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-revision 456 by ph10,
Fri Oct  2 08:53:31 2009 UTC
+revision 562 by ph10,
Sun Oct 31 14:06:43 2010 UTC
 Line 21 
 published by O'Reilly, covers regular ex
  description of PCRE's regular expressions is intended as reference material.
  .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,
  PCRE must be built to include UTF-8 support, and you must call
  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
  is also a special sequence that can be given at the start of a pattern:
 Line 42 
 in the main
  .\"
  page.
  .P
+ Another special sequence that may appear at the start of a pattern or in
+ combination with (*UTF8) is:
+ .sp
+   (*UCP)
+ .sp
+ This has the same effect as setting the PCRE_UCP option: it causes sequences
+ such as \ed and \ew to use Unicode properties to determine character types,
+ instead of recognizing only characters with codes less than 128 via a lookup
+ table.
+ .P
  The remainder of this document discusses the patterns that are supported by
  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
  From release 6.0, PCRE offers a second matching function,
-Line 56 
 discussed in the
+Line 66 
 discussed in the
  page.
  .
  .
+ .\" HTML <a name="newlines"></a>
  .SH "NEWLINE CONVENTIONS"
  .rs
  .sp
-Line 83 
 string with one of the following five se
+Line 94 
 string with one of the following five se
    (*ANYCRLF)   any of the three above
    (*ANY)       all Unicode newline sequences
  .sp
  These override the default and the options given to \fBpcre_compile()\fP or
  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
  newline sequence, the pattern
  .sp
-Line 95 
 Perl-compatible, are recognized only at
+Line 106 
 Perl-compatible, are recognized only at
  they must be in upper case. If more than one of them is present, the last one
  is used.
  .P
- The newline convention does not affect what the \eR 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 \eN. However, it does not
- this can be changed; see the description of \eR in the section entitled
+ affect what the \eR escape sequence matches. By default, this is any Unicode
+ newline sequence, for Perl compatibility. However, this can be changed; see the
+ description of \eR in the section entitled
  .\" HTML <a href="#newlineseq">
  .\" </a>
  "Newline sequences"
-Line 198 
 Perl, $ and @ cause variable interpolati
+Line 211 
 Perl, $ and @ cause variable interpolati
    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
  .sp
  The \eQ...\eE sequence is recognized both inside and outside character classes.
+ An isolated \eE that is not preceded by \eQ is ignored.
  .
  .
  .\" HTML <a name="digitsafterbackslash"></a>
-Line 217 
 one of the following escape sequences th
+Line 231 
 one of the following escape sequences th
    \en        linefeed (hex 0A)
    \er        carriage return (hex 0D)
    \et        tab (hex 09)
-   \eddd      character with octal code ddd, or backreference
+   \eddd      character with octal code ddd, or back reference
    \exhh      character with hex code hh
    \ex{hhh..} character with hex code hhh..
  .sp
-Line 295 
 zero, because no more than three octal d
+Line 309 
 zero, because no more than three octal d
  .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
- sequence \eb is interpreted as the backspace character (hex 08), and the
+ sequence \eb is interpreted as the backspace character (hex 08). The sequences
- sequences \eR and \eX are interpreted as the characters "R" and "X",
+ \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
- respectively. Outside a character class, these sequences have different
+ unrecognized escape sequences, they are treated as the literal characters "B",
- meanings
+ "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
- .\" HTML <a href="#uniextseq">
+ set. Outside a character class, these sequences have different meanings.
- .\" </a>
- (see below).
- .\"
  .
  .
  .SS "Absolute and relative back references"
-Line 333 
 syntax for referencing a subpattern as a
+Line 344 
 syntax for referencing a subpattern as a
  later.
  .\"
  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
  synonymous. The former is a back reference; the latter is a
  .\" HTML <a href="#subpatternsassubroutines">
  .\" </a>
  subroutine
-Line 341 
 subroutine
+Line 352 
 subroutine
  call.
  .
  .
+ .\" HTML <a name="genericchartypes"></a>
  .SS "Generic character types"
  .rs
  .sp
- 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:
  .sp
    \ed     any decimal digit
    \eD     any character that is not a decimal digit
-Line 358 
 following are always recognized:
+Line 369 
 following are always recognized:
    \ew     any "word" character
    \eW     any "non-word" character
  .sp
- Each pair of escape sequences partitions the complete set of characters into
+ There is also the single sequence \eN, 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
+ .\" HTML <a href="#fullstopdot">
+ .\" </a>
+ the "." metacharacter
+ .\"
+ when PCRE_DOTALL is not set.
  .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
  For compatibility with Perl, \es does not match the VT character (code 11).
-Line 372 
 are HT (9), LF (10), FF (12), CR (13), a
+Line 390 
 are HT (9), LF (10), FF (12), CR (13), a
  included in a Perl script, \es may match the VT character. In PCRE, it never
  does.
  .P
- In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
+ A "word" character is an underscore or any character that is a letter or digit.
- \ew, and always match \eD, \eS, and \eW. 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 \eb, because it is defined in terms of \ew
+ .\" HTML <a href="pcreapi.html#localesupport">
- and \eW.
+ .\" </a>
+ "Locale support"
+ .\"
+ in the
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ 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 \ew. The use of locales with
+ Unicode is discouraged.
+ .P
+ By default, in UTF-8 mode, characters with values greater than 128 never match
+ \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
+ their original meanings from before UTF-8 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:
+ .sp
+   \ed  any character that \ep{Nd} matches (decimal digit)
+   \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
+   \ew  any character that \ep{L} or \ep{N} matches, plus underscore
+ .sp
+ The upper case escapes match the inverse sets of characters. Note that \ed
+ matches only decimal digits, whereas \ew matches any Unicode digit, as well as
+ any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
+ \eB because they are defined in terms of \ew and \eW. Matching these sequences
+ is noticeably slower when PCRE_UCP is set.
  .P
  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
- other sequences, these do match certain high-valued codepoints in UTF-8 mode.
+ other sequences, which match only ASCII characters by default, these always
- The horizontal space characters are:
+ match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
+ set. The horizontal space characters are:
  .sp
    U+0009     Horizontal tab
    U+0020     Space
-Line 412 
 The vertical space characters are:
+Line 458 
 The vertical space characters are:
    U+0085     Next line
    U+2028     Line separator
    U+2029     Paragraph separator
- .P
- 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
- .\" HTML <a href="pcreapi.html#localesupport">
- .\" </a>
- "Locale support"
- .\"
- in the
- .\" HREF
- \fBpcreapi\fP
- .\"
- 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 \ew. The use of locales with Unicode
- is discouraged.
  .
  .
  .\" HTML <a name="newlineseq"></a>
-Line 468 
 one of the following sequences:
+Line 497 
 one of the following sequences:
    (*BSR_ANYCRLF)   CR, LF, or CRLF only
    (*BSR_UNICODE)   any Unicode newline sequence
  .sp
  These override the default and the options given to \fBpcre_compile()\fP or
  \fBpcre_compile2()\fP, but they can be overridden by options given to
  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
  which are not Perl-compatible, are recognized only at the very start of a
  pattern, and that they must be in upper case. If more than one of them is
  present, the last one is used. They can be combined with a change of newline
- convention, for example, a pattern can start with:
+ convention; for example, a pattern can start with:
  .sp
    (*ANY)(*BSR_ANYCRLF)
  .sp
- Inside a character class, \eR matches the letter "R".
+ They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
+ a character class, \eR is treated as an unrecognized escape sequence, and so
+ matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
  .
  .
  .\" HTML <a name="uniextseq"></a>
-Line 496 
 The extra escape sequences are:
+Line 527 
 The extra escape sequences are:
    \eX       an extended Unicode sequence
  .sp
  The property names represented by \fIxx\fP 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 \eP{Any} does not match any
+ in the
- characters, so always causes a match failure.
+ .\" HTML <a href="#extraprops">
+ .\" </a>
+ next section).
+ .\"
+ Other Perl properties such as "InMusicalSymbols" are not currently supported by
+ PCRE. Note that \eP{Any} does not match any characters, so always causes a
+ match failure.
  .P
  Sets of Unicode characters are defined as belonging to certain scripts. A
  character from one of these sets can be matched using a script name. For
-Line 513 
 Those that are not part of an identified
+Line 550 
 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 528 
 Cypriot,
+Line 569 
 Cypriot,
  Cyrillic,
  Deseret,
  Devanagari,
+ Egyptian_Hieroglyphs,
  Ethiopic,
  Georgian,
  Glagolitic,
-Line 540 
 Hangul,
+Line 582 
 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 557 
 Nko,
+Line 610 
 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 576 
 Thai,
+Line 638 
 Thai,
  Tibetan,
  Tifinagh,
  Ugaritic,
+ Vai,
  Yi.
  .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, \ep{^Lu} is the same as \eP{Lu}.
+ name. For example, \ep{^Lu} is the same as \eP{Lu}.
  .P
  If only one letter is specified with \ep or \eP, it includes all the general
  category properties that start with that letter. In this case, in the absence
-Line 680 
 non-UTF-8 mode \eX matches any one chara
+Line 743 
 non-UTF-8 mode \eX matches any one chara
  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 \ed and \ew 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 for \fBpcre_compile()\fP or by starting the pattern with
+ (*UCP).
+ .
+ .
+ .\" HTML <a name="extraprops"></a>
+ .SS PCRE's additional properties
+ .rs
+ .sp
+ 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 \ew and \es and POSIX character classes to use Unicode
+ properties. PCRE uses these non-standard, non-Perl properties internally when
+ PCRE_UCP is set. They are:
+ .sp
+   Xan   Any alphanumeric character
+   Xps   Any POSIX space character
+   Xsp   Any Perl space character
+   Xwd   Any Perl "word" character
+ .sp
+ 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.
  .
  .
  .\" HTML <a name="resetmatchstart"></a>
-Line 711 
 For example, when the pattern
+Line 798 
 For example, when the pattern
    (foo)\eKbar
  .sp
  matches "foobar", the first substring is still set to "foo".
+ .P
+ Perl documents that the use of \eK within assertions is "not well defined". In
+ PCRE, \eK is acted upon when it occurs inside positive assertions, but is
+ ignored in negative assertions.
  .
  .
  .\" HTML <a name="smallassertions"></a>
-Line 735 
 The backslashed assertions are:
+Line 826 
 The backslashed assertions are:
    \ez     matches only at the end of the subject
    \eG     matches at the first matching position in the subject
  .sp
- These assertions may not appear in character classes (but note that \eb has a
+ Inside a character class, \eb 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, \eB
+ matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
+ escape sequence" error is generated instead.
  .P
  A word boundary is a position in the subject string where the current character
  and the previous character do not both match \ew or \eW (i.e. one matches
  \ew and the other matches \eW), or the start or end of the string if the
- first or last character matches \ew, respectively. Neither PCRE nor Perl has a
+ first or last character matches \ew, respectively. In UTF-8 mode, the meanings
- separte "start of word" or "end of word" metasequence. However, whatever
+ of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
- follows \eb normally determines which it is. For example, the fragment
+ done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
- \eba matches "a" at the start of a word.
+ of word" or "end of word" metasequence. However, whatever follows \eb normally
+ determines which it is. For example, the fragment \eba matches "a" at the start
+ of a word.
  .P
  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
  dollar (described in the next section) in that they only ever match at the very
-Line 828 
 end of the subject in both modes, and if
+Line 924 
 end of the subject in both modes, and if
  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
  .
  .
- .SH "FULL STOP (PERIOD, DOT)"
+ .\" HTML <a name="fullstopdot"></a>
+ .SH "FULL STOP (PERIOD, DOT) AND \eN"
  .rs
  .sp
  Outside a character class, a dot in the pattern matches any one character in
-Line 850 
 to match it.
+Line 947 
 to match it.
  The handling of dot is entirely independent of the handling of circumflex and
  dollar, the only relationship being that they both involve newlines. Dot has no
  special meaning in a character class.
+ .P
+ The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
+ set. In other words, it matches any one character except one that signifies the
+ end of a line.
  .
  .
  .SH "MATCHING A SINGLE BYTE"
-Line 876 
 the lookbehind.
+Line 977 
 the lookbehind.
  .rs
  .sp
  An opening square bracket introduces a character class, terminated by a closing
  square bracket. A closing square bracket on its own is not special by default.
  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
  bracket causes a compile-time error. If a closing square bracket is required as
  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.
-Line 944 
 characters in both cases. In UTF-8 mode,
+Line 1045 
 characters in both cases. In UTF-8 mode,
  characters with values greater than 128 only when it is compiled with Unicode
  property support.
  .P
- The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
+ The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
- in a character class, and add the characters that they match to the class. For
+ \eW may also appear in a character class, and add the characters that they
- example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
+ match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
- conveniently be used with the upper case character types to specify a more
+ circumflex can conveniently be used with the upper case character types to
- restricted set of characters than the matching lower case type. For example,
+ specify a more restricted set of characters than the matching lower case type.
- the class [^\eW_] matches any letter or digit, but not underscore.
+ For example, the class [^\eW_] matches any letter or digit, but not underscore.
  .P
  The only metacharacters that are recognized in character classes are backslash,
  hyphen (only where it can be interpreted as specifying a range), circumflex
-Line 969 
 this notation. For example,
+Line 1070 
 this notation. For example,
    [01[:alpha:]%]
  .sp
  matches "0", "1", any alphabetic character, or "%". The supported class names
- are
+ are:
  .sp
    alnum    letters and digits
    alpha    letters
-Line 980 
 are
+Line 1081 
 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 \es)
    upper    upper case letters
    word     "word" characters (same as \ew)
-Line 1001 
 matches "1", "2", or any non-digit. PCRE
+Line 1102 
 matches "1", "2", or any non-digit. PCRE
  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
  supported, and an error is given if they are encountered.
  .P
- In UTF-8 mode, characters with values greater than 128 do not match any of
+ By default, in UTF-8 mode, 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 \fBpcre_compile()\fP, 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:
+ .sp
+   [:alnum:]  becomes  \ep{Xan}
+   [:alpha:]  becomes  \ep{L}
+   [:blank:]  becomes  \eh
+   [:digit:]  becomes  \ep{Nd}
+   [:lower:]  becomes  \ep{Ll}
+   [:space:]  becomes  \ep{Xps}
+   [:upper:]  becomes  \ep{Lu}
+   [:word:]   becomes  \ep{Xwd}
+ .sp
+ Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
+ classes are unchanged, and match only characters with code points less than
+.
  .
  .
  .SH "VERTICAL BAR"
-Line 1081 
 section entitled
+Line 1198 
 section entitled
  .\" </a>
  "Newline sequences"
  .\"
- above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
+ above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
- mode; this is equivalent to setting the PCRE_UTF8 option.
+ to set UTF-8 and Unicode property modes; they are equivalent to setting the
+ PCRE_UTF8 and the PCRE_UCP options, respectively.
  .
  .
  .\" HTML <a name="subpattern"></a>
-Line 1163 
 stored.
+Line 1281 
 stored.
    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
    # 1            2         2  3        2     3     4
  .sp
- A backreference to a numbered subpattern uses the most recent value that is set
+ A back reference to a numbered subpattern uses the most recent value that is
- for that number by any subpattern. The following pattern matches "abcabc" or
+ set for that number by any subpattern. The following pattern matches "abcabc"
- "defdef":
+ or "defdef":
  .sp
-   /(?|(abc)|(def))\1/
+   /(?|(abc)|(def))\e1/
  .sp
  In contrast, a recursive or "subroutine" call to a numbered subpattern always
  refers to the first one in the pattern with the given number. The following
  pattern matches "abcabc" or "defabc":
  .sp
    /(?|(abc)|(def))(?1)/
  .sp
+ If a
+ .\" HTML <a href="#conditions">
+ .\" </a>
+ condition test
+ .\"
+ 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
- An alternative approach to using the "branch reset" feature is to use
+ An alternative approach to using this "branch reset" feature is to use
  duplicate named subpatterns, as described in the next section.
  .
  .
-Line 1189 
 if an expression is modified, the number
+Line 1314 
 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
  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
  parentheses from other parts of the pattern, such as
  .\" HTML <a href="#backreferences">
  .\" </a>
- backreferences,
+ back references,
  .\"
  .\" HTML <a href="#recursion">
  .\" </a>
-Line 1216 
 extracting the name-to-number translatio
+Line 1342 
 extracting the name-to-number translatio
  is also a convenience function for extracting a captured substring by name.
  .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:
  .sp
    (?<DN>Mon|Fri|Sun)(?:day)?|
    (?<DN>Tue)(?:sday)?|
-Line 1234 
 subpattern, as described in the previous
+Line 1362 
 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
+ If you make a back reference to a non-unique named subpattern from elsewhere in
- details of the interfaces for handling named subpatterns, see the
+ 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
+ .\"
+ .\" HTML <a href="#conditions">
+ .\" </a>
+ section about conditions
+ .\"
+ 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
  .\" HREF
  \fBpcreapi\fP
  .\"
  documentation.
  .P
  \fBWarning:\fP 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.
  .
  .
  .SH REPETITION
-Line 1263 
 items:
+Line 1406 
 items:
    a character class
    a back reference (see next section)
    a parenthesized subpattern (unless it is an assertion)
    a recursive or "subroutine" call to a subpattern
  .sp
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
-Line 1374 
 worth setting PCRE_DOTALL in order to ob
+Line 1517 
 worth setting PCRE_DOTALL in order to ob
  alternatively using ^ to indicate anchoring explicitly.
  .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:
  .sp
-Line 1589 
 references to it always fail by default.
+Line 1732 
 references to it always fail by default.
  .sp
    (a|(bc))\e2
  .sp
  always fails if it starts to match "a" rather than "bc". However, if the
  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
  unset value matches an empty string.
  .P
  Because there may be many capturing parentheses in a pattern, all digits
-Line 1603 
 whitespace. Otherwise, the \eg{ syntax o
+Line 1746 
 whitespace. Otherwise, the \eg{ syntax o
  "Comments"
  .\"
  below) can be used.
- .P
+ .
+ .SS "Recursive back references"
+ .rs
+ .sp
  A back reference that occurs inside the parentheses to which it refers fails
  when the subpattern is first used, so, for example, (a\e1) never matches.
  However, such references can be useful inside repeated subpatterns. For
-Line 1617 
 to the previous iteration. In order for
+Line 1763 
 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
+ Back references of this type cause the group that they reference to be treated
+ as an
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ atomic group.
+ .\"
+ Once the whole group has been matched, a subsequent matching failure cannot
+ cause backtracking into the middle of the group.
  .
  .
  .\" HTML <a name="bigassertions"></a>
-Line 1712 
 In some cases, the Perl 5.10 escape sequ
+Line 1867 
 In some cases, the Perl 5.10 escape sequ
  .\" </a>
  (see above)
  .\"
  can be used instead of a lookbehind assertion to get round the fixed-length
  restriction.
  .P
  The implementation of lookbehind assertions is, for each alternative, to
-Line 1730 
 different numbers of bytes, are also not
+Line 1885 
 different numbers of bytes, are also not
  "Subroutine"
  .\"
  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
  as the subpattern matches a fixed-length string.
  .\" HTML <a href="#recursion">
  .\" </a>
  Recursion,
-Line 1803 
 characters that are not "999".
+Line 1958 
 characters that are not "999".
  .sp
  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 specific capturing subpattern has
  already been matched. The two possible forms of conditional subpattern are:
  .sp
    (?(condition)yes-pattern)
-Line 1811 
 already been matched. The two possible f
+Line 1966 
 already been matched. The two possible f
  .sp
  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:
+ .sp
+   (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
+ .sp
  .P
  There are four kinds of condition: references to subpatterns, references to
  recursion, a pseudo-condition called DEFINE, and assertions.
-Line 1821 
 recursion, a pseudo-condition called DEF
+Line 1983 
 recursion, a pseudo-condition called DEF
  .sp
  If the text between the parentheses consists of a sequence of digits, the
  condition is true if a capturing subpattern of that number has previously
  matched. If there is more than one capturing subpattern with the same number
  (see the earlier
  .\"
  .\" HTML <a href="#recursion">
  .\" </a>
-Line 1874 
 Rewriting the above example to use a nam
+Line 2036 
 Rewriting the above example to use a nam
  .sp
    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
  .sp
+ 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.
  .
  .SS "Checking for pattern recursion"
  .rs
-Line 1887 
 letter R, for example:
+Line 2052 
 letter R, for example:
  .sp
  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
  At "top level", all these recursion test conditions are false.
  .\" HTML <a href="#recursion">
  .\" </a>
- Recursive patterns
+ The syntax for recursive patterns
  .\"
- are described below.
+ is described below.
  .
  .SS "Defining subpatterns for use by reference only"
  .rs
-Line 1903 
 If the condition is the string (DEFINE),
+Line 2070 
 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
  .\" HTML <a href="#subpatternsassubroutines">
  .\" </a>
  "subroutines"
-Line 1944 
 dd-aaa-dd or dd-dd-dd, where aaa are let
+Line 2111 
 dd-aaa-dd or dd-dd-dd, where aaa are let
  .SH COMMENTS
  .rs
  .sp
- 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
- 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 \fBpcre_compile()\fP or by a special
+ sequence at the start of the pattern, as described in the section entitled
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ "Newline conventions"
+ .\"
+ above. Note that 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:
+ .sp
+   abc #comment \en still comment
+ .sp
+ On encountering the # character, \fBpcre_compile()\fP skips along, looking for
+ a newline in the pattern. The sequence \en 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.
  .
  .
  .\" HTML <a name="recursion"></a>
-Line 1980 
 this kind of recursion was subsequently
+Line 2168 
 this kind of recursion was subsequently
  .P
  A special item that consists of (? followed by a number greater than zero and a
  closing parenthesis is a recursive call of the subpattern of the given number,
  provided that it occurs inside that subpattern. (If not, it is a
  .\" HTML <a href="#subpatternsassubroutines">
  .\" </a>
  "subroutine"
-Line 1996 
 PCRE_EXTENDED option is set so that whit
+Line 2184 
 PCRE_EXTENDED option is set so that whit
  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. Note the use of a possessive quantifier
  to avoid backtracking into sequences of non-parentheses.
  .P
  If this were part of a larger pattern, you would not want to recurse the entire
-Line 2044 
 the match runs for a very long time inde
+Line 2232 
 the match runs for a very long time inde
  ways the + and * repeats can carve up the subject, and all have to be tested
  before failure can be reported.
  .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
  .\" HREF
  \fBpcrecallout\fP
  .\"
-Line 2055 
 documentation). If the pattern above is
+Line 2242 
 documentation). If the pattern above is
  .sp
    (ab(cd)ef)
  .sp
- 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
- .sp
+ matched at the top level, its final value is unset, even if it is (temporarily)
-   \e( ( ( [^()]++ | (?R) )* ) \e)
+ set at a deeper level.
-      ^                        ^
+ .P
-      ^                        ^
+ If there are more than 15 capturing parentheses in a pattern, PCRE has to
- .sp
+ obtain extra memory to store data during a recursion, which it does by using
- the string they capture is "ab(cd)ef", the contents of the top level
+ \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
- parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
+ be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
- has to obtain extra memory to store data during a recursion, which it does by
- using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
- memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
  .P
  Do not confuse the (?R) item with the condition (R), which tests for recursion.
  Consider this pattern, which matches text in angle brackets, allowing for
-Line 2087 
 is the actual recursive call.
+Line 2271 
 is the actual recursive call.
  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"):
  .sp
    ^(.|(.)(?1)\e2)$
  .sp
  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
  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
  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
-Line 2113 
 alternatives in the other order, things
+Line 2297 
 alternatives in the other order, things
  .sp
    ^((.)(?1)\e2|.)$
  .sp
  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
  To change the pattern so that matches all palindromic strings, not just those
  with an odd number of characters, it is tempting to change the pattern to this:
  .sp
    ^((.)(?1)\e2|.?)$
  .sp
  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:
  .sp
    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
  .sp
  If you want to match typical palindromic phrases, the pattern has to ignore all
  non-word characters, which can be done like this:
  .sp
-   ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
+   ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
  .sp
  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.
-Line 2177 
 matches "sense and sensibility" and "res
+Line 2361 
 matches "sense and sensibility" and "res
  is used, it does match "sense and responsibility" as well as the other two
  strings. Another example is given in the discussion of DEFINE above.
  .P
- Like recursive subpatterns, a "subroutine" call is always treated as an atomic
+ Like recursive subpatterns, a subroutine call is always treated as an atomic
  group. That is, once it has matched some of the subject string, it is never
  re-entered, even if it contains untried alternatives and there is a subsequent
- matching failure.
+ matching failure. Any capturing parentheses that are set during the subroutine
+ call revert to their previous values afterwards.
  .P
  When a subpattern is used as a subroutine, processing options such as
  case-independence are fixed when the subpattern is defined. They cannot be
-Line 2249 
 description of the interface to the call
+Line 2434 
 description of the interface to the call
  documentation.
  .
  .
+ .\" HTML <a name="backtrackcontrol"></a>
  .SH "BACKTRACKING CONTROL"
  .rs
  .sp
-Line 2264 
 a backtracking algorithm. With the excep
+Line 2450 
 a backtracking algorithm. With the excep
  failing negative assertion, they cause an error if encountered by
  \fBpcre_dfa_exec()\fP.
  .P
- If any of these verbs are used in an assertion subpattern, their effect is
+ If any of these verbs are used in an assertion or subroutine subpattern
- confined to that subpattern; it does not extend to the surrounding pattern.
+ (including recursive subpatterns), their effect is confined to that subpattern;
- Note that assertion subpatterns are processed as anchored at the point where
+ it does not extend to the surrounding pattern. Note that such subpatterns are
- they are tested.
+ processed as anchored at the point where they are tested.
  .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. An name is a sequence of
- are two kinds:
+ letters, digits, and underscores. 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
+ 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 \fBpcre_exec()\fP.
+ .
  .
  .SS "Verbs that act immediately"
  .rs
  .sp
- 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.
  .sp
     (*ACCEPT)
  .sp
-Line 2289 
 captured. (This feature was added to PCR
+Line 2487 
 captured. (This feature was added to PCR
  .sp
    A((?:A|B(*ACCEPT)|C)D)
  .sp
  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
  the outer parentheses.
  .sp
    (*FAIL) or (*F)
-Line 2305 
 callout feature, as for example in this
+Line 2503 
 callout feature, as for example in this
  A match with the string "aaaa" always fails, but the callout is taken before
  each backtrack happens (in this example, 10 times).
  .
+ .
+ .SS "Recording which path was taken"
+ .rs
+ .sp
+ 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).
+ .sp
+   (*MARK:NAME) or (*:NAME)
+ .sp
+ 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
+ When a match succeeds, the name of the last-encountered (*MARK) is passed back
+ to the caller via the \fIpcre_extra\fP data structure, as described in the
+ .\" HTML <a href="pcreapi.html#extradata">
+ .\" </a>
+ section on \fIpcre_extra\fP
+ .\"
+ in the
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ documentation. No data is returned for a partial match. Here is an example of
+ \fBpcretest\fP output, where the /K modifier requests the retrieval and
+ outputting of (*MARK) data:
+ .sp
+   /X(*MARK:A)Y|X(*MARK:B)Z/K
+   XY
+: XY
+   MK: A
+   XZ
+: XZ
+   MK: B
+ .sp
+ 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
+ A name may also be returned after a failed match if the final path through the
+ pattern involves (*MARK). However, unless (*MARK) used in conjunction with
+ (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
+ starting point for matching is advanced, the final check is often with an empty
+ string, causing a failure before (*MARK) is reached. For example:
+ .sp
+   /X(*MARK:A)Y|X(*MARK:B)Z/K
+   XP
+   No match
+ .sp
+ There are three potential starting points for this match (starting with X,
+ starting with P, and with an empty string). If the pattern is anchored, the
+ result is different:
+ .sp
+   /^X(*MARK:A)Y|^X(*MARK:B)Z/K
+   XP
+   No match, mark = B
+ .sp
+ PCRE's start-of-match optimizations can also interfere with this. For example,
+ if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
+ subject length for a match, a shorter subject will not be scanned at all.
+ .P
+ Note that similar anomalies (though different in detail) exist in Perl, no
+ doubt for the same reasons. The use of (*MARK) data after a failed match of an
+ unanchored pattern is not recommended, unless (*COMMIT) is involved.
+ .
+ .
  .SS "Verbs that act after backtracking"
  .rs
  .sp
  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
+ These verbs differ in exactly what kind of failure occurs when backtracking
+ reaches them.
  .sp
    (*COMMIT)
  .sp
- 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 starting point take place. Once (*COMMIT) has been
+ unanchored, no further attempts to find a match by advancing the starting point
- passed, \fBpcre_exec()\fP is committed to finding a match at the current
+ take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP 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:
  .sp
    a+(*COMMIT)b
  .sp
  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
- .sp
+ recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
-   (*PRUNE)
+ match failure.
- .sp
+ .P
- This verb causes the match to fail at the current position if the rest of the
+ Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
- pattern does not match. If the pattern is unanchored, the normal "bumpalong"
+ unless PCRE's start-of-match optimizations are turned off, as shown in this
- advance to the next starting character then happens. Backtracking can occur as
+ \fBpcretest\fP example:
- usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
+ .sp
- if there is no match to the right, backtracking cannot cross (*PRUNE).
+   /(*COMMIT)abc/
- In simple cases, the use of (*PRUNE) is just an alternative to an atomic
+   xyzabc
- group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
+: abc
- be expressed in any other way.
+   xyzabc\eY
+   No match
+ .sp
+ 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 \eY escape in the second subject, the match
+ starts at "x" and so the (*COMMIT) causes it to fail without trying any other
+ starting points.
+ .sp
+   (*PRUNE) or (*PRUNE:NAME)
+ .sp
+ 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) when the
+ match fails completely; the name is passed back if this is the final attempt.
+ (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
+ pattern (*PRUNE) has the same effect as (*COMMIT).
  .sp
    (*SKIP)
  .sp
- 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:
  .sp
    a+(*SKIP)b
  .sp
-Line 2352 
 effect as this example; although it woul
+Line 2648 
 effect as this example; although it woul
  first match attempt, the second attempt would start at the second character
  instead of skipping on to "c".
  .sp
-   (*THEN)
+   (*SKIP:NAME)
  .sp
- 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, normal "bumpalong" of one character happens (the
+ (*SKIP) is ignored).
+ .sp
+   (*THEN) or (*THEN:NAME)
+ .sp
+ This verb causes a skip to the next alternation in the innermost enclosing
+ group if the rest of the pattern does not match. That is, it cancels pending
+ backtracking, but only within the current alternation. Its name comes from the
+ observation that it can be used for a pattern-based if-then-else block:
  .sp
    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
  .sp
  If the COND1 pattern matches, FOO is tried (and possibly further items after
  the end of the group if FOO succeeds); on failure the matcher skips to the
- second alternative and tries COND2, without backtracking into COND1. If (*THEN)
+ 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 the
+ overall match fails. If (*THEN) is not directly inside an alternation, it acts
+ like (*PRUNE).
+ .
+ .P
+ The above verbs provide four different "strengths" of control when subsequent
+ matching fails. (*THEN) is the weakest, carrying on the match at the next
+ alternation. (*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
+ If more than one is present in a pattern, the "stongest" one wins. For example,
+ consider this pattern, where A, B, etc. are complex pattern fragments:
+ .sp
+   (A(*COMMIT)B(*THEN)C|D)
+ .sp
+ 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 alternation (that is, D) does not happen because (*COMMIT)
+ overrides.
  .
  .
  .SH "SEE ALSO"
  .rs
  .sp
  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
  \fBpcresyntax\fP(3), \fBpcre\fP(3).
  .
  .
-Line 2388 
 Cambridge CB2 3QH, England.
+Line 2714 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 30 September 2009
+ Last updated: 31 October 2010
- Copyright (c) 1997-2009 University of Cambridge.
+ Copyright (c) 1997-2010 University of Cambridge.
  .fi

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