single-character
- quantifiers have single-character abbreviations:
+ abbreviations:
  <pre>
    *    is equivalent to {0,}
    +    is equivalent to {1,}
-Line 1032 
 which matches one digit by preference, b
+Line 1278 
 which matches one digit by preference, b
  way the rest of the pattern matches.
  </P>
  <P>
- If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
+ If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
  the quantifiers are not greedy by default, but individual ones can be made
  greedy by following them with a question mark. In other words, it inverts the
  default behaviour.
-Line 1044 
 compiled pattern, in proportion to the s
+Line 1290 
 compiled pattern, in proportion to the s
  </P>
  <P>
  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
- to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
+ to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
  implicitly anchored, because whatever follows will be tried against every
  character position in the subject string, so there is no point in retrying the
  overall match at any position after the first. PCRE normally treats such a
-Line 1058 
 alternatively using ^ to indicate anchor
+Line 1304 
 alternatively using ^ to indicate anchor
  <P>
  However, there is one situation where the optimization cannot be used. When .*
  is inside capturing parentheses that are the subject of a backreference
- elsewhere in the pattern, a match at the start may fail, and a later one
+ elsewhere in the pattern, a match at the start may fail where a later one
- succeed. Consider, for example:
+ succeeds. Consider, for example:
  <pre>
    (.*)abc\1
  </pre>
-Line 1081 
 example, after
+Line 1327 
 example, after
  </pre>
  matches "aba" the value of the second captured substring is "b".
  <a name="atomicgroup"></a></P>
- <br><a name="SEC13" 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 and minimizing repetition, failure of what follows
+ With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- normally causes the repeated item to be re-evaluated to see if a different
+ repetition, failure of what follows normally causes the repeated item to be
- number of repeats allows the rest of the pattern to match. Sometimes it is
+ re-evaluated to see if a different number of repeats allows the rest of the
- useful to prevent this, either to change the nature of the match, or to cause
+ pattern to match. Sometimes it is useful to prevent this, either to change the
- it fail earlier than it otherwise might, when the author of the pattern knows
+ nature of the match, or to cause it fail earlier than it otherwise might, when
- there is no point in carrying on.
+ the author of the pattern knows there is no point in carrying on.
  </P>
  <P>
  Consider, for example, the pattern \d+foo when applied to the subject line
-Line 1102 
 item, and then with 4, and so on, before
+Line 1348 
 item, and then with 4, and so on, before
  that once a subpattern has matched, it is not to be re-evaluated in this way.
  </P>
  <P>
- If we use atomic grouping for the previous example, the matcher would give up
+ If we use atomic grouping for the previous example, the matcher gives up
  immediately on failing to match "foo" the first time. The notation is a kind of
  special parenthesis, starting with (?&#62; as in this example:
  <pre>
-Line 1135 
 previous example can be rewritten as
+Line 1381 
 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 or processing of a
+ atomic group. However, there is no difference in the meaning of a possessive
- possessive quantifier and the equivalent atomic group.
+ quantifier and the equivalent atomic group, though there may be a performance
+ difference; possessive quantifiers should be slightly faster.
+ </P>
+ <P>
+ The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
+ Jeffrey Friedl originated the idea (and the name) in the first edition of his
+ book. Mike McCloskey liked it, so implemented it when he built Sun's Java
+ package, and PCRE copied it from there. It ultimately found its way into Perl
+ at release 5.10.
  </P>
  <P>
- The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey
+ PCRE has an optimization that automatically "possessifies" certain simple
- Friedl originated the idea (and the name) in the first edition of his book.
+ pattern constructs. For example, the sequence A+B is treated as A++B because
- Mike McCloskey liked it, so implemented it when he built Sun's Java package,
+ there is no point in backtracking into a sequence of A's when B must follow.
- and PCRE copied it from there.
  </P>
  <P>
  When a pattern contains an unlimited repeat inside a subpattern that can itself
-Line 1173 
 an atomic group, like this:
+Line 1431 
 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="SEC14" 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 1190 
 when a repetition is involved and the su
+Line 1448 
 when a repetition is involved and the su
  in an earlier iteration.
  </P>
  <P>
- It is not possible to have a numerical "forward back reference" to subpattern
+ It is not possible to have a numerical "forward back reference" to a subpattern
- whose number is 10 or more. However, a back reference to any subpattern is
+ whose number is 10 or more using this syntax because a sequence such as \50 is
- possible using named parentheses (see below). See also the subsection entitled
+ interpreted as a character defined in octal. See the subsection entitled
  "Non-printing characters"
  <a href="#digitsafterbackslash">above</a>
- for further details of the handling of digits following a backslash.
+ for further details of the handling of digits following a backslash. There is
+ no such problem when named parentheses are used. A back reference to any
+ subpattern is possible using named parentheses (see below).
+ </P>
+ <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 an unsigned number or a negative
+ number, optionally enclosed in braces. These examples are all identical:
+ <pre>
+   (ring), \1
+   (ring), \g1
+   (ring), \g{1}
+ </pre>
+ An unsigned number specifies an absolute reference without the ambiguity that
+ is present in the older syntax. It is also useful when literal digits follow
+ the reference. A negative number is a relative reference. Consider this
+ example:
+ <pre>
+   (abc(def)ghi)\g{-1}
+ </pre>
+ The sequence \g{-1} is a reference to the most recently started capturing
+ subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
+ would be equivalent to \1. The use of relative references can be helpful in
+ long patterns, and also in patterns that are created by joining together
+ fragments that contain references within themselves.
  </P>
  <P>
  A back reference matches whatever actually matched the capturing subpattern in
-Line 1216 
 matches "rah rah" and "RAH RAH", but not
+Line 1499 
 matches "rah rah" and "RAH RAH", but not
  capturing subpattern is matched caselessly.
  </P>
  <P>
- Back references to named subpatterns use the Python syntax (?P=name). We could
+ There are several different ways of writing back references to named
- rewrite the above example as follows:
+ 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 1255 
 that the first iteration does not need t
+Line 1545 
 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="SEC15" 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 1337 
 lengths, but it is acceptable if rewritt
+Line 1627 
 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 width and then try 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
- match is deemed to fail.
+ assertion fails.
  </P>
  <P>
  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
  to appear in lookbehind assertions, because it makes it impossible to calculate
- the length of the lookbehind. The \X escape, which can match different numbers
+ the length of the lookbehind. The \X and \R escapes, which can match
- of bytes, is also not permitted.
+ different numbers of bytes, are also not permitted.
  </P>
  <P>
- Atomic groups can be used in conjunction with lookbehind assertions to specify
+ Possessive quantifiers can be used in conjunction with lookbehind assertions to
- efficient matching at the end of the subject string. Consider a simple pattern
+ specify efficient matching at the end of the subject string. Consider a simple
- such as
+ pattern such as
  <pre>
    abcd$
  </pre>
-Line 1367 
 then all but the last two characters, an
+Line 1663 
 then all but the last two characters, an
  covers the entire string, from right to left, so we are no better off. However,
  if the pattern is written as
  <pre>
-   ^(?&#62;.*)(?&#60;=abcd)
- </pre>
- or, equivalently, using the possessive quantifier syntax,
- <pre>
    ^.*+(?&#60;=abcd)
  </pre>
- there can be no backtracking for the .* item; it can match only the entire
+ there can be no backtracking for the .*+ item; it can match only the entire
  string. The subsequent lookbehind assertion does a single test on the last four
  characters. If it fails, the match fails immediately. For long strings, this
  approach makes a significant difference to the processing time.
-Line 1413 
 preceded by "foo", while
+Line 1705 
 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="SEC16" 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 1428 
 no-pattern (if present) is used. If ther
+Line 1720 
 no-pattern (if present) is used. If ther
  subpattern, a compile-time error occurs.
  </P>
  <P>
- There are three kinds of condition. If the text between the parentheses
+ There are four kinds of condition: references to subpatterns, references to
- consists of a sequence of digits, or a sequence of alphanumeric characters and
+ recursion, a pseudo-condition called DEFINE, and assertions.
- underscores, the condition is satisfied if the capturing subpattern of that
+ </P>
- number or name has previously matched. There is a possible ambiguity here,
+ <br><b>
- because subpattern names may consist entirely of digits. PCRE looks first for a
+ Checking for a used subpattern by number
- named subpattern; if it cannot find one and the text consists entirely of
+ </b><br>
- digits, it looks for a subpattern of that number, which must be greater than
+ <P>
- zero. Using subpattern names that consist entirely of digits is not
+ If the text between the parentheses consists of a sequence of digits, the
- recommended.
+ condition is true if the capturing subpattern of that number has previously
+ matched. An alternative notation is to precede the digits with a plus or minus
+ sign. In this case, the subpattern number is relative rather than absolute.
+ The most recently opened parentheses can be 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 1453 
 or not. If they did, that is, if subject
+Line 1750 
 or not. If they did, that is, if subject
  the condition is true, and so the yes-pattern is executed and a closing
  parenthesis is required. Otherwise, since no-pattern is not present, the
  subpattern matches nothing. In other words, this pattern matches a sequence of
- non-parentheses, optionally enclosed in parentheses. Rewriting it to use a
+ non-parentheses, optionally enclosed in parentheses.
- named subpattern gives this:
+ </P>
+ <P>
+ If you were embedding this pattern in a larger one, you could use a relative
+ reference:
  <pre>
-   (?P&#60;OPEN&#62; \( )?    [^()]+    (?(OPEN) \) )
+   ...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>
+ <P>
+ Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
+ subpattern by name. For compatibility with earlier versions of PCRE, which had
+ this facility before Perl, the syntax (?(name)...) is also recognized. However,
+ there is a possible ambiguity with this syntax, because subpattern names may
+ consist entirely of digits. PCRE looks first for a named subpattern; if it
+ cannot find one and the name consists entirely of digits, PCRE looks for a
+ subpattern of that number, which must be greater than zero. Using subpattern
+ names that consist entirely of digits is not recommended.
+ </P>
+ <P>
+ Rewriting the above example to use a named subpattern gives this:
+ <pre>
+   (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
+ </PRE>
+ </P>
+ <br><b>
+ Checking for pattern recursion
+ </b><br>
+ <P>
  If the condition is the string (R), and there is no subpattern with the name R,
- the condition is satisfied if a recursive call to the pattern or subpattern has
+ the condition is true if a recursive call to the whole pattern or any
- been made. At "top level", the condition is false. This is a PCRE extension.
+ subpattern has been made. If digits or a name preceded by ampersand follow the
- Recursive patterns are described in the next section.
+ letter R, for example:
+ <pre>
+   (?(R3)...) or (?(R&name)...)
+ </pre>
+ the condition is true if the most recent recursion is into the subpattern whose
+ number or name is given. This condition does not check the entire recursion
+ stack.
+ </P>
+ <P>
+ At "top level", all these recursion test conditions are false. Recursive
+ patterns are described below.
+ </P>
+ <br><b>
+ Defining subpatterns for use by reference only
+ </b><br>
+ <P>
+ If the condition is the string (DEFINE), and there is no subpattern with the
+ name DEFINE, the condition is always false. In this case, there may be only one
+ alternative in the subpattern. It is always skipped if control reaches this
+ point in the pattern; the idea of DEFINE is that it can be used to define
+ "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
+ is described below.) For example, a pattern to match an IPv4 address could be
+ written like this (ignore whitespace and line breaks):
+ <pre>
+   (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
+   \b (?&byte) (\.(?&byte)){3} \b
+ </pre>
+ 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.
+ </P>
+ <P>
+ 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
+ </b><br>
  <P>
- If the condition is not a sequence of digits or (R), it must be an assertion.
+ If the condition is not in any of the above formats, it must be an assertion.
  This may be a positive or negative lookahead or lookbehind assertion. Consider
  this pattern, again containing non-significant white space, and with the two
  alternatives on the second line:
-Line 1479 
 subject is matched against the first alt
+Line 1843 
 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="SEC17" 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 1490 
 If the PCRE_EXTENDED option is set, an u
+Line 1854 
 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="SEC18" 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
  be done is to use a pattern that matches up to some fixed depth of nesting. It
- is not possible to handle an arbitrary nesting depth. Perl provides a facility
+ is not possible to handle an arbitrary nesting depth.
- that allows regular expressions to recurse (amongst other things). It does this
+ </P>
- by interpolating Perl code in the expression at run time, and the code can
+ <P>
- refer to the expression itself. A Perl pattern to solve the parentheses problem
+ For some time, Perl has provided a facility that allows regular expressions to
- can be created like this:
+ recurse (amongst other things). It does this by interpolating Perl code in the
+ expression at run time, and the code can refer to the expression itself. A Perl
+ pattern using code interpolation to solve the parentheses problem can be
+ created like this:
  <pre>
    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
  </pre>
  The (?p{...}) item interpolates Perl code at run time, and in this case refers
- recursively to the pattern in which it appears. Obviously, PCRE cannot support
+ recursively to the pattern in which it appears.
- the interpolation of Perl code. Instead, it supports some special syntax for
- recursion of the entire pattern, and also for individual subpattern recursion.
  </P>
  <P>
- The special item that consists of (? followed by a number greater than zero and
+ Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
- a closing parenthesis is a recursive call of the subpattern of the given
+ supports special syntax for recursion of the entire pattern, and also for
- number, provided that it occurs inside that subpattern. (If not, it is a
+ individual subpattern recursion. After its introduction in PCRE and Python,
- "subroutine" call, which is described in the next section.) The special item
+ this kind of recursion was introduced into Perl at release 5.10.
- (?R) is a recursive call of the entire regular expression.
  </P>
  <P>
- A recursive subpattern call is always treated as an atomic group. That is, once
+ A special item that consists of (? followed by a number greater than zero and a
- it has matched some of the subject string, it is never re-entered, even if
+ closing parenthesis is a recursive call of the subpattern of the given number,
- it contains untried alternatives and there is a subsequent matching failure.
+ provided that it occurs inside that subpattern. (If not, it is a "subroutine"
+ call, which is described in the next section.) The special item (?R) or (?0) is
+ a recursive call of the entire regular 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
-Line 1538 
 pattern, so instead you could use this:
+Line 1910 
 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. For this, PCRE uses (?P&#62;name), which is an extension to
+ <P>
- the Python syntax that PCRE uses for named parentheses (Perl does not provide
+ In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- named parentheses). We could rewrite the above example as follows:
+ is made easier by the use of relative references. (A Perl 5.10 feature.)
- <pre>
+ Instead of (?1) in the pattern above you can write (?-2) to refer to the second
-   (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )
+ most recently opened parentheses preceding the recursion. In other words, a
- </pre>
+ negative number counts capturing parentheses leftwards from the point at which
- This particular example pattern contains nested unlimited repeats, and so the
+ it is encountered.
- 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
+ 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.
+ </P>
+ <P>
+ 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 1562 
 before failure can be reported.
+Line 1953 
 before failure can be reported.
  At the end of a match, the values set for any capturing subpatterns are those
  from the outermost level of the recursion at which the subpattern value is set.
  If you want to obtain intermediate values, a callout function can be used (see
- the next section and the
+ below and the
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation). If the pattern above is matched against
  <pre>
-Line 1593 
 In this pattern, (?(R) is the start of a
+Line 1984 
 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="SEC19" 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. An earlier example pointed out that the
+ subroutine in a programming language. The "called" subpattern may be defined
- 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 1608 
 matches "sense and sensibility" and "res
+Line 2006 
 matches "sense and sensibility" and "res
    (sens|respons)e and (?1)ibility
  </pre>
  is used, it does match "sense and responsibility" as well as the other two
- strings. Such references, if given numerically, must follow the subpattern to
+ strings. Another example is given in the discussion of DEFINE above.
- which they refer. However, named references can refer to later subpatterns.
  </P>
  <P>
  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
-Line 1617 
 group. That is, once it has matched some
+Line 2014 
 group. That is, once it has matched some
  re-entered, even if it contains untried alternatives and there is a subsequent
  matching failure.
  </P>
- <br><a name="SEC20" href="#TOC1">CALLOUTS</a><br>
+ <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))
+ </pre>
+ It matches "abcabc". It does not match "abcABC" because the change of
+ processing option does not affect the called subpattern.
+ </P>
+ <br><a name="SEC23" 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 1636 
 function is to be called. If you want to
+Line 2043 
 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 1652 
 description of the interface to the call
+Line 2059 
 description of the interface to the call
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation.
  </P>
+ <br><a name="SEC24" href="#TOC1">BACKTRACKING CONTROL</a><br>
  <P>
- Last updated: 06 June 2006
+ 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, they can be used
+ only when the pattern is to be matched using <b>pcre_exec()</b>, which uses a
+ backtracking algorithm. 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="SEC25" 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="SEC26" href="#TOC1">AUTHOR</a><br>
+ <P>
+ Philip Hazel
+ <br>
+ University Computing Service
+ <br>
+ Cambridge CB2 3QH, England.
+ <br>
+ </P>
+ <br><a name="SEC27" href="#TOC1">REVISION</a><br>
+ <P>
+ Last updated: 14 September 2007
+ <br>
+ Copyright &copy; 1997-2007 University of Cambridge.
  <br>
- Copyright &copy; 1997-2006 University of Cambridge.
  <p>
  Return to the <a href="index.html">PCRE index page</a>.
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