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

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-revision 416 by ph10,
Sat Apr 11 14:34:02 2009 UTC
+revision 903 by ph10,
Sat Jan 21 16:37:17 2012 UTC
 Line 18 
 man page, in case the conversion went wr
  <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
  <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
  <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
- <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
+ <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
- <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
+ <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE DATA UNIT</a>
  <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
  <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
  <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
 Line 61 
 description of PCRE's regular expression
  </P>
  <P>
  The original operation of PCRE was on strings of one-byte characters. However,
- there is now also support for UTF-8 character strings. To use this, you must
+ there is now also support for UTF-8 strings in the original library, and a
- build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
+ second library that supports 16-bit and UTF-16 character strings. To use these
- the PCRE_UTF8 option. There is also a special sequence that can be given at the
+ features, PCRE must be built to include appropriate support. When using UTF
- start of a pattern:
+ strings you must either call the compiling function with the PCRE_UTF8 or
+ PCRE_UTF16 option, or the pattern must start with one of these special
+ sequences:
  <pre>
    (*UTF8)
+   (*UTF16)
  </pre>
- Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
+ Starting a pattern with such a sequence is equivalent to setting the relevant
- option. This feature is not Perl-compatible. How setting UTF-8 mode affects
+ option. This feature is not Perl-compatible. How setting a UTF mode affects
  pattern matching is mentioned in several places below. There is also a summary
- of UTF-8 features in the
+ of features in the
- <a href="pcre.html#utf8support">section on UTF-8 support</a>
+ <a href="pcreunicode.html"><b>pcreunicode</b></a>
- in the main
- <a href="pcre.html"><b>pcre</b></a>
  page.
  </P>
  <P>
+ Another special sequence that may appear at the start of a pattern or in
+ combination with (*UTF8) or (*UTF16) is:
+ <pre>
+   (*UCP)
+ </pre>
+ This has the same effect as setting the PCRE_UCP option: it causes sequences
+ such as \d and \w to use Unicode properties to determine character types,
+ instead of recognizing only characters with codes less than 128 via a lookup
+ table.
+ </P>
+ <P>
+ If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
+ PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
+ also some more of these special sequences that are concerned with the handling
+ of newlines; they are described below.
+ </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.
+ PCRE when one its main matching functions, <b>pcre_exec()</b> (8-bit) or
- From release 6.0, PCRE offers a second matching function,
+ <b>pcre16_exec()</b> (16-bit), is used. PCRE also has alternative matching
- <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
+ functions, <b>pcre_dfa_exec()</b> and <b>pcre16_dfa_exec()</b>, which match using
- Perl-compatible. Some of the features discussed below are not available when
+ a different algorithm that is not Perl-compatible. Some of the features
- <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
+ discussed below are not available when DFA matching is used. The advantages and
- alternative function, and how it differs from the normal function, are
+ disadvantages of the alternative functions, and how they differ from the normal
- discussed in the
+ functions, are discussed in the
  <a href="pcrematching.html"><b>pcrematching</b></a>
  page.
- </P>
+ <a name="newlines"></a></P>
  <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
  <P>
  PCRE supports five different conventions for indicating line breaks in
-Line 111 
 string with one of the following five se
+Line 129 
 string with one of the following five se
    (*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
+ These override the default and the options given to the compiling function. For
  example, on a Unix system where LF is the default newline sequence, the pattern
  <pre>
    (*CR)a.b
-Line 123 
 they must be in upper case. If more than
+Line 141 
 they must be in upper case. If more than
  is used.
  </P>
  <P>
- The newline convention does not affect what the \R escape sequence matches. By
+ The newline convention affects the interpretation of the dot metacharacter when
- default, this is any Unicode newline sequence, for Perl compatibility. However,
+ PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
- this can be changed; see the description of \R in the section entitled
+ 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.
-Line 140 
 corresponding characters in the subject.
+Line 160 
 corresponding characters in the subject.
  </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
+ independently of case. In a UTF 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.
+ UTF support.
  </P>
  <P>
  The power of regular expressions comes from the ability to include alternatives
-Line 190 
 The following sections describe the use
+Line 210 
 The following sections describe the use
  <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
+ character that is not a number or a letter, it takes away any special meaning
- may have. This use of backslash as an escape character applies both inside and
+ that character may have. This use of backslash as an escape character applies
- outside character classes.
+ both inside and outside character classes.
  </P>
  <P>
  For example, if you want to match a * character, you write \* in the pattern.
-Line 202 
 non-alphanumeric with backslash to speci
+Line 222 
 non-alphanumeric with backslash to speci
  particular, if you want to match a backslash, you write \\.
  </P>
  <P>
+ In a UTF mode, only ASCII numbers and letters have any special meaning after a
+ backslash. All other characters (in particular, those whose codepoints are
+ greater than 127) are treated as literals.
+ </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 are ignored. An escaping backslash can
-Line 220 
 Perl, $ and @ cause variable interpolati
+Line 245 
 Perl, $ and @ cause variable interpolati
    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
  </pre>
  The \Q...\E sequence is recognized both inside and outside character classes.
+ An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
+ by \E later in the pattern, the literal interpretation continues to the end of
+ the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
+ a character class, this causes an error, because the character class is not
+ terminated.
  <a name="digitsafterbackslash"></a></P>
  <br><b>
  Non-printing characters
-Line 228 
 Non-printing characters
+Line 258 
 Non-printing characters
  A second use of backslash provides a way of encoding non-printing characters
  in patterns in a visible manner. There is no restriction on the appearance of
  non-printing characters, apart from the binary zero that terminates a pattern,
- but when a pattern is being prepared by text editing, it is usually easier to
+ but when a pattern is being prepared by text editing, it is often easier to use
- use one of the following escape sequences than the binary character it
+ one of the following escape sequences than the binary character it represents:
- represents:
  <pre>
    \a        alarm, that is, the BEL character (hex 07)
-   \cx       "control-x", where x is any character
+   \cx       "control-x", where x is any ASCII character
    \e        escape (hex 1B)
    \f        formfeed (hex 0C)
    \n        linefeed (hex 0A)
    \r        carriage return (hex 0D)
    \t        tab (hex 09)
-   \ddd      character with octal code ddd, or backreference
+   \ddd      character with octal code ddd, or back reference
    \xhh      character with hex code hh
-   \x{hhh..} character with hex code hhh..
+   \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
+   \uhhhh    character with hex code hhhh (JavaScript mode only)
  </pre>
  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
+ Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
-B.
+ \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
- </P>
+ than 127, a compile-time error occurs. This locks out non-ASCII characters in
- <P>
+ all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
- After \x, from zero to two hexadecimal digits are read (letters can be in
+ lower case letter is converted to upper case, and then the 0xc0 bits are
- upper or lower case). Any number of hexadecimal digits may appear between \x{
+ flipped.)
- and }, but the value of the character code must be less than 256 in non-UTF-8
+ </P>
- mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
+ <P>
- hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
+ By default, after \x, from zero to two hexadecimal digits are read (letters
- point, which is 10FFFF.
+ can be in upper or lower case). Any number of hexadecimal digits may appear
+ between \x{ and }, but the character code is constrained as follows:
+ <pre>
+-bit non-UTF mode    less than 0x100
+-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
+-bit non-UTF mode   less than 0x10000
+-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
+ </pre>
+ Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
+ "surrogate" codepoints).
  </P>
  <P>
  If characters other than hexadecimal digits appear between \x{ and }, or if
-Line 263 
 initial \x will be interpreted as a basi
+Line 302 
 initial \x will be interpreted as a basi
  following digits, giving a character whose value is zero.
  </P>
  <P>
+ If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
+ as just described only when it is followed by two hexadecimal digits.
+ Otherwise, it matches a literal "x" character. In JavaScript mode, support for
+ code points greater than 256 is provided by \u, which must be followed by
+ four hexadecimal digits; otherwise it matches a literal "u" character.
+ </P>
+ <P>
  Characters whose value is less than 256 can be defined by either of the two
- syntaxes for \x. There is no difference in the way they are handled. For
+ syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
- example, \xdc is exactly the same as \x{dc}.
+ way they are handled. For example, \xdc is exactly the same as \x{dc} (or
+ \u00dc in JavaScript mode).
  </P>
  <P>
  After \0 up to two further octal digits are read. If there are fewer than two
-Line 288 
 following the discussion of
+Line 335 
 following the discussion of
  Inside a character class, or if the decimal number is greater than 9 and there
  have not been that many capturing subpatterns, PCRE re-reads up to three octal
  digits following the backslash, and uses them to generate a data character. Any
- subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
+ subsequent digits stand for themselves. The value of the character is
- character specified in octal must be less than \400. In UTF-8 mode, values up
+ constrained in the same way as characters specified in hexadecimal.
- to \777 are permitted. For example:
+ For example:
  <pre>
    \040   is another way of writing a space
    \40    is the same, provided there are fewer than 40 previous capturing subpatterns
-Line 299 
 to \777 are permitted. For example:
+Line 346 
 to \777 are permitted. For example:
    \011   is always a tab
    \0113  is a tab followed by the character "3"
    \113   might be a back reference, otherwise the character with octal code 113
-   \377   might be a back reference, otherwise the byte consisting entirely of 1 bits
+   \377   might be a back reference, otherwise the value 255 (decimal)
    \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"
  </pre>
  Note that octal values of 100 or greater must not be introduced by a leading
-Line 307 
 zero, because no more than three octal d
+Line 354 
 zero, because no more than three octal d
  </P>
  <P>
  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
+ and outside character classes. In addition, inside a character class, \b is
- sequence \b is interpreted as the backspace character (hex 08), and the
+ interpreted as the backspace character (hex 08).
- sequences \R and \X are interpreted as the characters "R" and "X",
+ </P>
- respectively. Outside a character class, these sequences have different
+ <P>
- meanings
+ \N is not allowed in a character class. \B, \R, and \X are not special
- <a href="#uniextseq">(see below).</a>
+ inside a character class. Like other unrecognized escape sequences, they are
+ treated as the literal characters "B", "R", and "X" by default, but cause an
+ error if the PCRE_EXTRA option is set. Outside a character class, these
+ sequences have different meanings.
+ </P>
+ <br><b>
+ Unsupported escape sequences
+ </b><br>
+ <P>
+ In Perl, the sequences \l, \L, \u, and \U are recognized by its string
+ handler and used to modify the case of following characters. By default, PCRE
+ does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
+ option is set, \U matches a "U" character, and \u can be used to define a
+ character by code point, as described in the previous section.
  </P>
  <br><b>
  Absolute and relative back references
-Line 334 
 a number enclosed either in angle bracke
+Line 394 
 a number enclosed either in angle bracke
  syntax for referencing a subpattern as a "subroutine". Details are discussed
  <a href="#onigurumasubroutines">later.</a>
  Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
- synonymous. The former is a back reference; the latter is a subroutine call.
+ synonymous. The former is a back reference; the latter is a
- </P>
+ <a href="#subpatternsassubroutines">subroutine</a>
+ call.
+ <a name="genericchartypes"></a></P>
  <br><b>
  Generic character types
  </b><br>
  <P>
- Another use of backslash is for specifying generic character types. The
+ Another use of backslash is for specifying generic character types:
- following are always recognized:
  <pre>
    \d     any decimal digit
    \D     any character that is not a decimal digit
-Line 354 
 following are always recognized:
+Line 415 
 following are always recognized:
    \w     any "word" character
    \W     any "non-word" character
  </pre>
- Each pair of escape sequences partitions the complete set of characters into
+ There is also the single sequence \N, which matches a non-newline character.
- two disjoint sets. Any given character matches one, and only one, of each pair.
+ This is the same as
+ <a href="#fullstopdot">the "." metacharacter</a>
+ when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
+ PCRE does not support this.
  </P>
  <P>
- These character type sequences can appear both inside and outside character
+ Each pair of lower and upper case escape sequences partitions the complete set
+ of characters into two disjoint sets. Any given character matches one, and only
+ one, of each pair. The sequences can appear both inside and outside character
  classes. They each match one character of the appropriate type. If the current
- matching point is at the end of the subject string, all of them fail, since
+ matching point is at the end of the subject string, all of them fail, because
  there is no character to match.
  </P>
  <P>
-Line 371 
 included in a Perl script, \s may match
+Line 437 
 included in a Perl script, \s may match
  does.
  </P>
  <P>
- In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
+ A "word" character is an underscore or any character that is a letter or digit.
- \w, and always match \D, \S, and \W. This is true even when Unicode
+ By default, the definition of letters and digits is controlled by PCRE's
- character property support is available. These sequences retain their original
+ low-valued character tables, and may vary if locale-specific matching is taking
- meanings from before UTF-8 support was available, mainly for efficiency
+ place (see
- reasons. Note that this also affects \b, because it is defined in terms of \w
+ <a href="pcreapi.html#localesupport">"Locale support"</a>
- and \W.
+ in the
+ <a href="pcreapi.html"><b>pcreapi</b></a>
+ page). For example, in a French locale such as "fr_FR" in Unix-like systems,
+ or "french" in Windows, some character codes greater than 128 are used for
+ accented letters, and these are then matched by \w. The use of locales with
+ Unicode is discouraged.
  </P>
  <P>
- The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
+ By default, in a UTF mode, characters with values greater than 128 never match
- other sequences, these do match certain high-valued codepoints in UTF-8 mode.
+ \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
- The horizontal space characters are:
+ their original meanings from before UTF support was available, mainly for
+ efficiency reasons. However, if PCRE is compiled with Unicode property support,
+ and the PCRE_UCP option is set, the behaviour is changed so that Unicode
+ properties are used to determine character types, as follows:
+ <pre>
+   \d  any character that \p{Nd} matches (decimal digit)
+   \s  any character that \p{Z} matches, plus HT, LF, FF, CR
+   \w  any character that \p{L} or \p{N} matches, plus underscore
+ </pre>
+ The upper case escapes match the inverse sets of characters. Note that \d
+ matches only decimal digits, whereas \w matches any Unicode digit, as well as
+ any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
+ \B because they are defined in terms of \w and \W. Matching these sequences
+ is noticeably slower when PCRE_UCP is set.
+ </P>
+ <P>
+ The sequences \h, \H, \v, and \V are features that were added to Perl at
+ release 5.10. In contrast to the other sequences, which match only ASCII
+ characters by default, these always match certain high-valued codepoints,
+ whether or not PCRE_UCP is set. The horizontal space characters are:
  <pre>
    U+0009     Horizontal tab
    U+0020     Space
-Line 412 
 The vertical space characters are:
+Line 502 
 The vertical space characters are:
    U+0085     Next line
    U+2028     Line separator
    U+2029     Paragraph separator
- </PRE>
+ </pre>
- </P>
+ In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
- <P>
+ relevant.
- A "word" character is an underscore or any character less than 256 that is a
- letter or digit. The definition of letters and digits is controlled by PCRE's
- low-valued character tables, and may vary if locale-specific matching is taking
- place (see
- <a href="pcreapi.html#localesupport">"Locale support"</a>
- in the
- <a href="pcreapi.html"><b>pcreapi</b></a>
- page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- or "french" in Windows, some character codes greater than 128 are used for
- accented letters, and these are matched by \w. The use of locales with Unicode
- is discouraged.
  <a name="newlineseq"></a></P>
  <br><b>
  Newline sequences
  </b><br>
  <P>
  Outside a character class, by default, the escape sequence \R matches any
- Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
+ Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
- equivalent to the following:
+ following:
  <pre>
    (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
  </pre>
-Line 446 
 line, U+0085). The two-character sequenc
+Line 525 
 line, U+0085). The two-character sequenc
  cannot be split.
  </P>
  <P>
- In UTF-8 mode, two additional characters whose codepoints are greater than 255
+ In other modes, 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.
-Line 463 
 one of the following sequences:
+Line 542 
 one of the following sequences:
    (*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
+ These override the default and the options given to the compiling function, but
- they can be overridden by options given to <b>pcre_exec()</b>. Note that these
+ they can themselves be overridden by options given to a matching function. Note
- special settings, which are not Perl-compatible, are recognized only at the
+ that these special settings, which are not Perl-compatible, are recognized only
- very start of a pattern, and that they must be in upper case. If more than one
+ at the very start of a pattern, and that they must be in upper case. If more
- of them is present, the last one is used. They can be combined with a change of
+ than one of them is present, the last one is used. They can be combined with a
- newline convention, for example, a pattern can start with:
+ 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".
+ They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
+ sequences. Inside a character class, \R is treated as an unrecognized escape
+ sequence, and so matches the letter "R" by default, but causes an error if
+ PCRE_EXTRA is set.
  <a name="uniextseq"></a></P>
  <br><b>
  Unicode character properties
-Line 480 
 Unicode character properties
+Line 562 
 Unicode character properties
  <P>
  When PCRE is built with Unicode character property support, three additional
  escape sequences that match characters with specific properties are available.
- When not in UTF-8 mode, these sequences are of course limited to testing
+ When in 8-bit non-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>
-Line 489 
 The extra escape sequences are:
+Line 571 
 The extra escape sequences are:
    \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
+ script names, the general category properties, "Any", which matches any
- character (including newline). Other properties such as "InMusicalSymbols" are
+ character (including newline), and some special PCRE properties (described
- not currently supported by PCRE. Note that \P{Any} does not match any
+ in the
- characters, so always causes a match failure.
+ <a href="#extraprops">next section).</a>
+ Other Perl 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
-Line 508 
 Those that are not part of an identified
+Line 593 
 Those that are not part of an identified
  <P>
  Arabic,
  Armenian,
+ Avestan,
  Balinese,
+ Bamum,
  Bengali,
  Bopomofo,
  Braille,
  Buginese,
  Buhid,
  Canadian_Aboriginal,
+ Carian,
+ Cham,
  Cherokee,
  Common,
  Coptic,
-Line 523 
 Cypriot,
+Line 612 
 Cypriot,
  Cyrillic,
  Deseret,
  Devanagari,
+ Egyptian_Hieroglyphs,
  Ethiopic,
  Georgian,
  Glagolitic,
-Line 535 
 Hangul,
+Line 625 
 Hangul,
  Hanunoo,
  Hebrew,
  Hiragana,
+ Imperial_Aramaic,
  Inherited,
+ Inscriptional_Pahlavi,
+ Inscriptional_Parthian,
+ Javanese,
+ Kaithi,
  Kannada,
  Katakana,
+ Kayah_Li,
  Kharoshthi,
  Khmer,
  Lao,
  Latin,
+ Lepcha,
  Limbu,
  Linear_B,
+ Lisu,
+ Lycian,
+ Lydian,
  Malayalam,
+ Meetei_Mayek,
  Mongolian,
  Myanmar,
  New_Tai_Lue,
-Line 552 
 Nko,
+Line 653 
 Nko,
  Ogham,
  Old_Italic,
  Old_Persian,
+ Old_South_Arabian,
+ Old_Turkic,
+ Ol_Chiki,
  Oriya,
  Osmanya,
  Phags_Pa,
  Phoenician,
+ Rejang,
  Runic,
+ Samaritan,
+ Saurashtra,
  Shavian,
  Sinhala,
+ Sundanese,
  Syloti_Nagri,
  Syriac,
  Tagalog,
  Tagbanwa,
  Tai_Le,
+ Tai_Tham,
+ Tai_Viet,
  Tamil,
  Telugu,
  Thaana,
-Line 571 
 Thai,
+Line 681 
 Thai,
  Tibetan,
  Tifinagh,
  Ugaritic,
+ Vai,
  Yi.
  </P>
  <P>
- Each character has exactly one general category property, specified by a
+ Each character has exactly one Unicode general category property, specified by
- two-letter abbreviation. For compatibility with Perl, negation can be specified
+ a two-letter abbreviation. For compatibility with Perl, negation can be
- by including a circumflex between the opening brace and the property name. For
+ specified by including a circumflex between the opening brace and the property
- example, \p{^Lu} is the same as \P{Lu}.
+ 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
-Line 640 
 a modifier or "other".
+Line 751 
 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
+ U+DFFF. Such characters are not valid in Unicode strings and so
- cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
+ cannot be tested by PCRE, unless UTF validity checking has been turned off
- (see the discussion of PCRE_NO_UTF8_CHECK in the
+ (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
  <a href="pcreapi.html"><b>pcreapi</b></a>
- page).
+ page). Perl does not support the Cs property.
  </P>
  <P>
- The long synonyms for these properties that Perl supports (such as \p{Letter})
+ The long synonyms for property names that Perl supports (such as \p{Letter})
  are not supported by PCRE, nor is it permitted to prefix any of these
  properties with "Is".
  </P>
-Line 672 
 atomic group
+Line 783 
 atomic group
  <a href="#atomicgroup">(see below).</a>
  Characters with the "mark" property are typically accents that affect the
  preceding character. None of them have codepoints less than 256, so in
- non-UTF-8 mode \X matches any one character.
+-bit non-UTF-8 mode \X matches any one character.
+ </P>
+ <P>
+ Note that recent versions of Perl have changed \X to match what Unicode calls
+ an "extended grapheme cluster", which has a more complicated definition.
  </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.
+ properties in PCRE by default, though you can make them do so by setting the
+ PCRE_UCP option or by starting the pattern with (*UCP).
+ <a name="extraprops"></a></P>
+ <br><b>
+ PCRE's additional properties
+ </b><br>
+ <P>
+ As well as the standard Unicode properties described in the previous
+ section, PCRE supports four more that make it possible to convert traditional
+ escape sequences such as \w and \s and POSIX character classes to use Unicode
+ properties. PCRE uses these non-standard, non-Perl properties internally when
+ PCRE_UCP is set. They are:
+ <pre>
+   Xan   Any alphanumeric character
+   Xps   Any POSIX space character
+   Xsp   Any Perl space character
+   Xwd   Any Perl "word" character
+ </pre>
+ Xan matches characters that have either the L (letter) or the N (number)
+ property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
+ carriage return, and any other character that has the Z (separator) property.
+ Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
+ same characters as Xan, plus underscore.
  <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
+ The escape sequence \K causes any previously matched characters not to be
- matched characters not to be included in the final matched sequence. For
+ included in the final matched sequence. For example, the pattern:
- example, the pattern:
  <pre>
    foo\Kbar
  </pre>
-Line 702 
 For example, when the pattern
+Line 838 
 For example, when the pattern
    (foo)\Kbar
  </pre>
  matches "foobar", the first substring is still set to "foo".
+ </P>
+ <P>
+ Perl documents that the use of \K within assertions is "not well defined". In
+ PCRE, \K is acted upon when it occurs inside positive assertions, but is
+ ignored in negative assertions.
  <a name="smallassertions"></a></P>
  <br><b>
  Simple assertions
-Line 722 
 The backslashed assertions are:
+Line 863 
 The backslashed assertions are:
    \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
+ Inside a character class, \b has a different meaning; it matches the backspace
- different meaning, namely the backspace character, inside a character class).
+ character. If any other of these assertions appears in a character class, by
+ default it matches the corresponding literal character (for example, \B
+ matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
+ escape sequence" error is generated instead.
  </P>
  <P>
  A word boundary is a position in the subject string where the current character
  and the previous character do not both match \w or \W (i.e. one matches
  \w and the other matches \W), or the start or end of the string if the
- first or last character matches \w, respectively.
+ first or last character matches \w, respectively. In a UTF mode, the meanings
+ of \w and \W can be changed by setting the PCRE_UCP option. When this is
+ done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
+ of word" or "end of word" metasequence. However, whatever follows \b normally
+ determines which it is. For example, the fragment \ba matches "a" at the start
+ of a word.
  </P>
  <P>
  The \A, \Z, and \z assertions differ from the traditional circumflex and
-Line 816 
 PCRE_DOLLAR_ENDONLY option is ignored if
+Line 965 
 PCRE_DOLLAR_ENDONLY option is ignored if
  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 or not PCRE_MULTILINE is set.
- </P>
+ <a name="fullstopdot"></a></P>
- <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
+ <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</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.
+ line.
  </P>
  <P>
  When a line ending is defined as a single character, dot never matches that
-Line 842 
 The handling of dot is entirely independ
+Line 991 
 The handling of dot is entirely independ
  dollar, the only relationship being that they both involve newlines. Dot has no
  special meaning in a character class.
  </P>
- <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
  <P>
- Outside a character class, the escape sequence \C matches any one byte, both
+ The escape sequence \N behaves like a dot, except that it is not affected by
- in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
+ the PCRE_DOTALL option. In other words, it matches any character except one
- characters. The feature is provided in Perl in order to match individual bytes
+ that signifies the end of a line. Perl also uses \N to match characters by
- in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
+ name; PCRE does not support this.
- what remains in the string may be a malformed UTF-8 string. For this reason,
+ </P>
- the \C escape sequence is best avoided.
+ <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
+ <P>
+ Outside a character class, the escape sequence \C matches any one data unit,
+ whether or not a UTF mode is set. In the 8-bit library, one data unit is one
+ byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \C always
+ matches line-ending characters. The feature is provided in Perl in order to
+ match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
+ used. Because \C breaks up characters into individual data units, matching one
+ unit with \C in a UTF mode means that the rest of the string may start with a
+ malformed UTF character. This has undefined results, because PCRE assumes that
+ it is dealing with valid UTF strings (and by default it checks this at the
+ start of processing unless the PCRE_NO_UTF8_CHECK option is used).
  </P>
  <P>
  PCRE does not allow \C to appear in lookbehind assertions
- <a href="#lookbehind">(described below),</a>
+ <a href="#lookbehind">(described below)</a>
- because in UTF-8 mode this would make it impossible to calculate the length of
+ in a UTF mode, because this would make it impossible to calculate the length of
  the lookbehind.
+ </P>
+ <P>
+ In general, the \C escape sequence is best avoided. However, one
+ way of using it that avoids the problem of malformed UTF characters is to use a
+ lookahead to check the length of the next character, as in this pattern, which
+ could be used with a UTF-8 string (ignore white space and line breaks):
+ <pre>
+   (?| (?=[\x00-\x7f])(\C) |
+       (?=[\x80-\x{7ff}])(\C)(\C) |
+       (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
+       (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
+ </pre>
+ A group that starts with (?| resets the capturing parentheses numbers in each
+ alternative (see
+ <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
+ below). The assertions at the start of each branch check the next UTF-8
+ character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
+ character's individual bytes are then captured by the appropriate number of
+ groups.
  <a name="characterclass"></a></P>
  <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
  <P>
  An opening square bracket introduces a character class, terminated by a closing
- square bracket. A closing square bracket on its own is not special. If a
+ square bracket. A closing square bracket on its own is not special by default.
- closing square bracket is required as a member of the class, it should be the
+ However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
- first data character in the class (after an initial circumflex, if present) or
+ bracket causes a compile-time error. If a closing square bracket is required as
- escaped with a backslash.
+ a member of the class, it should be the first data character in the class
+ (after an initial circumflex, if present) or escaped with a backslash.
  </P>
  <P>
- A character class matches a single character in the subject. In UTF-8 mode, the
+ A character class matches a single character in the subject. In a UTF mode, the
- character may occupy more than one byte. A matched character must be in the set
+ character may be more than one data unit long. A matched character must be in
- of characters defined by the class, unless the first character in the class
+ the set of characters defined by the class, unless the first character in the
- definition is a circumflex, in which case the subject character must not be in
+ class definition is a circumflex, in which case the subject character must not
- the set defined by the class. If a circumflex is actually required as a member
+ be in the set defined by the class. If a circumflex is actually required as a
- of the class, ensure it is not the first character, or escape it with a
+ member of the class, ensure it is not the first character, or escape it with a
  backslash.
  </P>
  <P>
-Line 879 
 For example, the character class [aeiou]
+Line 1058 
 For example, the character class [aeiou]
  [^aeiou] matches any character that is not a lower case vowel. Note that a
  circumflex is just a convenient notation for specifying the characters that
  are in the class by enumerating those that are not. A class that starts with a
- circumflex is not an assertion: it still consumes a character from the subject
+ circumflex is not an assertion; it still consumes a character from the subject
  string, and therefore it fails if the current pointer is at the end of the
  string.
  </P>
  <P>
- In UTF-8 mode, characters with values greater than 255 can be included in a
+ In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
- class as a literal string of bytes, or by using the \x{ escaping mechanism.
+ included in a class as a literal string of data units, or by using the \x{
+ escaping mechanism.
  </P>
  <P>
  When caseless matching is set, any letters in a class represent both their
  upper case and lower case versions, so for example, a caseless [aeiou] matches
  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
- caseful version would. In UTF-8 mode, PCRE always understands the concept of
+ caseful version would. In a UTF 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
+ If you want to use caseless matching in a UTF mode for characters 128 and
- ensure that PCRE is compiled with Unicode property support as well as with
+ above, you must ensure that PCRE is compiled with Unicode property support as
- UTF-8 support.
+ well as with UTF support.
  </P>
  <P>
  Characters that might indicate line breaks are never treated in any special way
-Line 923 
 followed by two other characters. The oc
+Line 1103 
 followed by two other characters. The oc
  </P>
  <P>
  Ranges operate in the collating sequence of character values. They can also be
- used for characters specified numerically, for example [\000-\037]. In UTF-8
+ used for characters specified numerically, for example [\000-\037]. Ranges
- mode, ranges can include characters whose values are greater than 255, for
+ can include any characters that are valid for the current mode.
- example [\x{100}-\x{2ff}].
  </P>
  <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 in non-UTF-8 mode, if character
+ [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
  tables for a French locale are in use, [\xc8-\xcb] matches accented E
- characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
+ characters in both cases. In UTF modes, 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, \p, \P, \s, \S, \w, and \W may also appear
+ The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
- in a character class, and add the characters that they match to the class. For
+ \V, \w, and \W may appear in a character class, and add the characters that
- example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
+ they match to the class. For example, [\dABCDEF] matches any hexadecimal
- conveniently be used with the upper case character types to specify a more
+ digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
- restricted set of characters than the matching lower case type. For example,
+ and their upper case partners, just as it does when they appear outside a
- the class [^\W_] matches any letter or digit, but not underscore.
+ character class, as described in the section entitled
+ <a href="#genericchartypes">"Generic character types"</a>
+ above. The escape sequence \b has a different meaning inside a character
+ class; it matches the backspace character. The sequences \B, \N, \R, and \X
+ are not special inside a character class. Like any other unrecognized escape
+ sequences, they are treated as the literal characters "B", "N", "R", and "X" by
+ default, but cause an error if the PCRE_EXTRA option is set.
+ </P>
+ <P>
+ 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,
+ whereas [\w] includes underscore. A positive character class should be read as
+ "something OR something OR ..." and a negative class as "NOT something AND NOT
+ something AND NOT ...".
  </P>
  <P>
  The only metacharacters that are recognized in character classes are backslash,
-Line 961 
 this notation. For example,
+Line 1154 
 this notation. For example,
    [01[:alpha:]%]
  </pre>
  matches "0", "1", any alphabetic character, or "%". The supported class names
- are
+ are:
  <pre>
    alnum    letters and digits
    alpha    letters
-Line 972 
 are
+Line 1165 
 are
    graph    printing characters, excluding space
    lower    lower case letters
    print    printing characters, including space
-   punct    printing characters, excluding letters and digits
+   punct    printing characters, excluding letters and digits and space
    space    white space (not quite the same as \s)
    upper    upper case letters
    word     "word" characters (same as \w)
-Line 995 
 syntax [.ch.] and [=ch=] where "ch" is a
+Line 1188 
 syntax [.ch.] and [=ch=] where "ch" is a
  supported, and an error is given if they are encountered.
  </P>
  <P>
- In UTF-8 mode, characters with values greater than 128 do not match any of
+ By default, in UTF modes, characters with values greater than 128 do not match
- the POSIX character classes.
+ any of the POSIX character classes. However, if the PCRE_UCP option is passed
+ to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
+ character properties are used. This is achieved by replacing the POSIX classes
+ by other sequences, as follows:
+ <pre>
+   [:alnum:]  becomes  \p{Xan}
+   [:alpha:]  becomes  \p{L}
+   [:blank:]  becomes  \h
+   [:digit:]  becomes  \p{Nd}
+   [:lower:]  becomes  \p{Ll}
+   [:space:]  becomes  \p{Xps}
+   [:upper:]  becomes  \p{Lu}
+   [:word:]   becomes  \p{Xwd}
+ </pre>
+ Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
+ classes are unchanged, and match only characters with code points less than
+.
  </P>
  <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
  <P>
-Line 1046 
 extracted by the pcre_fullinfo()
+Line 1255 
 extracted by the pcre_fullinfo()
  </P>
  <P>
  An option change within a subpattern (see below for a description of
- subpatterns) affects only that part of the current pattern that follows it, so
+ subpatterns) affects only that part of the subpattern that follows it, so
  <pre>
    (a(?i)b)c
  </pre>
-Line 1064 
 behaviour otherwise.
+Line 1273 
 behaviour otherwise.
  </P>
  <P>
  <b>Note:</b> There are other PCRE-specific options that can be set by the
- application when the compile or match functions are called. In some cases the
+ application when the compiling or matching functions are called. In some cases
- pattern can contain special leading sequences such as (*CRLF) to override what
+ the pattern can contain special leading sequences such as (*CRLF) to override
- the application has set or what has been defaulted. Details are given in the
+ what the application has set or what has been defaulted. Details are given in
- section entitled
+ the section entitled
  <a href="#newlineseq">"Newline sequences"</a>
- above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
+ above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
- mode; this is equivalent to setting the PCRE_UTF8 option.
+ can be used to set UTF and Unicode property modes; they are equivalent to
+ setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
  <a name="subpattern"></a></P>
  <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
  <P>
-Line 1082 
 Turning part of a pattern into a subpatt
+Line 1292 
 Turning part of a pattern into a subpatt
  <pre>
    cat(aract|erpillar|)
  </pre>
- matches one of the words "cat", "cataract", or "caterpillar". Without the
+ matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
- parentheses, it would match "cataract", "erpillar" or an empty string.
+ match "cataract", "erpillar" or an empty string.
  <br>
  <br>
 . It sets up the subpattern as a capturing subpattern. This means that, when
  the whole pattern matches, that portion of the subject string that matched the
- subpattern is passed back to the caller via the <i>ovector</i> argument of
+ subpattern is passed back to the caller via the <i>ovector</i> argument of the
- <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
+ matching function. (This applies only to the traditional matching functions;
- from 1) to obtain numbers for the capturing subpatterns.
+ the DFA matching functions do not support capturing.)
  </P>
  <P>
- For example, if the string "the red king" is matched against the pattern
+ Opening parentheses are counted from left to right (starting from 1) to obtain
+ numbers for the capturing subpatterns. For example, if the string "the red
+ king" is matched against the pattern
  <pre>
    the ((red|white) (king|queen))
  </pre>
-Line 1125 
 match exactly the same set of strings. B
+Line 1337 
 match exactly the same set of strings. B
  from left to right, and options are not reset until the end of the subpattern
  is reached, an option setting in one branch does affect subsequent branches, so
  the above patterns match "SUNDAY" as well as "Saturday".
- </P>
+ <a name="dupsubpatternnumber"></a></P>
  <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
  <P>
  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
-Line 1141 
 at captured substring number one, whiche
+Line 1353 
 at captured substring number one, whiche
  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
+ parentheses that follow the subpattern start after the highest number used in
- branch. The following example is taken from the Perl documentation.
+ any branch. The following example is taken from the Perl documentation. The
- The numbers underneath show in which buffer the captured content will be
+ numbers underneath show in which buffer the captured content will be stored.
- 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
+ A back reference to a numbered subpattern uses the most recent value that is
- the first one in the pattern with the given number.
+ set for that number by any subpattern. The following pattern matches "abcabc"
+ or "defdef":
+ <pre>
+   /(?|(abc)|(def))\1/
+ </pre>
+ In contrast, a subroutine call to a numbered subpattern always refers to the
+ first one in the pattern with the given number. The following pattern matches
+ "abcabc" or "defabc":
+ <pre>
+   /(?|(abc)|(def))(?1)/
+ </pre>
+ If a
+ <a href="#conditions">condition test</a>
+ for a subpattern's having matched refers to a non-unique number, the test is
+ true if any of the subpatterns of that number have matched.
  </P>
  <P>
  An alternative approach to using this "branch reset" feature is to use
-Line 1165 
 if an expression is modified, the number
+Line 1390 
 if an expression is modified, the number
  difficulty, PCRE supports the naming of subpatterns. This feature was not
  added to Perl until release 5.10. Python had the feature earlier, and PCRE
  introduced it at release 4.0, using the Python syntax. PCRE now supports both
- the Perl and the Python syntax.
+ the Perl and the Python syntax. Perl allows identically numbered subpatterns to
+ have different names, but PCRE does not.
  </P>
  <P>
  In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
  (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
  parentheses from other parts of the pattern, such as
- <a href="#backreferences">backreferences,</a>
+ <a href="#backreferences">back references,</a>
  <a href="#recursion">recursion,</a>
  and
  <a href="#conditions">conditions,</a>
-Line 1186 
 is also a convenience function for extra
+Line 1412 
 is also a convenience function for extra
  </P>
  <P>
  By default, a name must be unique within a pattern, but it is possible to relax
- this constraint by setting the PCRE_DUPNAMES option at compile time. This can
+ this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
- be useful for patterns where only one instance of the named parentheses can
+ names are also always permitted for subpatterns with the same number, set up as
- match. Suppose you want to match the name of a weekday, either as a 3-letter
+ described in the previous section.) Duplicate names can be useful for patterns
- abbreviation or as the full name, and in both cases you want to extract the
+ where only one instance of the named parentheses can match. Suppose you want to
- abbreviation. This pattern (ignoring the line breaks) does the job:
+ match the name of a weekday, either as a 3-letter abbreviation or as the full
+ name, and in both cases you want to extract the abbreviation. This pattern
+ (ignoring the line breaks) does the job:
  <pre>
    (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
    (?&#60;DN&#62;Tue)(?:sday)?|
-Line 1205 
 subpattern, as described in the previous
+Line 1433 
 subpattern, as described in the previous
  <P>
  The convenience function for extracting the data by name returns the substring
  for the first (and in this example, the only) subpattern of that name that
- matched. This saves searching to find which numbered subpattern it was. If you
+ matched. This saves searching to find which numbered subpattern it was.
- make a reference to a non-unique named subpattern from elsewhere in the
+ </P>
- pattern, the one that corresponds to the lowest number is used. For further
+ <P>
- details of the interfaces for handling named subpatterns, see the
+ If you make a back reference to a non-unique named subpattern from elsewhere in
+ the pattern, the one that corresponds to the first occurrence of the name is
+ used. In the absence of duplicate numbers (see the previous section) this is
+ the one with the lowest number. If you use a named reference in a condition
+ test (see the
+ <a href="#conditions">section about conditions</a>
+ below), either to check whether a subpattern has matched, or to check for
+ recursion, all subpatterns with the same name are tested. If the condition is
+ true for any one of them, the overall condition is true. This is the same
+ behaviour as testing by number. For further details of the interfaces for
+ handling named subpatterns, see the
  <a href="pcreapi.html"><b>pcreapi</b></a>
  documentation.
  </P>
  <P>
  <b>Warning:</b> You cannot use different names to distinguish between two
- subpatterns with the same number (see the previous section) because PCRE uses
+ subpatterns with the same number because PCRE uses only the numbers when
- only the numbers when matching.
+ matching. For this reason, an error is given at compile time if different names
+ are given to subpatterns with the same number. However, you can give the same
+ name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
  </P>
  <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
  <P>
-Line 1225 
 items:
+Line 1465 
 items:
    a literal data character
    the dot metacharacter
    the \C escape sequence
-   the \X escape sequence (in UTF-8 mode with Unicode properties)
+   the \X escape sequence
    the \R escape sequence
-   an escape such as \d that matches a single character
+   an escape such as \d or \pL that matches a single character
    a character class
    a back reference (see next section)
-   a parenthesized subpattern (unless it is an assertion)
+   a parenthesized subpattern (including assertions)
+   a subroutine call to a subpattern (recursive or otherwise)
  </pre>
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
-Line 1256 
 quantifier, is taken as a literal charac
+Line 1497 
 quantifier, is taken as a literal charac
  quantifier, but a literal string of four characters.
  </P>
  <P>
- In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
+ In UTF modes, quantifiers apply to characters rather than to individual data
- bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
+ units. Thus, for example, \x{100}{2} matches two characters, each of
- which is represented by a two-byte sequence. Similarly, when Unicode property
+ which is represented by a two-byte sequence in a UTF-8 string. Similarly,
- support is available, \X{3} matches three Unicode extended sequences, each of
+ \X{3} matches three Unicode extended sequences, each of which may be several
- which may be several bytes long (and they may be of different lengths).
+ data units long (and they may be of different lengths).
  </P>
  <P>
  The quantifier {0} is permitted, causing the expression to behave as if the
  previous item and the quantifier were not present. This may be useful for
  subpatterns that are referenced as
  <a href="#subpatternsassubroutines">subroutines</a>
- from elsewhere in the pattern. Items other than subpatterns that have a {0}
+ from elsewhere in the pattern (but see also the section entitled
- quantifier are omitted from the compiled pattern.
+ <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
+ below). Items other than subpatterns that have a {0} quantifier are omitted
+ from the compiled pattern.
  </P>
  <P>
  For convenience, the three most common quantifiers have single-character
-Line 1348 
 alternatively using ^ to indicate anchor
+Line 1591 
 alternatively using ^ to indicate anchor
  </P>
  <P>
  However, there is one situation where the optimization cannot be used. When .*
- is inside capturing parentheses that are the subject of a backreference
+ is inside capturing parentheses that are the subject of a back reference
  elsewhere in the pattern, a match at the start may fail where a later one
  succeeds. Consider, for example:
  <pre>
-Line 1504 
 subpattern is possible using named paren
+Line 1747 
 subpattern is possible using named paren
  </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
+ backslash is to use the \g escape sequence. This escape must be followed by an
- Perl 5.10. This escape must be followed by an unsigned number or a negative
+ unsigned number or a negative number, optionally enclosed in braces. These
- number, optionally enclosed in braces. These examples are all identical:
+ examples are all identical:
  <pre>
    (ring), \1
    (ring), \g1
-Line 1520 
 example:
+Line 1763 
 example:
    (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}
+ subpattern before \g, that is, is it equivalent to \2 in this example.
- would be equivalent to \1. The use of relative references can be helpful in
+ Similarly, \g{-2} would be equivalent to \1. The use of relative references
- long patterns, and also in patterns that are created by joining together
+ can be helpful in long patterns, and also in patterns that are created by
- fragments that contain references within themselves.
+ joining together fragments that contain references within themselves.
  </P>
  <P>
  A back reference matches whatever actually matched the capturing subpattern in
-Line 1562 
 after the reference.
+Line 1805 
 after the reference.
  <P>
  There may be more than one back reference to the same subpattern. If a
  subpattern has not actually been used in a particular match, any back
- references to it always fail. For example, the pattern
+ references to it always fail by default. For example, the pattern
  <pre>
    (a|(bc))\2
  </pre>
- always fails if it starts to match "a" rather than "bc". Because there may be
+ always fails if it starts to match "a" rather than "bc". However, if the
- many capturing parentheses in a pattern, all digits following the backslash are
+ PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
- taken as part of a potential back reference number. If the pattern continues
+ unset value matches an empty string.
- with a digit character, some delimiter must be used to terminate the back
+ </P>
- reference. If the PCRE_EXTENDED option is set, this can be whitespace.
+ <P>
- Otherwise an empty comment (see
+ Because there may be many capturing parentheses in a pattern, all digits
+ following a backslash are taken as part of a potential back reference number.
+ If the pattern continues with a digit character, some delimiter must be used to
+ terminate the back reference. If the PCRE_EXTENDED option is set, this can be
+ whitespace. Otherwise, the \g{ syntax or an empty comment (see
  <a href="#comments">"Comments"</a>
  below) can be used.
  </P>
+ <br><b>
+ Recursive back references
+ </b><br>
  <P>
  A back reference that occurs inside the parentheses to which it refers fails
  when the subpattern is first used, so, for example, (a\1) never matches.
-Line 1589 
 to the previous iteration. In order for
+Line 1839 
 to the previous iteration. In order for
  that the first iteration does not need to match the back reference. This can be
  done using alternation, as in the example above, or by a quantifier with a
  minimum of zero.
+ </P>
+ <P>
+ Back references of this type cause the group that they reference to be treated
+ as an
+ <a href="#atomicgroup">atomic group.</a>
+ Once the whole group has been matched, a subsequent matching failure cannot
+ cause backtracking into the middle of the group.
  <a name="bigassertions"></a></P>
  <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
  <P>
-Line 1604 
 that look behind it. An assertion subpat
+Line 1861 
 that look behind it. An assertion subpat
  except that it does not cause the current matching position to be changed.
  </P>
  <P>
- Assertion subpatterns are not capturing subpatterns, and may not be repeated,
+ Assertion subpatterns are not capturing subpatterns. If such an assertion
- because it makes no sense to assert the same thing several times. If any kind
+ contains capturing subpatterns within it, these are counted for the purposes of
- of assertion contains capturing subpatterns within it, these are counted for
+ numbering the capturing subpatterns in the whole pattern. However, substring
- the purposes of numbering the capturing subpatterns in the whole pattern.
+ capturing is carried out only for positive assertions, because it does not make
- However, substring capturing is carried out only for positive assertions,
+ sense for negative assertions.
- because it does not make sense for negative assertions.
+ </P>
+ <P>
+ For compatibility with Perl, assertion subpatterns may be repeated; though
+ it makes no sense to assert the same thing several times, the side effect of
+ capturing parentheses may occasionally be useful. In practice, there only three
+ cases:
+ <br>
+ <br>
+ (1) If the quantifier is {0}, the assertion is never obeyed during matching.
+ However, it may contain internal capturing parenthesized groups that are called
+ from elsewhere via the
+ <a href="#subpatternsassubroutines">subroutine mechanism.</a>
+ <br>
+ <br>
+ (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
+ were {0,1}. At run time, the rest of the pattern match is tried with and
+ without the assertion, the order depending on the greediness of the quantifier.
+ <br>
+ <br>
+ (3) If the minimum repetition is greater than zero, the quantifier is ignored.
+ The assertion is obeyed just once when encountered during matching.
  </P>
  <br><b>
  Lookahead assertions
-Line 1639 
 lookbehind assertion is needed to achiev
+Line 1916 
 lookbehind assertion is needed to achiev
  If you want to force a matching failure at some point in a pattern, the most
  convenient way to do it is with (?!) because an empty string always matches, so
  an assertion that requires there not to be an empty string must always fail.
+ The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
  <a name="lookbehind"></a></P>
  <br><b>
  Lookbehind assertions
-Line 1662 
 is permitted, but
+Line 1940 
 is permitted, but
  </pre>
  causes an error at compile time. Branches that match different length strings
  are permitted only at the top level of a lookbehind assertion. This is an
- extension compared with Perl (at least for 5.8), which requires all branches to
+ extension compared with Perl, which requires all branches to match the same
- match the same length of string. An assertion such as
+ length of string. An assertion such as
  <pre>
    (?&#60;=ab(c|de))
  </pre>
  is not permitted, because its single top-level branch can match two different
- lengths, but it is acceptable if rewritten to use two top-level branches:
+ lengths, but it is acceptable to PCRE if rewritten to use two top-level
+ branches:
  <pre>
    (?&#60;=abc|abde)
  </pre>
- In some cases, the Perl 5.10 escape sequence \K
+ In some cases, the escape sequence \K
  <a href="#resetmatchstart">(see above)</a>
- can be used instead of a lookbehind assertion; this is not restricted to a
+ can be used instead of a lookbehind assertion to get round the fixed-length
- fixed-length.
+ restriction.
  </P>
  <P>
  The implementation of lookbehind assertions is, for each alternative, to
-Line 1684 
 match. If there are insufficient charact
+Line 1963 
 match. If there are insufficient charact
  assertion fails.
  </P>
  <P>
- PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
+ In a UTF mode, PCRE does not allow the \C escape (which matches a single data
- to appear in lookbehind assertions, because it makes it impossible to calculate
+ unit even in a UTF mode) to appear in lookbehind assertions, because it makes
- the length of the lookbehind. The \X and \R escapes, which can match
+ it impossible to calculate the length of the lookbehind. The \X and \R
- different numbers of bytes, are also not permitted.
+ escapes, which can match different numbers of data units, are also not
+ permitted.
+ </P>
+ <P>
+ <a href="#subpatternsassubroutines">"Subroutine"</a>
+ calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
+ as the subpattern matches a fixed-length string.
+ <a href="#recursion">Recursion,</a>
+ however, is not supported.
  </P>
  <P>
  Possessive quantifiers can be used in conjunction with lookbehind assertions to
- specify efficient matching at the end of the subject string. Consider a simple
+ specify efficient matching of fixed-length strings at the end of subject
- pattern such as
+ strings. Consider a simple pattern such as
  <pre>
    abcd$
  </pre>
-Line 1754 
 characters that are not "999".
+Line 2041 
 characters that are not "999".
  <P>
  It is possible to cause the matching process to obey a subpattern
  conditionally or to choose between two alternative subpatterns, depending on
- the result of an assertion, or whether a previous capturing subpattern matched
+ the result of an assertion, or whether a specific capturing subpattern has
- or not. The two possible forms of conditional subpattern are
+ already been matched. The two possible forms of conditional subpattern are:
  <pre>
    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)
  </pre>
  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.
+ subpattern, a compile-time error occurs. Each of the two alternatives may
+ itself contain nested subpatterns of any form, including conditional
+ subpatterns; the restriction to two alternatives applies only at the level of
+ the condition. This pattern fragment is an example where the alternatives are
+ complex:
+ <pre>
+   (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
+ </PRE>
  </P>
  <P>
  There are four kinds of condition: references to subpatterns, references to
-Line 1773 
 Checking for a used subpattern by number
+Line 2068 
 Checking for a used subpattern by number
  </b><br>
  <P>
  If the text between the parentheses consists of a sequence of digits, the
- condition is true if the capturing subpattern of that number has previously
+ condition is true if a capturing subpattern of that number has previously
- matched. An alternative notation is to precede the digits with a plus or minus
+ matched. If there is more than one capturing subpattern with the same number
- sign. In this case, the subpattern number is relative rather than absolute.
+ (see the earlier
- The most recently opened parentheses can be referenced by (?(-1), the next most
+ <a href="#recursion">section about duplicate subpattern numbers),</a>
- recent by (?(-2), and so on. In looping constructs it can also make sense to
+ the condition is true if any of them have matched. An alternative notation is
- refer to subsequent groups with constructs such as (?(+2).
+ 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. Inside
+ loops it can also make sense to refer to subsequent groups. The next
+ parentheses to be opened can be referenced as (?(+1), and so on. (The value
+ zero in any of these forms is not used; it provokes a compile-time error.)
  </P>
  <P>
  Consider the following pattern, which contains non-significant white space to
-Line 1790 
 three parts for ease of discussion:
+Line 2090 
 three parts for ease of discussion:
  The first part matches an optional opening parenthesis, and if that
  character is present, sets it as the first captured substring. The second part
  matches one or more characters that are not parentheses. The third part is a
- conditional subpattern that tests whether the first set of parentheses matched
+ conditional subpattern that tests whether or not the first set of parentheses
- or not. If they did, that is, if subject started with an opening parenthesis,
+ matched. If they did, that is, if subject started with an opening parenthesis,
  the condition is true, and so the yes-pattern is executed and a closing
  parenthesis is required. Otherwise, since no-pattern is not present, the
  subpattern matches nothing. In other words, this pattern matches a sequence of
-Line 1822 
 names that consist entirely of digits is
+Line 2122 
 names that consist entirely of digits is
  Rewriting the above example to use a named subpattern gives this:
  <pre>
    (?&#60;OPEN&#62; \( )?    [^()]+    (?(&#60;OPEN&#62;) \) )
+ </pre>
- </PRE>
+ If the name used in a condition of this kind is a duplicate, the test is
+ applied to all subpatterns of the same name, and is true if any one of them has
+ matched.
  </P>
  <br><b>
  Checking for pattern recursion
-Line 1836 
 letter R, for example:
+Line 2138 
 letter R, for example:
  <pre>
    (?(R3)...) or (?(R&name)...)
  </pre>
- the condition is true if the most recent recursion is into the subpattern whose
+ the condition is true if the most recent recursion is into a subpattern whose
  number or name is given. This condition does not check the entire recursion
- stack.
+ stack. If the name used in a condition of this kind is a duplicate, the test is
+ applied to all subpatterns of the same name, and is true if any one of them is
+ the most recent recursion.
  </P>
  <P>
- At "top level", all these recursion test conditions are false. Recursive
+ At "top level", all these recursion test conditions are false.
- patterns are described below.
+ <a href="#recursion">The syntax for recursive patterns</a>
- </P>
+ is described below.
+ <a name="subdefine"></a></P>
  <br><b>
  Defining subpatterns for use by reference only
  </b><br>
-Line 1852 
 If the condition is the string (DEFINE),
+Line 2157 
 If the condition is the string (DEFINE),
  name DEFINE, the condition is always false. In this case, there may be only one
  alternative in the subpattern. It is always skipped if control reaches this
  point in the pattern; the idea of DEFINE is that it can be used to define
- "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
+ subroutines that can be referenced from elsewhere. (The use of
- is described below.) For example, a pattern to match an IPv4 address could be
+ <a href="#subpatternsassubroutines">subroutines</a>
- written like this (ignore whitespace and line breaks):
+ is described below.) For example, a pattern to match an IPv4 address such as
+ "192.168.23.245" 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
-Line 1862 
 written like this (ignore whitespace and
+Line 2169 
 written like this (ignore whitespace and
  The first part of the pattern is a DEFINE group inside which a another group
  named "byte" is defined. This matches an individual component of an IPv4
  address (a number less than 256). When matching takes place, this part of the
- pattern is skipped because DEFINE acts like a false condition.
+ pattern is skipped because DEFINE acts like a false condition. The rest of the
- </P>
+ pattern uses references to the named group to match the four dot-separated
- <P>
+ components of an IPv4 address, insisting on a word boundary at each end.
- The rest of the pattern uses references to the named group to match the four
- dot-separated components of an IPv4 address, insisting on a word boundary at
- each end.
  </P>
  <br><b>
  Assertion conditions
-Line 1890 
 dd-aaa-dd or dd-dd-dd, where aaa are let
+Line 2194 
 dd-aaa-dd or dd-dd-dd, where aaa are let
  <a name="comments"></a></P>
  <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
  <P>
- The sequence (?# marks the start of a comment that continues up to the next
+ There are two ways of including comments in patterns that are processed by
- closing parenthesis. Nested parentheses are not permitted. The characters
+ PCRE. In both cases, the start of the comment must not be in a character class,
- that make up a comment play no part in the pattern matching at all.
+ nor in the middle of any other sequence of related characters such as (?: or a
+ subpattern name or number. The characters that make up a comment play no part
+ in the pattern matching.
  </P>
  <P>
- If the PCRE_EXTENDED option is set, an unescaped # character outside a
+ The sequence (?# marks the start of a comment that continues up to the next
- character class introduces a comment that continues to immediately after the
+ closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
- next newline in the pattern.
+ option is set, an unescaped # character also introduces a comment, which in
+ this case continues to immediately after the next newline character or
+ character sequence in the pattern. Which characters are interpreted as newlines
+ is controlled by the options passed to a compiling function or by a special
+ sequence at the start of the pattern, as described in the section entitled
+ <a href="#newlines">"Newline conventions"</a>
+ above. Note that the end of this type of comment is a literal newline sequence
+ in the pattern; escape sequences that happen to represent a newline do not
+ count. For example, consider this pattern when PCRE_EXTENDED is set, and the
+ default newline convention is in force:
+ <pre>
+   abc #comment \n still comment
+ </pre>
+ On encountering the # character, <b>pcre_compile()</b> skips along, looking for
+ a newline in the pattern. The sequence \n is still literal at this stage, so
+ it does not terminate the comment. Only an actual character with the code value
+x0a (the default newline) does so.
  <a name="recursion"></a></P>
  <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
  <P>
-Line 1922 
 recursively to the pattern in which it a
+Line 2244 
 recursively to the pattern in which it a
  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
  supports special syntax for recursion of the entire pattern, and also for
  individual subpattern recursion. After its introduction in PCRE and Python,
- this kind of recursion was introduced into Perl at release 5.10.
+ this kind of recursion was subsequently introduced into Perl at release 5.10.
  </P>
  <P>
  A special item that consists of (? followed by a number greater than zero and a
- closing parenthesis is a recursive call of the subpattern of the given number,
+ closing parenthesis is a recursive subroutine call of the subpattern of the
- provided that it occurs inside that subpattern. (If not, it is a "subroutine"
+ given number, provided that it occurs inside that subpattern. (If not, it is a
+ <a href="#subpatternsassubroutines">non-recursive subroutine</a>
  call, which is described in the next section.) The special item (?R) or (?0) is
  a recursive call of the entire regular expression.
  </P>
  <P>
- In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
- treated as an atomic group. That is, once it has matched some of the subject
- string, it is never re-entered, even if it contains untried alternatives and
- there is a subsequent matching failure.
- </P>
- <P>
  This PCRE pattern solves the nested parentheses problem (assume the
  PCRE_EXTENDED option is set so that white space is ignored):
  <pre>
-   \( ( (?&#62;[^()]+) | (?R) )* \)
+   \( ( [^()]++ | (?R) )* \)
  </pre>
  First it matches an opening parenthesis. Then it matches any number of
  substrings which can either be a sequence of non-parentheses, or a recursive
  match of the pattern itself (that is, a correctly parenthesized substring).
- Finally there is a closing parenthesis.
+ Finally there is a closing parenthesis. Note the use of a possessive quantifier
+ to avoid backtracking into sequences of non-parentheses.
  </P>
  <P>
  If this were part of a larger pattern, you would not want to recurse the entire
  pattern, so instead you could use this:
  <pre>
-   ( \( ( (?&#62;[^()]+) | (?1) )* \) )
+   ( \( ( [^()]++ | (?1) )* \) )
  </pre>
  We have put the pattern into parentheses, and caused the recursion to refer to
  them instead of the whole pattern.
  </P>
  <P>
  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- is made easier by the use of relative references. (A Perl 5.10 feature.)
+ is made easier by the use of relative references. Instead of (?1) in the
- Instead of (?1) in the pattern above you can write (?-2) to refer to the second
+ pattern above you can write (?-2) to refer to the second most recently opened
- most recently opened parentheses preceding the recursion. In other words, a
+ parentheses preceding the recursion. In other words, a negative number counts
- negative number counts capturing parentheses leftwards from the point at which
+ capturing parentheses leftwards from the point at which it is encountered.
- it is encountered.
  </P>
  <P>
  It is also possible to refer to subsequently opened parentheses, by writing
  references such as (?+2). However, these cannot be recursive because the
  reference is not inside the parentheses that are referenced. They are always
- "subroutine" calls, as described in the next section.
+ <a href="#subpatternsassubroutines">non-recursive subroutine</a>
+ calls, as described in the next section.
  </P>
  <P>
  An alternative approach is to use named parentheses instead. The Perl syntax
  for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
  could rewrite the above example as follows:
  <pre>
-   (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
+   (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
  </pre>
  If there is more than one subpattern with the same name, the earliest one is
  used.
  </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
+ unlimited repeats, and so the use of a possessive quantifier for matching
- non-parentheses is important when applying the pattern to strings that do not
+ strings of non-parentheses is important when applying the pattern to strings
- match. For example, when this pattern is applied to
+ that do not match. For example, when this pattern is applied to
  <pre>
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
  </pre>
- it yields "no match" quickly. However, if atomic grouping is not used,
+ it yields "no match" quickly. However, if a possessive quantifier is not used,
  the match runs for a very long time indeed because there are so many different
  ways the + and * repeats can carve up the subject, and all have to be tested
  before failure can be reported.
  </P>
  <P>
- At the end of a match, the values set for any capturing subpatterns are those
+ At the end of a match, the values of capturing parentheses are those from
- from the outermost level of the recursion at which the subpattern value is set.
+ the outermost level. If you want to obtain intermediate values, a callout
- If you want to obtain intermediate values, a callout function can be used (see
+ function can be used (see below and the
- below and the
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation). If the pattern above is matched against
  <pre>
    (ab(cd)ef)
  </pre>
- the value for the capturing parentheses is "ef", which is the last value taken
+ the value for the inner capturing parentheses (numbered 2) is "ef", which is
- on at the top level. If additional parentheses are added, giving
+ the last value taken on at the top level. If a capturing subpattern is not
- <pre>
+ matched at the top level, its final captured value is unset, even if it was
-   \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
+ (temporarily) set at a deeper level during the matching process.
-      ^                        ^
+ </P>
-      ^                        ^
+ <P>
- </pre>
+ If there are more than 15 capturing parentheses in a pattern, PCRE has to
- the string they capture is "ab(cd)ef", the contents of the top level
+ obtain extra memory to store data during a recursion, which it does by using
- parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
+ <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
- has to obtain extra memory to store data during a recursion, which it does by
+ be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
- using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
- memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
  </P>
  <P>
  Do not confuse the (?R) item with the condition (R), which tests for recursion.
-Line 2028 
 recursing), whereas any characters are p
+Line 2343 
 recursing), whereas any characters are p
  In this pattern, (?(R) is the start of a conditional subpattern, with two
  different alternatives for the recursive and non-recursive cases. The (?R) item
  is the actual recursive call.
+ <a name="recursiondifference"></a></P>
+ <br><b>
+ Differences in recursion processing between PCRE and Perl
+ </b><br>
+ <P>
+ Recursion processing in PCRE differs from Perl in two important ways. In PCRE
+ (like Python, but unlike Perl), a recursive subpattern call is always treated
+ as an atomic group. That is, once it has matched some of the subject string, it
+ is never re-entered, even if it contains untried alternatives and there is a
+ subsequent matching failure. This can be illustrated by the following pattern,
+ which purports to match a palindromic string that contains an odd number of
+ characters (for example, "a", "aba", "abcba", "abcdcba"):
+ <pre>
+   ^(.|(.)(?1)\2)$
+ </pre>
+ The idea is that it either matches a single character, or two identical
+ characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
+ it does not if the pattern is longer than three characters. Consider the
+ subject string "abcba":
+ </P>
+ <P>
+ At the top level, the first character is matched, but as it is not at the end
+ of the string, the first alternative fails; the second alternative is taken
+ and the recursion kicks in. The recursive call to subpattern 1 successfully
+ matches the next character ("b"). (Note that the beginning and end of line
+ tests are not part of the recursion).
+ </P>
+ <P>
+ Back at the top level, the next character ("c") is compared with what
+ subpattern 2 matched, which was "a". This fails. Because the recursion is
+ treated as an atomic group, there are now no backtracking points, and so the
+ entire match fails. (Perl is able, at this point, to re-enter the recursion and
+ try the second alternative.) However, if the pattern is written with the
+ alternatives in the other order, things are different:
+ <pre>
+   ^((.)(?1)\2|.)$
+ </pre>
+ This time, the recursing alternative is tried first, and continues to recurse
+ until it runs out of characters, at which point the recursion fails. But this
+ time we do have another alternative to try at the higher level. That is the big
+ difference: in the previous case the remaining alternative is at a deeper
+ recursion level, which PCRE cannot use.
+ </P>
+ <P>
+ To change the pattern so that it matches all palindromic strings, not just
+ those with an odd number of characters, it is tempting to change the pattern to
+ this:
+ <pre>
+   ^((.)(?1)\2|.?)$
+ </pre>
+ Again, this works in Perl, but not in PCRE, and for the same reason. When a
+ deeper recursion has matched a single character, it cannot be entered again in
+ order to match an empty string. The solution is to separate the two cases, and
+ write out the odd and even cases as alternatives at the higher level:
+ <pre>
+   ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
+ </pre>
+ If you want to match typical palindromic phrases, the pattern has to ignore all
+ non-word characters, which can be done like this:
+ <pre>
+   ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
+ </pre>
+ If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
+ man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
+ the use of the possessive quantifier *+ to avoid backtracking into sequences of
+ non-word characters. Without this, PCRE takes a great deal longer (ten times or
+ more) to match typical phrases, and Perl takes so long that you think it has
+ gone into a loop.
+ </P>
+ <P>
+ <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
+ string does not start with a palindrome that is shorter than the entire string.
+ For example, although "abcba" is correctly matched, if the subject is "ababa",
+ PCRE finds the palindrome "aba" at the start, then fails at top level because
+ the end of the string does not follow. Once again, it cannot jump back into the
+ recursion to try other alternatives, so the entire match fails.
+ </P>
+ <P>
+ The second way in which PCRE and Perl differ in their recursion processing is
+ in the handling of captured values. In Perl, when a subpattern is called
+ recursively or as a subpattern (see the next section), it has no access to any
+ values that were captured outside the recursion, whereas in PCRE these values
+ can be referenced. Consider this pattern:
+ <pre>
+   ^(.)(\1|a(?2))
+ </pre>
+ In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
+ then in the second group, when the back reference \1 fails to match "b", the
+ second alternative matches "a" and then recurses. In the recursion, \1 does
+ now match "b" and so the whole match succeeds. In Perl, the pattern fails to
+ match because inside the recursive call \1 cannot access the externally set
+ value.
  <a name="subpatternsassubroutines"></a></P>
  <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
+ If the syntax for a recursive subpattern call (either by number or by
  name) is used outside the parentheses to which it refers, it operates like a
- subroutine in a programming language. The "called" subpattern may be defined
+ subroutine in a programming language. The called subpattern may be defined
  before or after the reference. A numbered reference can be absolute or
  relative, as in these examples:
  <pre>
-Line 2054 
 is used, it does match "sense and respon
+Line 2461 
 is used, it does match "sense and respon
  strings. Another example is given in the discussion of DEFINE above.
  </P>
  <P>
- Like recursive subpatterns, a "subroutine" call is always treated as an atomic
+ All subroutine calls, whether recursive or not, are always treated as atomic
- group. That is, once it has matched some of the subject string, it is never
+ groups. That is, once a subroutine has matched some of the subject string, it
- re-entered, even if it contains untried alternatives and there is a subsequent
+ is never re-entered, even if it contains untried alternatives and there is a
- matching failure.
+ subsequent matching failure. Any capturing parentheses that are set during the
+ subroutine call revert to their previous values afterwards.
  </P>
  <P>
- When a subpattern is used as a subroutine, processing options such as
+ Processing options such as case-independence are fixed when a subpattern is
- case-independence are fixed when the subpattern is defined. They cannot be
+ defined, so if it is used as a subroutine, such options cannot be changed for
- changed for different calls. For example, consider this pattern:
+ different calls. For example, consider this pattern:
  <pre>
    (abc)(?i:(?-1))
  </pre>
-Line 2097 
 same pair of parentheses when there is a
+Line 2505 
 same pair of parentheses when there is a
  <P>
  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
  code. The feature is called "callout". The caller of PCRE provides an external
- function by putting its entry point in the global variable <i>pcre_callout</i>.
+ function by putting its entry point in the global variable <i>pcre_callout</i>
- By default, this variable contains NULL, which disables all calling out.
+ (8-bit library) or <i>pcre16_callout</i> (16-bit library). By default, this
+ variable contains NULL, which disables all calling out.
  </P>
  <P>
  Within a regular expression, (?C) indicates the points at which the external
-Line 2108 
 For example, this pattern has two callou
+Line 2517 
 For example, this pattern has two callou
  <pre>
    (?C1)abc(?C2)def
  </pre>
- If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
+ If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, 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
+ During matching, when PCRE reaches a callout point, the external function is
- set), the external function is called. It is provided with the number of the
+ called. It is provided with the number of the callout, the position in the
- callout, the position in the pattern, and, optionally, one item of data
+ pattern, and, optionally, one item of data originally supplied by the caller of
- originally supplied by the caller of <b>pcre_exec()</b>. The callout function
+ the matching function. The callout function may cause matching to proceed, to
- may cause matching to proceed, to backtrack, or to fail altogether. A complete
+ backtrack, or to fail altogether. A complete description of the interface to
- description of the interface to the callout function is given in the
+ the callout function is given in the
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation.
- </P>
+ <a name="backtrackcontrol"></a></P>
  <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
  <P>
  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
-Line 2132 
 remarks apply to the PCRE features descr
+Line 2541 
 remarks apply to the PCRE features descr
  </P>
  <P>
  Since these verbs are specifically related to backtracking, most of them can be
- used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
+ used only when the pattern is to be matched using one of the traditional
- a backtracking algorithm. With the exception of (*FAIL), which behaves like a
+ matching functions, which use a backtracking algorithm. With the exception of
- failing negative assertion, they cause an error if encountered by
+ (*FAIL), which behaves like a failing negative assertion, they cause an error
- <b>pcre_dfa_exec()</b>.
+ if encountered by a DFA matching function.
+ </P>
+ <P>
+ If any of these verbs are used in an assertion or in a subpattern that is
+ called as a subroutine (whether or not recursively), their effect is confined
+ to that subpattern; it does not extend to the surrounding pattern, with one
+ exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
+ a successful positive assertion <i>is</i> passed back when a match succeeds
+ (compare capturing parentheses in assertions). Note that such subpatterns are
+ processed as anchored at the point where they are tested. Note also that Perl's
+ treatment of subroutines is different in some cases.
  </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
+ parenthesis followed by an asterisk. They are generally of the form
- (*VERB:ARG) but PCRE does not support the use of arguments, so its general
+ (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
- form is just (*VERB). Any number of these verbs may occur in a pattern. There
+ depending on whether or not an argument is present. A name is any sequence of
- are two kinds:
+ characters that does not include a closing parenthesis. If the name is empty,
+ that is, if the closing parenthesis immediately follows the colon, the effect
+ is as if the colon were not there. Any number of these verbs may occur in a
+ pattern.
+ </P>
+ <P>
+ PCRE contains some optimizations that are used to speed up matching by running
+ some checks at the start of each match attempt. For example, it may know the
+ minimum length of matching subject, or that a particular character must be
+ present. When one of these optimizations suppresses the running of a match, any
+ included backtracking verbs will not, of course, be processed. You can suppress
+ the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
+ when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
+ pattern with (*NO_START_OPT).
+ </P>
+ <P>
+ Experiments with Perl suggest that it too has similar optimizations, sometimes
+ leading to anomalous results.
  </P>
  <br><b>
  Verbs that act immediately
  </b><br>
  <P>
- The following verbs act as soon as they are encountered:
+ The following verbs act as soon as they are encountered. They may not be
+ followed by a name.
  <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
+ pattern. However, when it is inside a subpattern that is called as a
- immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
+ subroutine, only that subpattern is ended successfully. Matching then continues
- capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
+ at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
- captured. For example:
+ far is captured. For example:
  <pre>
-   A(A|B(*ACCEPT)|C)D
+   A((?:A|B(*ACCEPT)|C)D)
  </pre>
- This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
+ This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
- captured.
+ the outer parentheses.
  <pre>
    (*FAIL) or (*F)
  </pre>
- This verb causes the match to fail, forcing backtracking to occur. It is
+ This verb causes a matching failure, 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
-Line 2177 
 A match with the string "aaaa" always fa
+Line 2614 
 A match with the string "aaaa" always fa
  each backtrack happens (in this example, 10 times).
  </P>
  <br><b>
+ Recording which path was taken
+ </b><br>
+ <P>
+ There is one verb whose main purpose is to track how a match was arrived at,
+ though it also has a secondary use in conjunction with advancing the match
+ starting point (see (*SKIP) below).
+ <pre>
+   (*MARK:NAME) or (*:NAME)
+ </pre>
+ A name is always required with this verb. There may be as many instances of
+ (*MARK) as you like in a pattern, and their names do not have to be unique.
+ </P>
+ <P>
+ When a match succeeds, the name of the last-encountered (*MARK) on the matching
+ path is passed back to the caller as described in the section entitled
+ <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
+ in the
+ <a href="pcreapi.html"><b>pcreapi</b></a>
+ documentation. Here is an example of <b>pcretest</b> output, where the /K
+ modifier requests the retrieval and outputting of (*MARK) data:
+ <pre>
+     re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
+   data&#62; XY
+: XY
+   MK: A
+   XZ
+: XZ
+   MK: B
+ </pre>
+ The (*MARK) name is tagged with "MK:" in this output, and in this example it
+ indicates which of the two alternatives matched. This is a more efficient way
+ of obtaining this information than putting each alternative in its own
+ capturing parentheses.
+ </P>
+ <P>
+ If (*MARK) is encountered in a positive assertion, its name is recorded and
+ passed back if it is the last-encountered. This does not happen for negative
+ assertions.
+ </P>
+ <P>
+ After a partial match or a failed match, the name of the last encountered
+ (*MARK) in the entire match process is returned. For example:
+ <pre>
+     re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
+   data&#62; XP
+   No match, mark = B
+ </pre>
+ Note that in this unanchored example the mark is retained from the match
+ attempt that started at the letter "X". Subsequent match attempts starting at
+ "P" and then with an empty string do not get as far as the (*MARK) item, but
+ nevertheless do not reset it.
+ </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.
+ with what follows, but if there is no subsequent match, causing a backtrack to
- The verbs differ in exactly what kind of failure occurs.
+ the verb, a failure is forced. That is, backtracking cannot pass to the left of
+ the verb. However, when one of these verbs appears inside an atomic group, its
+ effect is confined to that group, because once the group has been matched,
+ there is never any backtracking into it. In this situation, backtracking can
+ "jump back" to the left of the entire atomic group. (Remember also, as stated
+ above, that this localization also applies in subroutine calls and assertions.)
+ </P>
+ <P>
+ These verbs differ in exactly what kind of failure occurs when backtracking
+ reaches them.
  <pre>
    (*COMMIT)
  </pre>
- This verb causes the whole match to fail outright if the rest of the pattern
+ This verb, which may not be followed by a name, causes the whole match to fail
- does not match. Even if the pattern is unanchored, no further attempts to find
+ outright if the rest of the pattern does not match. Even if the pattern is
- a match by advancing the start point take place. Once (*COMMIT) has been
+ unanchored, no further attempts to find a match by advancing the starting point
- passed, <b>pcre_exec()</b> is committed to finding a match at the current
+ take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
- starting point, or not at all. For example:
+ 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."
+ dynamic anchor, or "I've started, so I must finish." The name of the most
- <pre>
+ recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
-   (*PRUNE)
+ match failure.
- </pre>
+ </P>
- This verb causes the match to fail at the current position if the rest of the
+ <P>
- pattern does not match. If the pattern is unanchored, the normal "bumpalong"
+ Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
- advance to the next starting character then happens. Backtracking can occur as
+ unless PCRE's start-of-match optimizations are turned off, as shown in this
- usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
+ <b>pcretest</b> example:
- if there is no match to the right, backtracking cannot cross (*PRUNE).
+ <pre>
- In simple cases, the use of (*PRUNE) is just an alternative to an atomic
+     re&#62; /(*COMMIT)abc/
- group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
+   data&#62; xyzabc
- be expressed in any other way.
+: abc
+   xyzabc\Y
+   No match
+ </pre>
+ PCRE knows that any match must start with "a", so the optimization skips along
+ the subject to "a" before running the first match attempt, which succeeds. When
+ the optimization is disabled by the \Y escape in the second subject, the match
+ starts at "x" and so the (*COMMIT) causes it to fail without trying any other
+ starting points.
+ <pre>
+   (*PRUNE) or (*PRUNE:NAME)
+ </pre>
+ This verb causes the match to fail at the current starting position in the
+ subject 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), before it is
+ reached, 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.
+ The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
+ anchored pattern (*PRUNE) has the same effect as (*COMMIT).
  <pre>
    (*SKIP)
  </pre>
- This verb is like (*PRUNE), except that if the pattern is unanchored, the
+ This verb, when given without a name, is like (*PRUNE), except that if the
- "bumpalong" advance is not to the next character, but to the position in the
+ pattern is unanchored, the "bumpalong" advance is not to the next character,
- subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
+ but to the position in the subject where (*SKIP) was encountered. (*SKIP)
- was matched leading up to it cannot be part of a successful match. Consider:
+ 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
+ effect as this example; although it would suppress backtracking during the
  first match attempt, the second attempt would start at the second character
  instead of skipping on to "c".
  <pre>
-   (*THEN)
+   (*SKIP:NAME)
  </pre>
- This verb causes a skip to the next alternation if the rest of the pattern does
+ When (*SKIP) has an associated name, its behaviour is modified. If the
- not match. That is, it cancels pending backtracking, but only within the
+ following pattern fails to match, the previous path through the pattern is
- current alternation. Its name comes from the observation that it can be used
+ searched for the most recent (*MARK) that has the same name. If one is found,
- for a pattern-based if-then-else block:
+ the "bumpalong" advance is to the subject position that corresponds to that
+ (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
+ matching name is found, the (*SKIP) is ignored.
+ <pre>
+   (*THEN) or (*THEN:NAME)
+ </pre>
+ This verb causes a skip to the next innermost alternative if the rest of the
+ pattern does not match. That is, it cancels pending backtracking, but only
+ within the current alternative. 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
+ 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)
+ second alternative and tries COND2, without backtracking into COND1. The
- is used outside of any alternation, it acts exactly like (*PRUNE).
+ behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
+ If (*THEN) is not inside an alternation, it acts like (*PRUNE).
+ </P>
+ <P>
+ Note that a subpattern that does not contain a | character is just a part of
+ the enclosing alternative; it is not a nested alternation with only one
+ alternative. The effect of (*THEN) extends beyond such a subpattern to the
+ enclosing alternative. Consider this pattern, where A, B, etc. are complex
+ pattern fragments that do not contain any | characters at this level:
+ <pre>
+   A (B(*THEN)C) | D
+ </pre>
+ If A and B are matched, but there is a failure in C, matching does not
+ backtrack into A; instead it moves to the next alternative, that is, D.
+ However, if the subpattern containing (*THEN) is given an alternative, it
+ behaves differently:
+ <pre>
+   A (B(*THEN)C | (*FAIL)) | D
+ </pre>
+ The effect of (*THEN) is now confined to the inner subpattern. After a failure
+ in C, matching moves to (*FAIL), which causes the whole subpattern to fail
+ because there are no more alternatives to try. In this case, matching does now
+ backtrack into A.
+ </P>
+ <P>
+ Note also that a conditional subpattern is not considered as having two
+ alternatives, because only one is ever used. In other words, the | character in
+ a conditional subpattern has a different meaning. Ignoring white space,
+ consider:
+ <pre>
+   ^.*? (?(?=a) a | b(*THEN)c )
+ </pre>
+ If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
+ it initially matches zero characters. The condition (?=a) then fails, the
+ character "b" is matched, but "c" is not. At this point, matching does not
+ backtrack to .*? as might perhaps be expected from the presence of the |
+ character. The conditional subpattern is part of the single alternative that
+ comprises the whole pattern, and so the match fails. (If there was a backtrack
+ into .*?, allowing it to match "b", the match would succeed.)
+ </P>
+ <P>
+ The verbs just described provide four different "strengths" of control when
+ subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
+ next alternative. (*PRUNE) comes next, failing the match at the current
+ starting position, but allowing an advance to the next character (for an
+ unanchored pattern). (*SKIP) is similar, except that the advance may be more
+ than one character. (*COMMIT) is the strongest, causing the entire match to
+ fail.
+ </P>
+ <P>
+ If more than one such verb is present in a pattern, the "strongest" one wins.
+ For example, consider this pattern, where A, B, etc. are complex pattern
+ fragments:
+ <pre>
+   (A(*COMMIT)B(*THEN)C|D)
+ </pre>
+ Once A has matched, PCRE is committed to this match, at the current starting
+ position. If subsequently B matches, but C does not, the normal (*THEN) action
+ of trying the next alternative (that is, D) does not happen because (*COMMIT)
+ overrides.
  </P>
  <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
  <P>
- <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
+ <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
+ <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>.
  </P>
  <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
  <P>
-Line 2253 
 Cambridge CB2 3QH, England.
+Line 2843 
 Cambridge CB2 3QH, England.
  </P>
  <br><a name="SEC28" href="#TOC1">REVISION</a><br>
  <P>
- Last updated: 11 April 2009
+ Last updated: 09 January 2012
  <br>
- Copyright &copy; 1997-2009 University of Cambridge.
+ Copyright &copy; 1997-2012 University of Cambridge.
  <br>
  <p>
  Return to the <a href="index.html">PCRE index page</a>.

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