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-revision 73 by nigel,
Sat Feb 24 21:40:30 2007 UTC
+revision 91 by nigel,
Sat Feb 24 21:41:34 2007 UTC
 Line 3
  <title>pcrepattern specification</title>
  </head>
  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
- This HTML document has been generated automatically from the original man page.
+ <h1>pcrepattern man page</h1>
- If there is any nonsense in it, please consult the man page, in case the
+ <p>
- conversion went wrong.<br>
+ Return to the <a href="index.html">PCRE index page</a>.
+ </p>
+ <p>
+ This page is part of the PCRE HTML documentation. It was generated automatically
+ from the original man page. If there is any nonsense in it, please consult the
+ man page, in case the conversion went wrong.
+ <br>
  <ul>
  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
  <li><a name="TOC2" href="#SEC2">BACKSLASH</a>
  <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>
  <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>
  <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>
- <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS</a>
+ <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a>
  <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>
  <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>
  <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>
-Line 32 
 conversion went wrong.<br>
+Line 38 
 conversion went wrong.<br>
  <P>
  The syntax and semantics of the regular expressions supported by PCRE are
  described below. Regular expressions are also described in the Perl
- documentation and in a number of other books, some of which have copious
+ documentation and in a number of books, some of which have copious examples.
- examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
+ Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
- O'Reilly, covers them in great detail. The description here is intended as
+ regular expressions in great detail. This description of PCRE's regular
- reference documentation.
+ expressions is intended as reference material.
  </P>
  <P>
- The basic operation of PCRE is on strings of bytes. However, there is also
+ The original operation of PCRE was on strings of one-byte characters. However,
- support for UTF-8 character strings. To use this support you must build PCRE to
+ there is now also support for UTF-8 character strings. To use this, you must
- include UTF-8 support, and then call <b>pcre_compile()</b> with the PCRE_UTF8
+ build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
- option. How this affects the pattern matching is mentioned in several places
+ the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
- below. There is also a summary of UTF-8 features in the
+ places below. There is also a summary of UTF-8 features in the
  <a href="pcre.html#utf8support">section on UTF-8 support</a>
  in the main
  <a href="pcre.html"><b>pcre</b></a>
  page.
  </P>
  <P>
+ The remainder of this document discusses the patterns that are supported by
+ 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,
+ and how it differs from the normal function, are discussed in the
+ <a href="pcrematching.html"><b>pcrematching</b></a>
+ page.
+ </P>
+ <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
  corresponding characters in the subject. As a trivial example, the pattern
- </P>
- <P>
  <pre>
    The quick brown fox
- </PRE>
+ </pre>
+ matches a portion of a subject string that is identical to itself. When
+ caseless matching is specified (the PCRE_CASELESS option), letters are matched
+ independently of case. In UTF-8 mode, PCRE always understands the concept of
+ case for characters whose values are less than 128, so caseless matching is
+ always possible. For characters with higher values, the concept of case is
+ supported if PCRE is compiled with Unicode property support, but not otherwise.
+ If you want to use caseless matching for characters 128 and above, you must
+ ensure that PCRE is compiled with Unicode property support as well as with
+ UTF-8 support.
  </P>
  <P>
- matches a portion of a subject string that is identical to itself. The power of
+ The power of regular expressions comes from the ability to include alternatives
- regular expressions comes from the ability to include alternatives and
+ and repetitions in the pattern. These are encoded in the pattern by the use of
- repetitions in the pattern. These are encoded in the pattern by the use of
+ <i>metacharacters</i>, which do not stand for themselves but instead are
- <i>meta-characters</i>, which do not stand for themselves but instead are
  interpreted in some special way.
  </P>
  <P>
- There are two different sets of meta-characters: those that are recognized
+ 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 meta-characters are
+ recognized in square brackets. Outside square brackets, the metacharacters are
  as follows:
- </P>
- <P>
  <pre>
    \      general escape character with several uses
    ^      assert start of string (or line, in multiline mode)
-Line 88 
 as follows:
+Line 108 
 as follows:
    +      1 or more quantifier
           also "possessive quantifier"
    {      start min/max quantifier
- </PRE>
+ </pre>
- </P>
- <P>
  Part of a pattern that is in square brackets is called a "character class". In
- a character class the only meta-characters are:
+ a character class the only metacharacters are:
- </P>
- <P>
  <pre>
    \      general escape character
    ^      negate the class, but only if the first character
    -      indicates character range
-   [      POSIX character class (only if followed by POSIX
+   [      POSIX character class (only if followed by POSIX syntax)
-            syntax)
    ]      terminates the character class
- </PRE>
+ </pre>
- </P>
+ The following sections describe the use of each of the metacharacters.
- <P>
- The following sections describe the use of each of the meta-characters.
  </P>
  <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>
  <P>
  The backslash character has several uses. Firstly, if it is followed by a
- non-alphameric character, it takes away any special meaning that character may
+ non-alphanumeric character, it takes away any special meaning that character
- have. This use of backslash as an escape character applies both inside and
+ may have. This use of backslash as an escape character applies both inside and
  outside character classes.
  </P>
  <P>
  For example, if you want to match a * character, you write \* in the pattern.
  This escaping action applies whether or not the following character would
- otherwise be interpreted as a meta-character, so it is always safe to precede a
+ otherwise be interpreted as a metacharacter, so it is always safe to precede a
- non-alphameric with backslash to specify that it stands for itself. In
+ non-alphanumeric with backslash to specify that it stands for itself. In
  particular, if you want to match a backslash, you write \\.
  </P>
  <P>
  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
  pattern (other than in a character class) and characters between a # outside
- a character class and the next newline character are ignored. An escaping
+ a character class and the next newline are ignored. An escaping backslash can
- backslash can be used to include a whitespace or # character as part of the
+ be used to include a whitespace or # character as part of the pattern.
- pattern.
  </P>
  <P>
  If you want to remove the special meaning from a sequence of characters, you
  can do so by putting them between \Q and \E. This is different from Perl in
  that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
  Perl, $ and @ cause variable interpolation. Note the following examples:
- </P>
- <P>
  <pre>
    Pattern            PCRE matches   Perl matches
- </PRE>
- </P>
+   \Qabc$xyz\E        abc$xyz        abc followed by the contents of $xyz
- <P>
- <pre>
-   \Qabc$xyz\E        abc$xyz        abc followed by the
-                                       contents of $xyz
    \Qabc\$xyz\E       abc\$xyz       abc\$xyz
    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
- </PRE>
+ </pre>
- </P>
- <P>
  The \Q...\E sequence is recognized both inside and outside character classes.
- </P>
+ <a name="digitsafterbackslash"></a></P>
+ <br><b>
+ Non-printing characters
+ </b><br>
  <P>
  A second use of backslash provides a way of encoding non-printing characters
  in patterns in a visible manner. There is no restriction on the appearance of
-Line 157 
 non-printing characters, apart from the
+Line 164 
 non-printing characters, apart from the
  but when a pattern is being prepared by text editing, it is usually easier to
  use one of the following escape sequences than the binary character it
  represents:
- </P>
- <P>
  <pre>
    \a        alarm, that is, the BEL character (hex 07)
    \cx       "control-x", where x is any character
-Line 169 
 represents:
+Line 174 
 represents:
    \t        tab (hex 09)
    \ddd      character with octal code ddd, or backreference
    \xhh      character with hex code hh
-   \x{hhh..} character with hex code hhh... (UTF-8 mode only)
+   \x{hhh..} character with hex code hhh..
- </PRE>
+ </pre>
- </P>
- <P>
  The precise effect of \cx is as follows: if x is a lower case letter, it
  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
  Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
-Line 180 
 Thus \cz becomes hex 1A, but \c{ becomes
+Line 183 
 Thus \cz becomes hex 1A, but \c{ becomes
  </P>
  <P>
  After \x, from zero to two hexadecimal digits are read (letters can be in
- upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
+ upper or lower case). Any number of hexadecimal digits may appear between \x{
- appear between \x{ and }, but the value of the character code must be less
+ and }, but the value of the character code must be less than 256 in non-UTF-8
- than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
+ mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
- other than hexadecimal digits appear between \x{ and }, or if there is no
+ is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{
- terminating }, this form of escape is not recognized. Instead, the initial
+ and }, or if there is no terminating }, this form of escape is not recognized.
- \x will be interpreted as a basic hexadecimal escape, with no following
+ Instead, the initial \x will be interpreted as a basic hexadecimal escape,
- digits, giving a byte whose value is zero.
+ 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
- syntaxes for \x when PCRE is in UTF-8 mode. There is no difference in the
+ syntaxes for \x. There is no difference in the way they are handled. For
- way they are handled. For example, \xdc is exactly the same as \x{dc}.
+ example, \xdc is exactly the same as \x{dc}.
  </P>
  <P>
- After \0 up to two further octal digits are read. In both cases, if there
+ After \0 up to two further octal digits are read. If there are fewer than two
- are fewer than two digits, just those that are present are used. Thus the
+ digits, just those that are present are used. Thus the sequence \0\x\07
- sequence \0\x\07 specifies two binary zeros followed by a BEL character
+ specifies two binary zeros followed by a BEL character (code value 7). Make
- (code value 7). Make sure you supply two digits after the initial zero if the
+ sure you supply two digits after the initial zero if the pattern character that
- character that follows is itself an octal digit.
+ follows is itself an octal digit.
  </P>
  <P>
  The handling of a backslash followed by a digit other than 0 is complicated.
-Line 206 
 Outside a character class, PCRE reads it
+Line 209 
 Outside a character class, PCRE reads it
  number. If the number is less than 10, or if there have been at least that many
  previous capturing left parentheses in the expression, the entire sequence is
  taken as a <i>back reference</i>. A description of how this works is given
- later, following the discussion of parenthesized subpatterns.
+ <a href="#backreferences">later,</a>
+ following the discussion of
+ <a href="#subpattern">parenthesized subpatterns.</a>
  </P>
  <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, and generates a single byte from the least
+ digits following the backslash, ane uses them to generate a data character. Any
- significant 8 bits of the value. Any subsequent digits stand for themselves.
+ subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
- For example:
+ character specified in octal must be less than \400. In UTF-8 mode, values up
- </P>
+ to \777 are permitted. For example:
- <P>
  <pre>
    \040   is another way of writing a space
-   \40    is the same, provided there are fewer than 40
+   \40    is the same, provided there are fewer than 40 previous capturing subpatterns
-             previous capturing subpatterns
    \7     is always a back reference
-   \11    might be a back reference, or another way of
+   \11    might be a back reference, or another way of writing a tab
-             writing a tab
    \011   is always a tab
    \0113  is a tab followed by the character "3"
-   \113   might be a back reference, otherwise the
+   \113   might be a back reference, otherwise the character with octal code 113
-             character with octal code 113
+   \377   might be a back reference, otherwise the byte consisting entirely of 1 bits
-   \377   might be a back reference, otherwise
+   \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"
-             the byte consisting entirely of 1 bits
+ </pre>
-   \81    is either a back reference, or a binary zero
-             followed by the two characters "8" and "1"
- </PRE>
- </P>
- <P>
  Note that octal values of 100 or greater must not be introduced by a leading
  zero, because no more than three octal digits are ever read.
  </P>
  <P>
- All the sequences that define a single byte value or a single UTF-8 character
+ All the sequences that define a single character value can be used both inside
- (in UTF-8 mode) can be used both inside and outside character classes. In
+ and outside character classes. In addition, inside a character class, the
- addition, inside a character class, the sequence \b is interpreted as the
+ sequence \b is interpreted as the backspace character (hex 08), and the
- backspace character (hex 08). Outside a character class it has a different
+ sequence \X is interpreted as the character "X". Outside a character class,
- meaning (see below).
+ these sequences have different meanings
- </P>
+ <a href="#uniextseq">(see below).</a>
- <P>
+ </P>
- The third use of backslash is for specifying generic character types:
+ <br><b>
- </P>
+ Generic character types
+ </b><br>
  <P>
+ The third 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
 Line 255 
 The third use of backslash is for specif
    \S     any character that is not a whitespace character
    \w     any "word" character
    \W     any "non-word" character
- </PRE>
+ </pre>
- </P>
- <P>
  Each pair of escape sequences partitions the complete set of characters into
  two disjoint sets. Any given character matches one, and only one, of each pair.
  </P>
  <P>
- In UTF-8 mode, characters with values greater than 255 never match \d, \s, or
+ These character type sequences can appear both inside and outside character
- \w, and always match \D, \S, and \W.
+ classes. They each match one character of the appropriate type. If the current
+ matching point is at the end of the subject string, all of them fail, since
+ there is no character to match.
  </P>
  <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).
+ 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.)
  </P>
  <P>
- A "word" character is any letter or digit or the underscore character, that is,
+ A "word" character is an underscore or any character less than 256 that is a
- any character which can be part of a Perl "word". The definition of letters and
+ letter or digit. The definition of letters and digits is controlled by PCRE's
- digits is controlled by PCRE's character tables, and may vary if locale-
+ low-valued character tables, and may vary if locale-specific matching is taking
- specific matching is taking place (see
+ 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" (French) locale, some character codes greater
+ page). For example, in the "fr_FR" (French) locale, some character codes
- than 128 are used for accented letters, and these are matched by \w.
+ greater than 128 are used for accented letters, and these are matched by \w.
  </P>
  <P>
- These character type sequences can appear both inside and outside character
+ In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
- classes. They each match one character of the appropriate type. If the current
+ \w, and always match \D, \S, and \W. This is true even when Unicode
- matching point is at the end of the subject string, all of them fail, since
+ character property support is available. The use of locales with Unicode is
- there is no character to match.
+ discouraged.
- </P>
+ <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
+ is selected. They 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
+   \X       an extended Unicode sequence
+ </pre>
+ The property names represented by <i>xx</i> above are limited to the Unicode
+ script names, the general category properties, and "Any", which matches any
+ character (including newline). Other properties such as "InMusicalSymbols" are
+ not currently supported by PCRE. Note that \P{Any} does not match any
+ characters, so always causes a match failure.
+ </P>
+ <P>
+ Sets of Unicode characters are defined as belonging to certain scripts. A
+ character from one of these sets can be matched using a script name. For
+ example:
+ <pre>
+   \p{Greek}
+   \P{Han}
+ </pre>
+ Those that are not part of an identified script are lumped together as
+ "Common". The current list of scripts is:
+ </P>
+ <P>
+ Arabic,
+ Armenian,
+ Bengali,
+ Bopomofo,
+ Braille,
+ Buginese,
+ Buhid,
+ Canadian_Aboriginal,
+ Cherokee,
+ Common,
+ Coptic,
+ Cypriot,
+ Cyrillic,
+ Deseret,
+ Devanagari,
+ Ethiopic,
+ Georgian,
+ Glagolitic,
+ Gothic,
+ Greek,
+ Gujarati,
+ Gurmukhi,
+ Han,
+ Hangul,
+ Hanunoo,
+ Hebrew,
+ Hiragana,
+ Inherited,
+ Kannada,
+ Katakana,
+ Kharoshthi,
+ Khmer,
+ Lao,
+ Latin,
+ Limbu,
+ Linear_B,
+ Malayalam,
+ Mongolian,
+ Myanmar,
+ New_Tai_Lue,
+ Ogham,
+ Old_Italic,
+ Old_Persian,
+ Oriya,
+ Osmanya,
+ Runic,
+ Shavian,
+ Sinhala,
+ Syloti_Nagri,
+ Syriac,
+ Tagalog,
+ Tagbanwa,
+ Tai_Le,
+ Tamil,
+ Telugu,
+ Thaana,
+ Thai,
+ Tibetan,
+ Tifinagh,
+ Ugaritic,
+ Yi.
+ </P>
+ <P>
+ Each character has exactly one general category property, specified by a
+ two-letter abbreviation. For compatibility with Perl, negation can be specified
+ by including a circumflex between the opening brace and the property name. For
+ example, \p{^Lu} is the same as \P{Lu}.
+ </P>
+ <P>
+ If only one letter is specified with \p or \P, it includes all the general
+ category properties that start with that letter. In this case, in the absence
+ of negation, the curly brackets in the escape sequence are optional; these two
+ examples have the same effect:
+ <pre>
+   \p{L}
+   \pL
+ </pre>
+ The following general category property codes are supported:
+ <pre>
+   C     Other
+   Cc    Control
+   Cf    Format
+   Cn    Unassigned
+   Co    Private use
+   Cs    Surrogate
+   L     Letter
+   Ll    Lower case letter
+   Lm    Modifier letter
+   Lo    Other letter
+   Lt    Title case letter
+   Lu    Upper case letter
+   M     Mark
+   Mc    Spacing mark
+   Me    Enclosing mark
+   Mn    Non-spacing mark
+   N     Number
+   Nd    Decimal number
+   Nl    Letter number
+   No    Other number
+   P     Punctuation
+   Pc    Connector punctuation
+   Pd    Dash punctuation
+   Pe    Close punctuation
+   Pf    Final punctuation
+   Pi    Initial punctuation
+   Po    Other punctuation
+   Ps    Open punctuation
+   S     Symbol
+   Sc    Currency symbol
+   Sk    Modifier symbol
+   Sm    Mathematical symbol
+   So    Other symbol
+   Z     Separator
+   Zl    Line separator
+   Zp    Paragraph separator
+   Zs    Space separator
+ </pre>
+ The special property L& is also supported: it matches a character that has
+ the Lu, Ll, or Lt property, in other words, a letter that is not classified as
+ a modifier or "other".
+ </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".
+ </P>
+ <P>
+ No character that is in the Unicode table has the Cn (unassigned) property.
+ Instead, this property is assumed for any code point that is not in the
+ Unicode table.
+ </P>
+ <P>
+ Specifying caseless matching does not affect these escape sequences. For
+ example, \p{Lu} always matches only upper case letters.
+ </P>
+ <P>
+ The \X escape matches any number of Unicode characters that form an extended
+ Unicode sequence. \X is equivalent to
+ <pre>
+   (?&#62;\PM\pM*)
+ </pre>
+ That is, it matches a character without the "mark" property, followed by zero
+ or more characters with the "mark" property, and treats the sequence as an
+ atomic group
+ <a href="#atomicgroup">(see below).</a>
+ Characters with the "mark" property are typically accents that affect the
+ preceding 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="smallassertions"></a></P>
+ <br><b>
+ Simple assertions
+ </b><br>
  <P>
  The fourth 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 below. The backslashed
+ subpatterns for more complicated assertions is described
- assertions are
+ <a href="#bigassertions">below.</a>
- </P>
+ The backslashed assertions are:
- <P>
  <pre>
    \b     matches at a word boundary
    \B     matches when not at a word boundary
-Line 302 
 assertions are
+Line 496 
 assertions are
    \Z     matches at end of subject or before newline at end
    \z     matches at end of subject
    \G     matches at first matching position in subject
- </PRE>
+ </pre>
- </P>
- <P>
  These assertions may not appear in character classes (but note that \b has a
  different meaning, namely the backspace character, inside a character class).
  </P>
-Line 316 
 first or last character matches \w, resp
+Line 508 
 first or last character matches \w, resp
  </P>
  <P>
  The \A, \Z, and \z assertions differ from the traditional circumflex and
- dollar (described below) in that they only ever match at the very start and end
+ dollar (described in the next section) in that they only ever match at the very
- of the subject string, whatever options are set. Thus, they are independent of
+ start and end of the subject string, whatever options are set. Thus, they are
- multiline mode.
+ independent of multiline mode. These three assertions are not affected by the
- </P>
+ PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
- <P>
+ circumflex and dollar metacharacters. However, if the <i>startoffset</i>
- They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the
+ argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
- <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero, indicating that
+ at a point other than the beginning of the subject, \A can never match. The
- matching is to start at a point other than the beginning of the subject, \A
+ difference between \Z and \z is that \Z matches before a newline at the end
- can never match. The difference between \Z and \z is that \Z matches before
+ of the string as well as at the very end, whereas \z matches only at the end.
- a newline that is the last character of the string as well as at the end of the
- string, whereas \z matches only at the end.
  </P>
  <P>
  The \G assertion is true only when the current matching position is at the
-Line 351 
 regular expression.
+Line 541 
 regular expression.
  <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
  <P>
  Outside a character class, in the default matching mode, the circumflex
- character is an assertion which is true only if the current matching point is
+ character is an assertion that is true only if the current matching point is
  at the start of the subject string. If the <i>startoffset</i> argument of
  <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
  option is unset. Inside a character class, circumflex has an entirely different
- meaning (see below).
+ meaning
+ <a href="#characterclass">(see below).</a>
  </P>
  <P>
  Circumflex need not be the first character of the pattern if a number of
-Line 367 
 constrained to match only at the start o
+Line 558 
 constrained to match only at the start o
  to be anchored.)
  </P>
  <P>
- A dollar character is an assertion which is true only if the current matching
+ A dollar character is an assertion that is true only if the current matching
  point is at the end of the subject string, or immediately before a newline
- character that is the last character in the string (by default). Dollar need
+ at the end of the string (by default). Dollar need not be the last character of
- not be the last character of the pattern if a number of alternatives are
+ the pattern if a number of alternatives are involved, but it should be the last
- involved, but it should be the last item in any branch in which it appears.
+ item in any branch in which it appears. Dollar has no special meaning in a
- Dollar has no special meaning in a character class.
+ character class.
  </P>
  <P>
  The meaning of dollar can be changed so that it matches only at the very end of
-Line 381 
 does not affect the \Z assertion.
+Line 572 
 does not affect the \Z assertion.
  </P>
  <P>
  The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set. When this is the case, they match immediately
+ PCRE_MULTILINE option is set. When this is the case, a circumflex matches
- after and immediately before an internal newline character, respectively, in
+ immediately after internal newlines as well as at the start of the subject
- addition to matching at the start and end of the subject string. For example,
+ string. It does not match after a newline that ends the string. A dollar
- the pattern /^abc$/ matches the subject string "def\nabc" in multiline mode,
+ matches before any newlines in the string, as well as at the very end, when
- but not otherwise. Consequently, patterns that are anchored in single line mode
+ PCRE_MULTILINE is set. When newline is specified as the two-character
- because all branches start with ^ are not anchored in multiline mode, and a
+ sequence CRLF, isolated CR and LF characters do not indicate newlines.
- match for circumflex is possible when the <i>startoffset</i> argument of
+ </P>
- <b>pcre_exec()</b> is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
+ <P>
- PCRE_MULTILINE is set.
+ For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
+ \n represents a newline) in multiline mode, but not otherwise. Consequently,
+ patterns that are anchored in single line mode because all branches start with
+ ^ are not anchored in multiline mode, and a match for circumflex is possible
+ when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
+ PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
  </P>
  <P>
  Note that the sequences \A, \Z, and \z can be used to match the start and
  end of the subject in both modes, and if all branches of a pattern start with
- \A it is always anchored, whether PCRE_MULTILINE is set or not.
+ \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>
  <P>
  Outside a character class, a dot in the pattern matches any one character in
- the subject, including a non-printing character, but not (by default) newline.
+ the subject string except (by default) a character that signifies the end of a
- In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
+ line. In UTF-8 mode, the matched character may be more than one byte long. When
- byte long, except (by default) for newline. If the PCRE_DOTALL option is set,
+ a line ending is defined as a single character (CR or LF), dot never matches
- dots match newlines as well. The handling of dot is entirely independent of the
+ that character; when the two-character sequence CRLF is used, dot does not
- handling of circumflex and dollar, the only relationship being that they both
+ match CR if it is immediately followed by LF, but otherwise it matches all
- involve newline characters. Dot has no special meaning in a character class.
+ characters (including isolated CRs and LFs).
+ </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
+ is defined as the two-character sequence CRLF, 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>
  <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 a newline. The
+ in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The
  feature is provided in Perl in order to match individual bytes in UTF-8 mode.
  Because it breaks up UTF-8 characters into individual bytes, what remains in
- the string may be a malformed UTF-8 string. For this reason it is best avoided.
+ the string may be a malformed UTF-8 string. For this reason, the \C escape
+ sequence is best avoided.
  </P>
  <P>
- PCRE does not allow \C to appear in lookbehind assertions (see below), because
+ PCRE does not allow \C to appear in lookbehind assertions
- in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.
+ <a href="#lookbehind">(described below),</a>
- </P>
+ because in UTF-8 mode this would make it impossible to calculate the length of
- <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS</a><br>
+ the lookbehind.
+ <a name="characterclass"></a></P>
+ <br><a name="SEC6" 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 438 
 backslash.
+Line 647 
 backslash.
  <P>
  For example, the character class [aeiou] matches any lower case vowel, while
  [^aeiou] matches any character that is not a lower case vowel. Note that a
- circumflex is just a convenient notation for specifying the characters which
+ circumflex is just a convenient notation for specifying the characters that
- are in the class by enumerating those that are not. It is not an assertion: it
+ are in the class by enumerating those that are not. A class that starts with a
- still consumes a character from the subject string, and fails if the current
+ circumflex is not an assertion: it still consumes a character from the subject
- pointer is at the end of the string.
+ string, and therefore it fails if the current pointer is at the end of the
+ string.
  </P>
  <P>
  In UTF-8 mode, characters with values greater than 255 can be included in a
-Line 451 
 class as a literal string of bytes, or b
+Line 661 
 class as a literal string of bytes, or b
  When caseless matching is set, any letters in a class represent both their
  upper case and lower case versions, so for example, a caseless [aeiou] matches
  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
- caseful version would. PCRE does not support the concept of case for characters
+ caseful version would. In UTF-8 mode, PCRE always understands the concept of
- with values greater than 255.
+ case for characters whose values are less than 128, so caseless matching is
+ always possible. For characters with higher values, the concept of case is
+ supported if PCRE is compiled with Unicode property support, but not otherwise.
+ If you want to use caseless matching for characters 128 and above, you must
+ ensure that PCRE is compiled with Unicode property support as well as with
+ UTF-8 support.
  </P>
  <P>
- The newline character is never treated in any special way in character classes,
+ Characters that might indicate line breaks (CR and LF) are never treated in any
- whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
+ special way when matching character classes, whatever line-ending sequence is
- such as [^a] will always match a newline.
+ in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is
+ used. A class such as [^a] always matches one of these characters.
  </P>
  <P>
  The minus (hyphen) character can be used to specify a range of characters in a
-Line 471 
 It is not possible to have the literal c
+Line 687 
 It is not possible to have the literal c
  range. A pattern such as [W-]46] is interpreted as a class of two characters
  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
- the end of range, so [W-\]46] is interpreted as a single class containing a
+ the end of range, so [W-\]46] is interpreted as a class containing a range
- range followed by two separate characters. The octal or hexadecimal
+ followed by two other characters. The octal or hexadecimal representation of
- representation of "]" can also be used to end a range.
+ "]" can also be used to end a range.
  </P>
  <P>
  Ranges operate in the collating sequence of character values. They can also be
-Line 484 
 example [\x{100}-\x{2ff}].
+Line 700 
 example [\x{100}-\x{2ff}].
  <P>
  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 if character tables for the "fr"
+ [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
- locale are in use, [\xc8-\xcb] matches accented E characters in both cases.
+ tables for the "fr_FR" 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.
  </P>
  <P>
- The character types \d, \D, \s, \S, \w, and \W may also appear in a
+ The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
- character class, and add the characters that they match to the class. For
+ in a character class, and add the characters that they match to the class. For
  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
  conveniently be used with the upper case character types to specify a more
  restricted set of characters than the matching lower case type. For example,
  the class [^\W_] matches any letter or digit, but not underscore.
  </P>
  <P>
- All non-alphameric characters other than \, -, ^ (at the start) and the
+ The only metacharacters that are recognized in character classes are backslash,
- terminating ] are non-special in character classes, but it does no harm if they
+ hyphen (only where it can be interpreted as specifying a range), circumflex
- are escaped.
+ (only at the start), opening square bracket (only when it can be interpreted as
+ introducing a POSIX class name - see the next section), and the terminating
+ closing square bracket. However, escaping other non-alphanumeric characters
+ does no harm.
  </P>
  <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
  <P>
- Perl supports the POSIX notation for character classes, which uses names
+ Perl supports the POSIX notation for character classes. This uses names
  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
  this notation. For example,
- </P>
- <P>
  <pre>
    [01[:alpha:]%]
- </PRE>
+ </pre>
- </P>
- <P>
  matches "0", "1", any alphabetic character, or "%". The supported class names
  are
- </P>
- <P>
  <pre>
    alnum    letters and digits
    alpha    letters
-Line 531 
 are
+Line 747 
 are
    upper    upper case letters
    word     "word" characters (same as \w)
    xdigit   hexadecimal digits
- </PRE>
+ </pre>
- </P>
- <P>
  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
  space (32). Notice that this list includes the VT character (code 11). This
  makes "space" different to \s, which does not include VT (for Perl
-Line 543 
 compatibility).
+Line 757 
 compatibility).
  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
 .8. Another Perl extension is negation, which is indicated by a ^ character
  after the colon. For example,
- </P>
- <P>
  <pre>
    [12[:^digit:]]
- </PRE>
+ </pre>
- </P>
- <P>
  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
  supported, and an error is given if they are encountered.
  </P>
  <P>
- In UTF-8 mode, characters with values greater than 255 do not match any of
+ 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>
  <P>
  Vertical bar characters are used to separate alternative patterns. For example,
  the pattern
- </P>
- <P>
  <pre>
    gilbert|sullivan
- </PRE>
+ </pre>
- </P>
- <P>
  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
- and an empty alternative is permitted (matching the empty string).
+ and an empty alternative is permitted (matching the empty string). The matching
- The matching process tries each alternative in turn, from left to right,
+ process tries each alternative in turn, from left to right, and the first one
- and the first one that succeeds is used. If the alternatives are within a
+ that succeeds is used. If the alternatives are within a subpattern
- subpattern (defined below), "succeeds" means matching the rest of the main
+ <a href="#subpattern">(defined below),</a>
- pattern as well as the alternative in the subpattern.
+ "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>
  <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
  Perl option letters enclosed between "(?" and ")". The option letters are
- </P>
- <P>
  <pre>
    i  for PCRE_CASELESS
    m  for PCRE_MULTILINE
    s  for PCRE_DOTALL
    x  for PCRE_EXTENDED
- </PRE>
+ </pre>
- </P>
- <P>
  For example, (?im) sets caseless, multiline matching. It is also possible to
  unset these options by preceding the letter with a hyphen, and a combined
  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
-Line 608 
 the global options (and it will therefor
+Line 811 
 the global options (and it will therefor
  <P>
  An option change within a subpattern affects only that part of the current
  pattern that follows it, so
- </P>
- <P>
  <pre>
    (a(?i)b)c
- </PRE>
+ </pre>
- </P>
- <P>
  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
  By this means, options can be made to have different settings in different
  parts of the pattern. Any changes made in one alternative do carry on
  into subsequent branches within the same subpattern. For example,
- </P>
- <P>
  <pre>
    (a(?i)b|c)
- </PRE>
+ </pre>
- </P>
- <P>
  matches "ab", "aB", "c", and "C", even though when matching "C" the first
  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_UNGREEDY and PCRE_EXTRA can be changed in the
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- same way as the Perl-compatible options by using the characters U and X
+ changed in the same way as the Perl-compatible options by using the characters
- respectively. The (?X) flag setting is special in that it must always occur
+ J, U and X respectively.
- earlier in the pattern than any of the additional features it turns on, even
+ <a name="subpattern"></a></P>
- when it is at top level. It is best put at the start.
- </P>
  <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>
  <P>
  Subpatterns are delimited by parentheses (round brackets), which can be nested.
- Marking part of a pattern as a subpattern does two things:
+ Turning part of a pattern into a subpattern does two things:
- </P>
+ <br>
- <P>
+ <br>
 . It localizes a set of alternatives. For example, the pattern
- </P>
- <P>
  <pre>
    cat(aract|erpillar|)
- </PRE>
+ </pre>
- </P>
- <P>
  matches one of the words "cat", "cataract", or "caterpillar". Without the
  parentheses, it would match "cataract", "erpillar" or the empty string.
- </P>
+ <br>
- <P>
+ <br>
-. It sets up the subpattern as a capturing subpattern (as defined above).
+. It sets up the subpattern as a capturing subpattern. This means that, when
- When the whole pattern matches, that portion of the subject string that matched
+ the whole pattern matches, that portion of the subject string that matched the
- the subpattern is passed back to the caller via the <i>ovector</i> argument of
+ subpattern is passed back to the caller via the <i>ovector</i> argument of
  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
- from 1) to obtain the numbers of the capturing subpatterns.
+ from 1) to obtain numbers for the capturing subpatterns.
  </P>
  <P>
  For example, if the string "the red king" is matched against the pattern
- </P>
- <P>
  <pre>
    the ((red|white) (king|queen))
- </PRE>
+ </pre>
- </P>
- <P>
  the captured substrings are "red king", "red", and "king", and are numbered 1,
 , and 3, respectively.
  </P>
-Line 681 
 capturing requirement. If an opening par
+Line 866 
 capturing requirement. If an opening par
  and a colon, the subpattern does not do any capturing, and is not counted when
  computing the number of any subsequent capturing subpatterns. For example, if
  the string "the white queen" is matched against the pattern
- </P>
- <P>
  <pre>
    the ((?:red|white) (king|queen))
- </PRE>
+ </pre>
- </P>
- <P>
  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
  of nesting of all subpatterns, both capturing and non-capturing, is 200.
-Line 696 
 of nesting of all subpatterns, both capt
+Line 877 
 of nesting of all subpatterns, both capt
  As a convenient shorthand, if any option settings are required at the start of
  a non-capturing subpattern, the option letters may appear between the "?" and
  the ":". Thus the two patterns
- </P>
- <P>
  <pre>
    (?i:saturday|sunday)
    (?:(?i)saturday|sunday)
- </PRE>
+ </pre>
- </P>
- <P>
  match exactly the same set of strings. Because alternative branches are tried
  from left to right, and options are not reset until the end of the subpattern
  is reached, an option setting in one branch does affect subsequent branches, so
-Line 713 
 the above patterns match "SUNDAY" as wel
+Line 890 
 the above patterns match "SUNDAY" as wel
  <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 the
+ if an expression is modified, the numbers may change. To help with this
  difficulty, PCRE supports the naming of subpatterns, something that Perl does
- not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of
+ not provide. The Python syntax (?P&#60;name&#62;...) is used. References to capturing
- alphanumeric characters and underscores, and must be unique within a pattern.
+ parentheses from other parts of the pattern, such as
- </P>
+ <a href="#backreferences">backreferences,</a>
- <P>
+ <a href="#recursion">recursion,</a>
- Named capturing parentheses are still allocated numbers as well as names. The
+ and
- PCRE API provides function calls for extracting the name-to-number translation
+ <a href="#conditions">conditions,</a>
- table from a compiled pattern. For further details see the
+ 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
+ API provides function calls for extracting the name-to-number translation table
+ from a compiled pattern. There 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
+ this constraint by setting the PCRE_DUPNAMES option at compile time. This can
+ be useful for patterns where only one instance of the named parentheses can
+ match. Suppose you want to match the name of a weekday, either as a 3-letter
+ abbreviation or as the full name, and in both cases you want to extract the
+ abbreviation. This pattern (ignoring the line breaks) does the job:
+ <pre>
+   (?P&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
+   (?P&#60;DN&#62;Tue)(?:sday)?|
+   (?P&#60;DN&#62;Wed)(?:nesday)?|
+   (?P&#60;DN&#62;Thu)(?:rsday)?|
+   (?P&#60;DN&#62;Sat)(?:urday)?
+ </pre>
+ There are five capturing substrings, but only one is ever set after a match.
+ The convenience function for extracting the data by name returns the substring
+ for the first, and in this example, the only, subpattern of that name that
+ matched. This saves searching to find which numbered subpattern it was. If you
+ make a reference to a non-unique named subpattern from elsewhere in the
+ pattern, the one that corresponds to the lowest number is used. For further
+ details of the interfaces for handling named subpatterns, see the
  <a href="pcreapi.html"><b>pcreapi</b></a>
  documentation.
  </P>
-Line 729 
 documentation.
+Line 935 
 documentation.
  <P>
  Repetition is specified by quantifiers, which can follow any of the following
  items:
- </P>
- <P>
  <pre>
    a literal data character
    the . metacharacter
    the \C escape sequence
-   escapes such as \d that match single characters
+   the \X escape sequence (in UTF-8 mode with Unicode properties)
+   an escape such as \d that matches a single character
    a character class
    a back reference (see next section)
    a parenthesized subpattern (unless it is an assertion)
- </PRE>
+ </pre>
- </P>
- <P>
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
  separated by a comma. The numbers must be less than 65536, and the first must
  be less than or equal to the second. For example:
- </P>
- <P>
  <pre>
    z{2,4}
- </PRE>
+ </pre>
- </P>
- <P>
  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
  character. If the second number is omitted, but the comma is present, there is
  no upper limit; if the second number and the comma are both omitted, the
  quantifier specifies an exact number of required matches. Thus
- </P>
- <P>
  <pre>
    [aeiou]{3,}
- </PRE>
+ </pre>
- </P>
- <P>
  matches at least 3 successive vowels, but may match many more, while
- </P>
- <P>
  <pre>
    \d{8}
- </PRE>
+ </pre>
- </P>
- <P>
  matches exactly 8 digits. An opening curly bracket that appears in a position
  where a quantifier is not allowed, or one that does not match the syntax of a
  quantifier, is taken as a literal character. For example, {,6} is not a
-Line 780 
 quantifier, but a literal string of four
+Line 971 
 quantifier, but a literal string of four
  <P>
  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
  bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
- which is represented by a two-byte sequence.
+ which is represented by a two-byte sequence. Similarly, when Unicode property
+ support is available, \X{3} matches three Unicode extended sequences, each of
+ which may be several bytes long (and they may be of different lengths).
  </P>
  <P>
  The quantifier {0} is permitted, causing the expression to behave as if the
-Line 789 
 previous item and the quantifier were no
+Line 982 
 previous item and the quantifier were no
  <P>
  For convenience (and historical compatibility) the three most common
  quantifiers have single-character abbreviations:
- </P>
- <P>
  <pre>
    *    is equivalent to {0,}
    +    is equivalent to {1,}
    ?    is equivalent to {0,1}
- </PRE>
+ </pre>
- </P>
- <P>
  It is possible to construct infinite loops by following a subpattern that can
  match no characters with a quantifier that has no upper limit, for example:
- </P>
- <P>
  <pre>
    (a?)*
- </PRE>
+ </pre>
- </P>
- <P>
  Earlier versions of Perl and PCRE used to give an error at compile time for
  such patterns. However, because there are cases where this can be useful, such
  patterns are now accepted, but if any repetition of the subpattern does in fact
-Line 816 
 match no characters, the loop is forcibl
+Line 1001 
 match no characters, the loop is forcibl
  By default, the quantifiers are "greedy", that is, they match as much as
  possible (up to the maximum number of permitted times), without causing the
  rest of the pattern to fail. The classic example of where this gives problems
- is in trying to match comments in C programs. These appear between the
+ is in trying to match comments in C programs. These appear between /* and */
- sequences /* and */ and within the sequence, individual * and / characters may
+ and within the comment, individual * and / characters may appear. An attempt to
- appear. An attempt to match C comments by applying the pattern
+ match C comments by applying the pattern
- </P>
- <P>
  <pre>
    /\*.*\*/
- </PRE>
+ </pre>
- </P>
- <P>
  to the string
- </P>
- <P>
  <pre>
-   /* first command */  not comment  /* second comment */
+   /* first comment */  not comment  /* second comment */
- </PRE>
+ </pre>
- </P>
- <P>
  fails, because it matches the entire string owing to the greediness of the .*
  item.
  </P>
-Line 841 
 item.
+Line 1018 
 item.
  However, if a quantifier is followed by a question mark, it ceases to be
  greedy, and instead matches the minimum number of times possible, so the
  pattern
- </P>
- <P>
  <pre>
    /\*.*?\*/
- </PRE>
+ </pre>
- </P>
- <P>
  does the right thing with the C comments. The meaning of the various
  quantifiers is not otherwise changed, just the preferred number of matches.
  Do not confuse this use of question mark with its use as a quantifier in its
  own right. Because it has two uses, it can sometimes appear doubled, as in
- </P>
- <P>
  <pre>
    \d??\d
- </PRE>
+ </pre>
- </P>
- <P>
  which matches one digit by preference, but can match two if that is the only
  way the rest of the pattern matches.
  </P>
-Line 870 
 default behaviour.
+Line 1039 
 default behaviour.
  </P>
  <P>
  When a parenthesized subpattern is quantified with a minimum repeat count that
- is greater than 1 or with a limited maximum, more store is required for the
+ is greater than 1 or with a limited maximum, more memory is required for the
  compiled pattern, in proportion to the size of the minimum or maximum.
  </P>
  <P>
-Line 891 
 However, there is one situation where th
+Line 1060 
 However, there is one situation where th
  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
  succeed. Consider, for example:
- </P>
- <P>
  <pre>
    (.*)abc\1
- </PRE>
+ </pre>
- </P>
- <P>
  If the subject is "xyz123abc123" the match point is the fourth character. For
  this reason, such a pattern is not implicitly anchored.
  </P>
  <P>
  When a capturing subpattern is repeated, the value captured is the substring
  that matched the final iteration. For example, after
- </P>
- <P>
  <pre>
    (tweedle[dume]{3}\s*)+
- </PRE>
+ </pre>
- </P>
- <P>
  has matched "tweedledum tweedledee" the value of the captured substring is
  "tweedledee". However, if there are nested capturing subpatterns, the
  corresponding captured values may have been set in previous iterations. For
  example, after
- </P>
- <P>
  <pre>
    /(a|(b))+/
- </PRE>
+ </pre>
- </P>
- <P>
  matches "aba" the value of the second captured substring is "b".
- </P>
+ <a name="atomicgroup"></a></P>
  <br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
  <P>
  With both maximizing and minimizing repetition, failure of what follows
-Line 935 
 there is no point in carrying on.
+Line 1092 
 there is no point in carrying on.
  </P>
  <P>
  Consider, for example, the pattern \d+foo when applied to the subject line
- </P>
- <P>
  <pre>
 bar
- </PRE>
+ </pre>
- </P>
- <P>
  After matching all 6 digits and then failing to match "foo", the normal
  action of the matcher is to try again with only 5 digits matching the \d+
  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
-Line 952 
 that once a subpattern has matched, it i
+Line 1105 
 that once a subpattern has matched, it i
  If we use atomic grouping for the previous example, the matcher would give 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:
- </P>
- <P>
  <pre>
    (?&#62;\d+)foo
- </PRE>
+ </pre>
- </P>
- <P>
  This kind of parenthesis "locks up" the  part of the pattern it contains once
  it has matched, and a failure further into the pattern is prevented from
  backtracking into it. Backtracking past it to previous items, however, works as
-Line 983 
 group is just a single repeated item, as
+Line 1132 
 group is just a single repeated item, as
  notation, called a "possessive quantifier" can be used. This consists of an
  additional + character following a quantifier. Using this notation, the
  previous example can be rewritten as
- </P>
- <P>
  <pre>
-   \d++bar
+   \d++foo
- </PRE>
+ </pre>
- </P>
- <P>
  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
  possessive quantifier and the equivalent atomic group.
  </P>
  <P>
- The possessive quantifier syntax is an extension to the Perl syntax. It
+ The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey
- originates in Sun's Java package.
+ 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.
  </P>
  <P>
  When a pattern contains an unlimited repeat inside a subpattern that can itself
  be repeated an unlimited number of times, the use of an atomic group is the
  only way to avoid some failing matches taking a very long time indeed. The
  pattern
- </P>
- <P>
  <pre>
    (\D+|&#60;\d+&#62;)*[!?]
- </PRE>
+ </pre>
- </P>
- <P>
  matches an unlimited number of substrings that either consist of non-digits, or
  digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
  quickly. However, if it is applied to
- </P>
- <P>
  <pre>
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
- </PRE>
+ </pre>
- </P>
- <P>
  it takes a long time before reporting failure. This is because the string can
- be divided between the two repeats in a large number of ways, and all have to
+ be divided between the internal \D+ repeat and the external * repeat in a
- be tried. (The example used [!?] rather than a single character at the end,
+ large number of ways, and all have to be tried. (The example uses [!?] rather
- because both PCRE and Perl have an optimization that allows for fast failure
+ than a single character at the end, because both PCRE and Perl have an
- when a single character is used. They remember the last single character that
+ optimization that allows for fast failure when a single character is used. They
- is required for a match, and fail early if it is not present in the string.)
+ remember the last single character that is required for a match, and fail early
- If the pattern is changed to
+ if it is not present in the string.) If the pattern is changed so that it uses
- </P>
+ an atomic group, like this:
- <P>
  <pre>
    ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
- </PRE>
+ </pre>
- </P>
- <P>
  sequences of non-digits cannot be broken, and failure happens quickly.
- </P>
+ <a name="backreferences"></a></P>
  <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>
  <P>
  Outside a character class, a backslash followed by a digit greater than 0 (and
-Line 1049 
 However, if the decimal number following
+Line 1185 
 However, if the decimal number following
  always taken as a back reference, and causes an error only if there are not
  that many capturing left parentheses in the entire pattern. In other words, the
  parentheses that are referenced need not be to the left of the reference for
- numbers less than 10. See the section entitled "Backslash" above for further
+ numbers less than 10. A "forward back reference" of this type can make sense
- details of the handling of digits following a backslash.
+ when a repetition is involved and the subpattern to the right has participated
+ in an earlier iteration.
+ </P>
+ <P>
+ It is not possible to have a numerical "forward back reference" to subpattern
+ whose number is 10 or more. However, a back reference to any subpattern is
+ possible using named parentheses (see below). See also the subsection entitled
+ "Non-printing characters"
+ <a href="#digitsafterbackslash">above</a>
+ for further details of the handling of digits following a backslash.
  </P>
  <P>
  A back reference matches whatever actually matched the capturing subpattern in
-Line 1058 
 the current subject string, rather than
+Line 1203 
 the current subject string, rather than
  itself (see
  <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
  below for a way of doing that). So the pattern
- </P>
- <P>
  <pre>
    (sens|respons)e and \1ibility
- </PRE>
+ </pre>
- </P>
- <P>
  matches "sense and sensibility" and "response and responsibility", but not
  "sense and responsibility". If caseful matching is in force at the time of the
  back reference, the case of letters is relevant. For example,
- </P>
- <P>
  <pre>
    ((?i)rah)\s+\1
- </PRE>
+ </pre>
- </P>
- <P>
  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
  capturing subpattern is matched caselessly.
  </P>
  <P>
  Back references to named subpatterns use the Python syntax (?P=name). We could
  rewrite the above example as follows:
- </P>
- <P>
  <pre>
-   (?&#60;p1&#62;(?i)rah)\s+(?P=p1)
+   (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
- </PRE>
+ </pre>
+ A subpattern that is referenced by name may appear in the pattern before or
+ after the reference.
  </P>
  <P>
  There may be more than one back reference to the same subpattern. If a
  subpattern has not actually been used in a particular match, any back
  references to it always fail. For example, the pattern
- </P>
- <P>
  <pre>
    (a|(bc))\2
- </PRE>
+ </pre>
- </P>
- <P>
  always fails if it starts to match "a" rather than "bc". Because there may be
  many capturing parentheses in a pattern, all digits following the backslash are
  taken as part of a potential back reference number. If the pattern continues
  with a digit character, some delimiter must be used to terminate the back
  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
- Otherwise an empty comment can be used.
+ Otherwise an empty comment (see
+ <a href="#comments">"Comments"</a>
+ below) can be used.
  </P>
  <P>
  A back reference that occurs inside the parentheses to which it refers fails
  when the subpattern is first used, so, for example, (a\1) never matches.
  However, such references can be useful inside repeated subpatterns. For
  example, the pattern
- </P>
- <P>
  <pre>
    (a|b\1)+
- </PRE>
+ </pre>
- </P>
- <P>
  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
  the subpattern, the back reference matches the character string corresponding
  to the previous iteration. In order for this to work, the pattern must be such
  that the first iteration does not need to match the back reference. This can be
  done using alternation, as in the example above, or by a quantifier with a
  minimum of zero.
- </P>
+ <a name="bigassertions"></a></P>
  <br><a name="SEC15" 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
- assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above.
+ assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
+ <a href="#smallassertions">above.</a>
+ </P>
+ <P>
  More complicated assertions are coded as subpatterns. There are two kinds:
  those that look ahead of the current position in the subject string, and those
- that look behind it.
+ that look behind it. An assertion subpattern is matched in the normal way,
+ except that it does not cause the current matching position to be changed.
  </P>
  <P>
- An assertion subpattern is matched in the normal way, except that it does not
+ Assertion subpatterns are not capturing subpatterns, and may not be repeated,
- cause the current matching position to be changed. Lookahead assertions start
+ because it makes no sense to assert the same thing several times. If any kind
- with (?= for positive assertions and (?! for negative assertions. For example,
+ of assertion contains capturing subpatterns within it, these are counted for
+ the purposes of numbering the capturing subpatterns in the whole pattern.
+ However, substring capturing is carried out only for positive assertions,
+ because it does not make sense for negative assertions.
  </P>
+ <br><b>
+ Lookahead assertions
+ </b><br>
  <P>
+ Lookahead assertions start with (?= for positive assertions and (?! for
+ negative assertions. For example,
  <pre>
    \w+(?=;)
- </PRE>
+ </pre>
- </P>
- <P>
  matches a word followed by a semicolon, but does not include the semicolon in
  the match, and
- </P>
- <P>
  <pre>
    foo(?!bar)
- </PRE>
+ </pre>
- </P>
- <P>
  matches any occurrence of "foo" that is not followed by "bar". Note that the
  apparently similar pattern
- </P>
- <P>
  <pre>
    (?!foo)bar
- </PRE>
+ </pre>
- </P>
- <P>
  does not find an occurrence of "bar" that is preceded by something other than
  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
  (?!foo) is always true when the next three characters are "bar". A
- lookbehind assertion is needed to achieve this effect.
+ lookbehind assertion is needed to achieve the other effect.
  </P>
  <P>
  If you want to force a matching failure at some point in a pattern, the most
  convenient way to do it is with (?!) because an empty string always matches, so
  an assertion that requires there not to be an empty string must always fail.
- </P>
+ <a name="lookbehind"></a></P>
+ <br><b>
+ Lookbehind assertions
+ </b><br>
  <P>
  Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
  negative assertions. For example,
- </P>
- <P>
  <pre>
    (?&#60;!foo)bar
- </PRE>
+ </pre>
- </P>
- <P>
  does find an occurrence of "bar" that is not preceded by "foo". The contents of
  a lookbehind assertion are restricted such that all the strings it matches must
- have a fixed length. However, if there are several alternatives, they do not
+ have a fixed length. However, if there are several top-level alternatives, they
- all have to have the same fixed length. Thus
+ do not all have to have the same fixed length. Thus
- </P>
- <P>
  <pre>
    (?&#60;=bullock|donkey)
- </PRE>
+ </pre>
- </P>
- <P>
  is permitted, but
- </P>
- <P>
  <pre>
    (?&#60;!dogs?|cats?)
- </PRE>
+ </pre>
- </P>
- <P>
  causes an error at compile time. Branches that match different length strings
  are permitted only at the top level of a lookbehind assertion. This is an
  extension compared with Perl (at least for 5.8), which requires all branches to
  match the same length of string. An assertion such as
- </P>
- <P>
  <pre>
    (?&#60;=ab(c|de))
- </PRE>
+ </pre>
- </P>
- <P>
  is not permitted, because its single top-level branch can match two different
  lengths, but it is acceptable if rewritten to use two top-level branches:
- </P>
- <P>
  <pre>
    (?&#60;=abc|abde)
- </PRE>
+ </pre>
- </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
  match. If there are insufficient characters before the current position, the
-Line 1229 
 match is deemed to fail.
+Line 1345 
 match is deemed to fail.
  <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 length of the lookbehind. The \X escape, which can match different numbers
+ of bytes, is also not permitted.
  </P>
  <P>
  Atomic groups can be used in conjunction with lookbehind assertions to specify
  efficient matching at the end of the subject string. Consider a simple pattern
  such as
- </P>
- <P>
  <pre>
    abcd$
- </PRE>
+ </pre>
- </P>
- <P>
  when applied to a long string that does not match. Because matching proceeds
  from left to right, PCRE will look for each "a" in the subject and then see if
  what follows matches the rest of the pattern. If the pattern is specified as
- </P>
- <P>
  <pre>
    ^.*abcd$
- </PRE>
+ </pre>
- </P>
- <P>
  the initial .* matches the entire string at first, but when this fails (because
  there is no following "a"), it backtracks to match all but the last character,
  then all but the last two characters, and so on. Once again the search for "a"
  covers the entire string, from right to left, so we are no better off. However,
  if the pattern is written as
- </P>
- <P>
  <pre>
    ^(?&#62;.*)(?&#60;=abcd)
- </PRE>
+ </pre>
- </P>
+ or, equivalently, using the possessive quantifier syntax,
- <P>
- or, equivalently,
- </P>
- <P>
  <pre>
    ^.*+(?&#60;=abcd)
- </PRE>
+ </pre>
- </P>
- <P>
  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.
  </P>
+ <br><b>
+ Using multiple assertions
+ </b><br>
  <P>
  Several assertions (of any sort) may occur in succession. For example,
- </P>
- <P>
  <pre>
    (?&#60;=\d{3})(?&#60;!999)foo
- </PRE>
+ </pre>
- </P>
- <P>
  matches "foo" preceded by three digits that are not "999". Notice that each of
  the assertions is applied independently at the same point in the subject
  string. First there is a check that the previous three characters are all
-Line 1293 
 digits, and then there is a check that t
+Line 1393 
 digits, and then there is a check that t
  This pattern does <i>not</i> match "foo" preceded by six characters, the first
  of which are digits and the last three of which are not "999". For example, it
  doesn't match "123abcfoo". A pattern to do that is
- </P>
- <P>
  <pre>
    (?&#60;=\d{3}...)(?&#60;!999)foo
- </PRE>
+ </pre>
- </P>
- <P>
  This time the first assertion looks at the preceding six characters, checking
  that the first three are digits, and then the second assertion checks that the
  preceding three characters are not "999".
  </P>
  <P>
  Assertions can be nested in any combination. For example,
- </P>
- <P>
  <pre>
    (?&#60;=(?&#60;!foo)bar)baz
- </PRE>
+ </pre>
- </P>
- <P>
  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
  preceded by "foo", while
- </P>
- <P>
  <pre>
    (?&#60;=\d{3}(?!999)...)foo
- </PRE>
+ </pre>
- </P>
+ is another pattern that matches "foo" preceded by three digits and any three
- <P>
- is another pattern which matches "foo" preceded by three digits and any three
  characters that are not "999".
- </P>
+ <a name="conditions"></a></P>
- <P>
- Assertion subpatterns are not capturing subpatterns, and may not be repeated,
- because it makes no sense to assert the same thing several times. If any kind
- of assertion contains capturing subpatterns within it, these are counted for
- the purposes of numbering the capturing subpatterns in the whole pattern.
- However, substring capturing is carried out only for positive assertions,
- because it does not make sense for negative assertions.
- </P>
  <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
  <P>
  It is possible to cause the matching process to obey a subpattern
  conditionally or to choose between two alternative subpatterns, depending on
  the result of an assertion, or whether a previous capturing subpattern matched
  or not. The two possible forms of conditional subpattern are
- </P>
- <P>
  <pre>
    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)
- </PRE>
+ </pre>
- </P>
- <P>
  If the condition is satisfied, the yes-pattern is used; otherwise the
  no-pattern (if present) is used. If there are more than two alternatives in the
  subpattern, a compile-time error occurs.
  </P>
  <P>
  There are three kinds of condition. If the text between the parentheses
- consists of a sequence of digits, the condition is satisfied if the capturing
+ consists of a sequence of digits, or a sequence of alphanumeric characters and
- subpattern of that number has previously matched. The number must be greater
+ underscores, the condition is satisfied if the capturing subpattern of that
- than zero. Consider the following pattern, which contains non-significant white
+ number or name has previously matched. There is a possible ambiguity here,
- space to make it more readable (assume the PCRE_EXTENDED option) and to divide
+ because subpattern names may consist entirely of digits. PCRE looks first for a
- it into three parts for ease of discussion:
+ named subpattern; if it cannot find one and the text consists entirely of
+ digits, it 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>
+ Consider the following pattern, which contains non-significant white space to
+ make it more readable (assume the PCRE_EXTENDED option) and to divide it into
+ three parts for ease of discussion:
  <pre>
    ( \( )?    [^()]+    (?(1) \) )
- </PRE>
+ </pre>
- </P>
- <P>
  The first part matches an optional opening parenthesis, and if that
  character is present, sets it as the first captured substring. The second part
  matches one or more characters that are not parentheses. The third part is a
-Line 1373 
 or not. If they did, that is, if subject
+Line 1453 
 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.
+ non-parentheses, optionally enclosed in parentheses. Rewriting it to use a
- </P>
+ named subpattern gives this:
- <P>
+ <pre>
- If the condition is the string (R), it is satisfied if a recursive call to the
+   (?P&#60;OPEN&#62; \( )?    [^()]+    (?(OPEN) \) )
- pattern or subpattern has been made. At "top level", the condition is false.
+ </pre>
- This is a PCRE extension. Recursive patterns are described in the next section.
+ 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
+ been made. At "top level", the condition is false. This is a PCRE extension.
+ Recursive patterns are described in the next section.
  </P>
  <P>
  If the condition is not a sequence of digits or (R), 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:
- </P>
- <P>
  <pre>
    (?(?=[^a-z]*[a-z])
    \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
- </PRE>
+ </pre>
- </P>
- <P>
  The condition is a positive lookahead assertion that matches an optional
  sequence of non-letters followed by a letter. In other words, it tests for the
  presence of at least one letter in the subject. If a letter is found, the
  subject is matched against the first alternative; otherwise it is matched
  against the second. This pattern matches strings in one of the two forms
  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
- </P>
+ <a name="comments"></a></P>
  <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>
  <P>
- The sequence (?# marks the start of a comment which continues up to the next
+ The sequence (?# marks the start of a comment that continues up to the next
  closing parenthesis. Nested parentheses are not permitted. The characters
  that make up a comment play no part in the pattern matching at all.
  </P>
  <P>
  If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues up to the next newline
+ character class introduces a comment that continues to immediately after the
- character in the pattern.
+ next newline in the pattern.
- </P>
+ <a name="recursion"></a></P>
  <br><a name="SEC18" 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 has provided an
+ is not possible to handle an arbitrary nesting depth. Perl provides a facility
- experimental facility that allows regular expressions to recurse (amongst other
+ that allows regular expressions to recurse (amongst other things). It does this
- things). It does this by interpolating Perl code in the expression at run time,
+ by interpolating Perl code in the expression at run time, and the code can
- and the code can refer to the expression itself. A Perl pattern to solve the
+ refer to the expression itself. A Perl pattern to solve the parentheses problem
- parentheses problem can be created like this:
+ can be created like this:
- </P>
- <P>
  <pre>
    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
- </PRE>
+ </pre>
- </P>
- <P>
  The (?p{...}) item interpolates Perl code at run time, and in this case refers
  recursively to the pattern in which it appears. Obviously, PCRE cannot support
  the interpolation of Perl code. Instead, it supports some special syntax for
-Line 1441 
 number, provided that it occurs inside t
+Line 1516 
 number, provided that it occurs inside t
  (?R) is a recursive call of the entire regular expression.
  </P>
  <P>
- For example, this PCRE pattern solves the nested parentheses problem (assume
+ A recursive subpattern call is always treated as an atomic group. That is, once
- the PCRE_EXTENDED option is set so that white space is ignored):
+ it has matched some of the subject string, it is never re-entered, even if
+ it contains untried alternatives and there is a subsequent matching failure.
  </P>
  <P>
+ This PCRE pattern solves the nested parentheses problem (assume the
+ PCRE_EXTENDED option is set so that white space is ignored):
  <pre>
    \( ( (?&#62;[^()]+) | (?R) )* \)
- </PRE>
+ </pre>
- </P>
- <P>
  First it matches an opening parenthesis. Then it matches any number of
  substrings which can either be a sequence of non-parentheses, or a recursive
- match of the pattern itself (that is a correctly parenthesized substring).
+ match of the pattern itself (that is, a correctly parenthesized substring).
  Finally there is a closing parenthesis.
  </P>
  <P>
  If this were part of a larger pattern, you would not want to recurse the entire
  pattern, so instead you could use this:
- </P>
- <P>
  <pre>
    ( \( ( (?&#62;[^()]+) | (?1) )* \) )
- </PRE>
+ </pre>
- </P>
- <P>
  We have put the pattern into parentheses, and caused the recursion to refer to
  them instead of the whole pattern. In a larger pattern, keeping track of
  parenthesis numbers can be tricky. It may be more convenient to use named
  parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to
  the Python syntax that PCRE uses for named parentheses (Perl does not provide
  named parentheses). We could rewrite the above example as follows:
- </P>
- <P>
  <pre>
    (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )
- </PRE>
+ </pre>
- </P>
- <P>
  This particular example pattern 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
- </P>
- <P>
  <pre>
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
- </PRE>
+ </pre>
- </P>
- <P>
  it yields "no match" quickly. However, if atomic grouping is not used,
  the match runs for a very long time indeed because there are so many different
  ways the + and * repeats can carve up the subject, and all have to be tested
-Line 1498 
 before failure can be reported.
+Line 1562 
 before failure can be reported.
  At the end of a match, the values set for any capturing subpatterns are those
  from the outermost level of the recursion at which the subpattern value is set.
  If you want to obtain intermediate values, a callout function can be used (see
- below and the
+ the next section and the
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation). If the pattern above is matched against
- </P>
- <P>
  <pre>
    (ab(cd)ef)
- </PRE>
+ </pre>
- </P>
- <P>
  the value for the capturing parentheses is "ef", which is the last value taken
  on at the top level. If additional parentheses are added, giving
- </P>
- <P>
  <pre>
    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
       ^                        ^
       ^                        ^
- </PRE>
+ </pre>
- </P>
- <P>
  the string they capture is "ab(cd)ef", the contents of the top level
  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
  has to obtain extra memory to store data during a recursion, which it does by
-Line 1530 
 Do not confuse the (?R) item with the co
+Line 1586 
 Do not confuse the (?R) item with the co
  Consider this pattern, which matches text in angle brackets, allowing for
  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
  recursing), whereas any characters are permitted at the outer level.
- </P>
- <P>
  <pre>
    &#60; (?: (?(R) \d++  | [^&#60;&#62;]*+) | (?R)) * &#62;
- </PRE>
+ </pre>
- </P>
- <P>
  In this pattern, (?(R) is the start of a conditional subpattern, with two
  different alternatives for the recursive and non-recursive cases. The (?R) item
  is the actual recursive call.
- </P>
+ <a name="subpatternsassubroutines"></a></P>
- <a name="subpatternsassubroutines"></a><br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
+ <br><a name="SEC19" 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
  pattern
- </P>
- <P>
  <pre>
    (sens|respons)e and \1ibility
- </PRE>
+ </pre>
- </P>
- <P>
  matches "sense and sensibility" and "response and responsibility", but not
  "sense and responsibility". If instead the pattern
- </P>
- <P>
  <pre>
    (sens|respons)e and (?1)ibility
- </PRE>
+ </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
+ which they refer. However, named references can refer to later subpatterns.
  </P>
  <P>
- is used, it does match "sense and responsibility" as well as the other two
+ Like recursive subpatterns, a "subroutine" call is always treated as an atomic
- strings. Such references must, however, follow the subpattern to which they
+ group. That is, once it has matched some of the subject string, it is never
- refer.
+ 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>
-Line 1585 
 Within a regular expression, (?C) indica
+Line 1635 
 Within a regular expression, (?C) indica
  function is to be called. If you want to identify different callout points, you
  can put a number less than 256 after the letter C. The default value is zero.
  For example, this pattern has two callout points:
- </P>
- <P>
  <pre>
    (?C1)\dabc(?C2)def
- </PRE>
+ </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
+.
  </P>
  <P>
  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
  set), the external function is called. It is provided with the number of the
- callout, and, optionally, one item of data originally supplied by the caller of
+ callout, the position in the pattern, and, optionally, one item of data
- <b>pcre_exec()</b>. The callout function may cause matching to backtrack, or to
+ originally supplied by the caller of <b>pcre_exec()</b>. The callout function
- fail altogether. A complete description of the interface to the callout
+ may cause matching to proceed, to backtrack, or to fail altogether. A complete
- function is given in the
+ description of the interface to the callout function is given in the
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation.
  </P>
  <P>
- Last updated: 03 February 2003
+ Last updated: 06 June 2006
  <br>
- Copyright &copy; 1997-2003 University of Cambridge.
+ Copyright &copy; 1997-2006 University of Cambridge.
+ <p>
+ Return to the <a href="index.html">PCRE index page</a>.
+ </p>

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