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-revision 184 by ph10,
Tue Jun 19 13:26:46 2007 UTC
+revision 514 by ph10,
Mon May  3 12:54:22 2010 UTC
 Line 4 
 PCRE - Perl-compatible regular expressio
  .SH "PCRE REGULAR EXPRESSION DETAILS"
  .rs
  .sp
- The syntax and semantics of the regular expressions supported by PCRE are
+ The syntax and semantics of the regular expressions that are supported by PCRE
- described below. Regular expressions are also described in the Perl
+ are described in detail below. There is a quick-reference syntax summary in the
- documentation and in a number of books, some of which have copious examples.
+ .\" HREF
- Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
+ \fBpcresyntax\fP
- regular expressions in great detail. This description of PCRE's regular
+ .\"
- expressions is intended as reference material.
+ page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
+ also supports some alternative regular expression syntax (which does not
+ conflict with the Perl syntax) in order to provide some compatibility with
+ regular expressions in Python, .NET, and Oniguruma.
+ .P
+ Perl's regular expressions are described in its own documentation, and
+ regular expressions in general are covered in a number of books, some of which
+ have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
+ published by O'Reilly, covers regular expressions in great detail. This
+ description of PCRE's regular expressions is intended as reference material.
  .P
  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 character strings. To use this,
- build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
+ PCRE must be built to include UTF-8 support, and you must call
- the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
+ \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
- places below. There is also a summary of UTF-8 features in the
+ is also a special sequence that can be given at the start of a pattern:
+ .sp
+   (*UTF8)
+ .sp
+ Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
+ option. This feature is not Perl-compatible. How setting UTF-8 mode affects
+ pattern matching is mentioned in several places below. There is also a summary
+ of UTF-8 features in the
  .\" HTML <a href="pcre.html#utf8support">
  .\" </a>
  section on UTF-8 support
-Line 40 
 discussed in the
+Line 56 
 discussed in the
  page.
  .
  .
+ .SH "NEWLINE CONVENTIONS"
+ .rs
+ .sp
+ PCRE supports five different conventions for indicating line breaks in
+ strings: a single CR (carriage return) character, a single LF (linefeed)
+ character, the two-character sequence CRLF, any of the three preceding, or any
+ Unicode newline sequence. The
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ page has
+ .\" HTML <a href="pcreapi.html#newlines">
+ .\" </a>
+ further discussion
+ .\"
+ about newlines, and shows how to set the newline convention in the
+ \fIoptions\fP arguments for the compiling and matching functions.
+ .P
+ It is also possible to specify a newline convention by starting a pattern
+ string with one of the following five sequences:
+ .sp
+   (*CR)        carriage return
+   (*LF)        linefeed
+   (*CRLF)      carriage return, followed by linefeed
+   (*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
+   (*CR)a.b
+ .sp
+ changes the convention to CR. That pattern matches "a\enb" because LF is no
+ longer a newline. Note that these special settings, which are not
+ Perl-compatible, are recognized only at the very start of a pattern, and that
+ they must be in upper case. If more than one of them is present, the last one
+ is used.
+ .P
+ The newline convention affects the interpretation of the dot metacharacter when
+ PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
+ 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"
+ .\"
+ below. A change of \eR setting can be combined with a change of newline
+ convention.
+ .
+ .
  .SH "CHARACTERS AND METACHARACTERS"
  .rs
  .sp
-Line 141 
 The \eQ...\eE sequence is recognized bot
+Line 209 
 The \eQ...\eE sequence is recognized bot
  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:
  .sp
    \ea        alarm, that is, the BEL character (hex 07)
    \ecx       "control-x", where x is any character
    \ee        escape (hex 1B)
    \ef        formfeed (hex 0C)
-   \en        newline (hex 0A)
+   \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 164 
 Thus \ecz becomes hex 1A, but \ec{ becom
+Line 231 
 Thus \ecz becomes hex 1A, but \ec{ becom
  After \ex, from zero to two hexadecimal digits are read (letters can be in
  upper or lower case). Any number of hexadecimal digits may appear between \ex{
  and }, but the value of the character code must be less than 256 in non-UTF-8
- mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
+ mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
- is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
+ hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
- and }, or if there is no terminating }, this form of escape is not recognized.
+ point, which is 10FFFF.
- Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
+ .P
- with no following digits, giving a character whose value is zero.
+ If characters other than hexadecimal digits appear between \ex{ and }, or if
+ there is no terminating }, this form of escape is not recognized. Instead, the
+ initial \ex will be interpreted as a basic hexadecimal escape, with no
+ following digits, giving a character whose value is zero.
  .P
  Characters whose value is less than 256 can be defined by either of the two
  syntaxes for \ex. There is no difference in the way they are handled. For
-Line 227 
 zero, because no more than three octal d
+Line 297 
 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"
  .rs
  .sp
- The sequence \eg followed by a positive or negative number, optionally enclosed
+ The sequence \eg followed by an unsigned or a negative number, optionally
- in braces, is an absolute or relative back reference. A named back reference
+ enclosed in braces, is an absolute or relative back reference. A named back
- can be coded as \eg{name}. Back references are discussed
+ reference can be coded as \eg{name}. Back references are discussed
  .\" HTML <a href="#backreferences">
  .\" </a>
  later,
-Line 254 
 parenthesized subpatterns.
+Line 321 
 parenthesized subpatterns.
  .\"
  .
  .
+ .SS "Absolute and relative subroutine calls"
+ .rs
+ .sp
+ For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
+ a number enclosed either in angle brackets or single quotes, is an alternative
+ syntax for referencing a subpattern as a "subroutine". Details are discussed
+ .\" HTML <a href="#onigurumasubroutines">
+ .\" </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
+ .\"
+ call.
+ .
+ .
  .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 271 
 following are always recognized:
+Line 356 
 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
+ 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.
  .P
  These character type sequences can appear both inside and outside character
  classes. They each match one character of the appropriate type. If the current
-Line 289 
 In UTF-8 mode, characters with values gr
+Line 383 
 In UTF-8 mode, characters with values gr
  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
  character property support is available. These sequences retain their original
  meanings from before UTF-8 support was available, mainly for efficiency
- reasons.
+ reasons. Note that this also affects \eb, because it is defined in terms of \ew
+ and \eW.
  .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.
-Line 343 
 accented letters, and these are matched
+Line 438 
 accented letters, and these are matched
  is discouraged.
  .
  .
+ .\" HTML <a name="newlineseq"></a>
  .SS "Newline sequences"
  .rs
  .sp
- Outside a character class, the escape sequence \eR matches any Unicode newline
+ Outside a character class, by default, the escape sequence \eR matches any
- sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
+ Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
- the following:
+ equivalent to the following:
  .sp
    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
  .sp
-Line 368 
 are added: LS (line separator, U+2028) a
+Line 464 
 are added: LS (line separator, U+2028) a
  Unicode character property support is not needed for these characters to be
  recognized.
  .P
- Inside a character class, \eR matches the letter "R".
+ It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
+ complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
+ either at compile time or when the pattern is matched. (BSR is an abbrevation
+ for "backslash R".) This can be made the default when PCRE is built; if this is
+ the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
+ It is also possible to specify these settings by starting a pattern string with
+ one of the following sequences:
+ .sp
+   (*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:
+ .sp
+   (*ANY)(*BSR_ANYCRLF)
+ .sp
+ 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 403 
 Those that are not part of an identified
+Line 522 
 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 418 
 Cypriot,
+Line 541 
 Cypriot,
  Cyrillic,
  Deseret,
  Devanagari,
+ Egyptian_Hieroglyphs,
  Ethiopic,
  Georgian,
  Glagolitic,
-Line 430 
 Hangul,
+Line 554 
 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 447 
 Nko,
+Line 582 
 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 466 
 Thai,
+Line 610 
 Thai,
  Tibetan,
  Tifinagh,
  Ugaritic,
+ Vai,
  Yi.
  .P
  Each character has exactly one general category property, specified by a
-Line 531 
 The special property L& is also supporte
+Line 676 
 The special property L& is also supporte
  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
  a modifier or "other".
  .P
- The long synonyms for these properties that Perl supports (such as \ep{Letter})
+ The Cs (Surrogate) property applies only to characters in the range U+D800 to
+ U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
+ cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
+ (see the discussion of PCRE_NO_UTF8_CHECK in the
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ page). Perl does not support the Cs property.
+ .P
+ The long synonyms for property names that Perl supports (such as \ep{Letter})
  are not supported by PCRE, nor is it permitted to prefix any of these
  properties with "Is".
  .P
-Line 555 
 atomic group
+Line 709 
 atomic group
  (see below).
  .\"
  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 \eX matches any one character.
  .P
  Matching characters by Unicode property is not fast, because PCRE has to search
-Line 592 
 For example, when the pattern
+Line 746 
 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 616 
 The backslashed assertions are:
+Line 774 
 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.
+ first or last character matches \ew, respectively. Neither PCRE nor Perl has a
+ separte "start 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 706 
 end of the subject in both modes, and if
+Line 870 
 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 728 
 to match it.
+Line 893 
 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 754 
 the lookbehind.
+Line 923 
 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. 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
  A character class matches a single character in the subject. In UTF-8 mode, the
- character may occupy more than one byte. A matched character must be in the set
+ character may be more than one byte long. A matched character must be in the
- of characters defined by the class, unless the first character in the class
+ set of characters defined by the class, unless the first character in the class
  definition is a circumflex, in which case the subject character must not be in
  the set defined by the class. If a circumflex is actually required as a member
  of the class, ensure it is not the first character, or escape it with a
-Line 771 
 For example, the character class [aeiou]
+Line 941 
 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
-Line 785 
 caseful version would. In UTF-8 mode, PC
+Line 955 
 caseful version would. In UTF-8 mode, PC
  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 UTF8-mode for characters 128 and above,
- ensure that PCRE is compiled with Unicode property support as well as with
+ you must ensure that PCRE is compiled with Unicode property support as well as
- UTF-8 support.
+ with UTF-8 support.
  .P
  Characters that might indicate line breaks are never treated in any special way
  when matching character classes, whatever line-ending sequence is in use, and
-Line 906 
 alternative in the subpattern.
+Line 1076 
 alternative in the subpattern.
  .rs
  .sp
  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
- PCRE_EXTENDED options can be changed from within the pattern by a sequence of
+ PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
- Perl option letters enclosed between "(?" and ")". The option letters are
+ the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
+ The option letters are
  .sp
    i  for PCRE_CASELESS
    m  for PCRE_MULTILINE
-Line 921 
 PCRE_MULTILINE while unsetting PCRE_DOTA
+Line 1092 
 PCRE_MULTILINE while unsetting PCRE_DOTA
  permitted. If a letter appears both before and after the hyphen, the option is
  unset.
  .P
- When an option change occurs at top level (that is, not inside subpattern
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- parentheses), the change applies to the remainder of the pattern that follows.
+ changed in the same way as the Perl-compatible options by using the characters
- If the change is placed right at the start of a pattern, PCRE extracts it into
+ J, U and X respectively.
- the global options (and it will therefore show up in data extracted by the
+ .P
- \fBpcre_fullinfo()\fP function).
+ When one of these option changes occurs at top level (that is, not inside
+ subpattern parentheses), the change applies to the remainder of the pattern
+ that follows. If the change is placed right at the start of a pattern, PCRE
+ extracts it into the global options (and it will therefore show up in data
+ extracted by the \fBpcre_fullinfo()\fP function).
  .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
-Line 944 
 branch is abandoned before the option se
+Line 1119 
 branch is abandoned before the option se
  option settings happen at compile time. There would be some very weird
  behaviour otherwise.
  .P
- The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
+ \fBNote:\fP There are other PCRE-specific options that can be set by the
- changed in the same way as the Perl-compatible options by using the characters
+ application when the compile or match functions are called. In some cases the
- J, U and X respectively.
+ pattern can contain special leading sequences such as (*CRLF) to override what
+ the application has set or what has been defaulted. Details are given in the
+ section entitled
+ .\" HTML <a href="#newlineseq">
+ .\" </a>
+ "Newline sequences"
+ .\"
+ above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
+ mode; this is equivalent to setting the PCRE_UTF8 option.
  .
  .
  .\" HTML <a name="subpattern"></a>
-Line 1001 
 is reached, an option setting in one bra
+Line 1184 
 is reached, an option setting in one bra
  the above patterns match "SUNDAY" as well as "Saturday".
  .
  .
+ .\" HTML <a name="dupsubpatternnumber"></a>
  .SH "DUPLICATE SUBPATTERN NUMBERS"
  .rs
  .sp
-Line 1026 
 stored.
+Line 1210 
 stored.
    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
    # 1            2         2  3        2     3     4
  .sp
- 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":
+ .sp
+   /(?|(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 this "branch reset" feature is to use
  duplicate named subpatterns, as described in the next section.
-Line 1042 
 if an expression is modified, the number
+Line 1243 
 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 1069 
 extracting the name-to-number translatio
+Line 1271 
 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 1087 
 subpattern, as described in the previous
+Line 1291 
 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 because PCRE uses only the numbers when
+ 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 1112 
 items:
+Line 1335 
 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 1143 
 support is available, \eX{3} matches thr
+Line 1367 
 support is available, \eX{3} matches thr
  which may be several bytes long (and they may be of different lengths).
  .P
  The quantifier {0} is permitted, causing the expression to behave as if the
- previous item and the quantifier were not present.
+ previous item and the quantifier were not present. This may be useful for
+ subpatterns that are referenced as
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ subroutines
+ .\"
+ from elsewhere in the pattern. Items other than subpatterns that have a {0}
+ quantifier are omitted from the compiled pattern.
  .P
  For convenience, the three most common quantifiers have single-character
  abbreviations:
-Line 1215 
 worth setting PCRE_DOTALL in order to ob
+Line 1446 
 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 1290 
 previous example can be rewritten as
+Line 1521 
 previous example can be rewritten as
  .sp
    \ed++foo
  .sp
+ Note that a possessive quantifier can be used with an entire group, for
+ example:
+ .sp
+   (abc|xyz){2,3}+
+ .sp
  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
  option is ignored. They are a convenient notation for the simpler forms of
  atomic group. However, there is no difference in the meaning of a possessive
-Line 1364 
 subpattern is possible using named paren
+Line 1600 
 subpattern is possible using named paren
  .P
  Another way of avoiding the ambiguity inherent in the use of digits following a
  backslash is to use the \eg escape sequence, which is a feature introduced in
- Perl 5.10. This escape must be followed by a positive or a negative number,
+ Perl 5.10. This escape must be followed by an unsigned number or a negative
- optionally enclosed in braces. These examples are all identical:
+ number, optionally enclosed in braces. These examples are all identical:
  .sp
    (ring), \e1
    (ring), \eg1
    (ring), \eg{1}
  .sp
- A positive number specifies an absolute reference without the ambiguity that is
+ An unsigned number specifies an absolute reference without the ambiguity that
- present in the older syntax. It is also useful when literal digits follow the
+ is present in the older syntax. It is also useful when literal digits follow
- reference. A negative number is a relative reference. Consider this example:
+ the reference. A negative number is a relative reference. Consider this
+ example:
  .sp
    (abc(def)ghi)\eg{-1}
  .sp
-Line 1420 
 after the reference.
+Line 1657 
 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
  .sp
    (a|(bc))\e2
  .sp
- 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.
+ Because there may be many capturing parentheses in a pattern, all digits
- Otherwise an empty comment (see
+ 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 \eg{ syntax or an empty comment (see
  .\" HTML <a href="#comments">
  .\" </a>
  "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 1449 
 to the previous iteration. In order for
+Line 1692 
 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 1502 
 lookbehind assertion is needed to achiev
+Line 1754 
 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 Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
+ synonym for (?!).
  .
  .
  .\" HTML <a name="lookbehind"></a>
-Line 1526 
 is permitted, but
+Line 1780 
 is permitted, but
  .sp
  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 (5.8 and 5.10), which requires all branches to
  match the same length of string. An assertion such as
  .sp
    (?<=ab(c|de))
  .sp
  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:
  .sp
    (?<=abc|abde)
  .sp
-Line 1541 
 In some cases, the Perl 5.10 escape sequ
+Line 1796 
 In some cases, the Perl 5.10 escape sequ
  .\" </a>
  (see above)
  .\"
- 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
  The implementation of lookbehind assertions is, for each alternative, to
  temporarily move the current position back by the fixed length and then try to
-Line 1554 
 to appear in lookbehind assertions, beca
+Line 1809 
 to appear in lookbehind assertions, beca
  the length of the lookbehind. The \eX and \eR escapes, which can match
  different numbers of bytes, are also not permitted.
  .P
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ "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,
+ .\"
+ however, is not supported.
+ .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
  .sp
    abcd$
  .sp
-Line 1620 
 characters that are not "999".
+Line 1887 
 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 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:
  .sp
    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)
-Line 1637 
 recursion, a pseudo-condition called DEF
+Line 1904 
 recursion, a pseudo-condition called DEF
  .rs
  .sp
  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
+ .\"
- recent by (?(-2), and so on. In looping constructs it can also make sense to
+ .\" HTML <a href="#recursion">
- refer to subsequent groups with constructs such as (?(+2).
+ .\" </a>
+ section about duplicate subpattern numbers),
+ .\"
+ the condition is true if any of them have been set. An alternative notation is
+ to precede the digits with a plus or minus sign. In this case, the subpattern
+ number is relative rather than absolute. The most recently opened parentheses
+ can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
+ looping constructs it can also make sense to refer to subsequent groups with
+ constructs such as (?(+2).
  .P
  Consider the following pattern, which contains non-significant white space to
  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
-Line 1683 
 Rewriting the above example to use a nam
+Line 1958 
 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 1694 
 letter R, for example:
+Line 1972 
 letter R, for example:
  .sp
    (?(R3)...) or (?(R&name)...)
  .sp
- 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
- 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.
+ .\" HTML <a href="#recursion">
+ .\" </a>
+ The syntax for recursive patterns
+ .\"
+ is described below.
  .
  .SS "Defining subpatterns for use by reference only"
  .rs
-Line 1708 
 If the condition is the string (DEFINE),
+Line 1992 
 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
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ "subroutines"
+ .\"
  is described below.) For example, a pattern to match an IPv4 address could be
  written like this (ignore whitespace and line breaks):
  .sp
-Line 1718 
 written like this (ignore whitespace and
+Line 2006 
 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
- The rest of the pattern uses references to the named group to match the four
+ components of an IPv4 address, insisting on a word boundary at each end.
- dot-separated components of an IPv4 address, insisting on a word boundary at
- each end.
  .
  .SS "Assertion conditions"
  .rs
-Line 1779 
 recursively to the pattern in which it a
+Line 2065 
 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
  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 "subroutine"
+ provided that it occurs inside that subpattern. (If not, it is a
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ "subroutine"
+ .\"
  call, which is described in the next section.) The special item (?R) or (?0) is
  a recursive call of the entire regular expression.
  .P
- 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
  This PCRE pattern solves the nested parentheses problem (assume the
  PCRE_EXTENDED option is set so that white space is ignored):
  .sp
-   \e( ( (?>[^()]+) | (?R) )* \e)
+   \e( ( [^()]++ | (?R) )* \e)
  .sp
  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
  If this were part of a larger pattern, you would not want to recurse the entire
  pattern, so instead you could use this:
  .sp
-   ( \e( ( (?>[^()]+) | (?1) )* \e) )
+   ( \e( ( [^()]++ | (?1) )* \e) )
  .sp
  We have put the pattern into parentheses, and caused the recursion to refer to
  them instead of the whole pattern.
  .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 (a Perl 5.10 feature).
  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
  most recently opened parentheses preceding the recursion. In other words, a
  negative number counts capturing parentheses leftwards from the point at which
-Line 1820 
 it is encountered.
+Line 2106 
 it is encountered.
  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.
+ .\" HTML <a href="#subpatternsassubroutines">
+ .\" </a>
+ "subroutine"
+ .\"
+ calls, as described in the next section.
  .P
  An alternative approach is to use named parentheses instead. The Perl syntax
  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
  could rewrite the above example as follows:
  .sp
-   (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
+   (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
  .sp
  If there is more than one subpattern with the same name, the earliest one is
  used.
  .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
  .sp
    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
  .sp
- 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
- 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 1854 
 documentation). If the pattern above is
+Line 2143 
 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 1879 
 different alternatives for the recursive
+Line 2165 
 different alternatives for the recursive
  is the actual recursive call.
  .
  .
+ .\" HTML <a name="recursiondifference"></a>
+ .SS "Recursion difference from Perl"
+ .rs
+ .sp
+ 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
+ alternatives in the other order, things are different:
+ .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*+\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.
+ .P
+ \fBWARNING\fP: 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.
+ .
+ .
  .\" HTML <a name="subpatternsassubroutines"></a>
  .SH "SUBPATTERNS AS SUBROUTINES"
  .rs
-Line 1905 
 matches "sense and sensibility" and "res
+Line 2262 
 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 1920 
 It matches "abcabc". It does not match "
+Line 2278 
 It matches "abcabc". It does not match "
  processing option does not affect the called subpattern.
  .
  .
+ .\" HTML <a name="onigurumasubroutines"></a>
+ .SH "ONIGURUMA SUBROUTINE SYNTAX"
+ .rs
+ .sp
+ For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
+ a number enclosed either in angle brackets or single quotes, is an alternative
+ syntax for referencing a subpattern as a subroutine, possibly recursively. Here
+ are two of the examples used above, rewritten using this syntax:
+ .sp
+   (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
+   (sens|respons)e and \eg'1'ibility
+ .sp
+ PCRE supports an extension to Oniguruma: if a number is preceded by a
+ plus or a minus sign it is taken as a relative reference. For example:
+ .sp
+   (abc)(?i:\eg<-1>)
+ .sp
+ Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
+ synonymous. The former is a back reference; the latter is a subroutine call.
+ .
+ .
  .SH CALLOUTS
  .rs
  .sp
-Line 1956 
 description of the interface to the call
+Line 2335 
 description of the interface to the call
  documentation.
  .
  .
+ .\" HTML <a name="backtrackcontrol"></a>
+ .SH "BACKTRACKING CONTROL"
+ .rs
+ .sp
+ Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
+ are described in the Perl documentation as "experimental and subject to change
+ or removal in a future version of Perl". It goes on to say: "Their usage in
+ production code should be noted to avoid problems during upgrades." The same
+ remarks apply to the PCRE features described in this section.
+ .P
+ Since these verbs are specifically related to backtracking, most of them can be
+ used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
+ a backtracking algorithm. With the exception of (*FAIL), which behaves like a
+ 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 or subroutine subpattern
+ (including recursive subpatterns), their effect is confined to that subpattern;
+ it does not extend to the surrounding pattern. Note that such subpatterns are
+ 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. They are generally of the form
+ (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
+ depending on whether or not an argument is present. An name is a sequence of
+ 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. They may not be
+ followed by a name.
+ .sp
+    (*ACCEPT)
+ .sp
+ This verb causes the match to end successfully, skipping the remainder of the
+ pattern. When inside a recursion, only the innermost pattern is ended
+ immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
+ captured. (This feature was added to PCRE at release 8.00.) For example:
+ .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)
+ .sp
+ This verb causes the match to fail, forcing backtracking to occur. It is
+ equivalent to (?!) but easier to read. The Perl documentation notes that it is
+ probably useful only when combined with (?{}) or (??{}). Those are, of course,
+ Perl features that are not present in PCRE. The nearest equivalent is the
+ callout feature, as for example in this pattern:
+ .sp
+   a+(?C)(*FAIL)
+ .sp
+ 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, causing a backtrack to
+ 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, which may not be followed by a name, causes the whole match to fail
+ outright if the rest of the pattern does not match. Even if the pattern is
+ unanchored, no further attempts to find a match by advancing the starting point
+ take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
+ 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." The name of the most
+ recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
+ match failure.
+ .P
+ Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
+ unless PCRE's start-of-match optimizations are turned off, as shown in this
+ \fBpcretest\fP example:
+ .sp
+   /(*COMMIT)abc/
+   xyzabc
+: abc
+   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, when given without a name, is like (*PRUNE), except that if the
+ pattern is unanchored, the "bumpalong" advance is not to the next character,
+ but to the position in the subject where (*SKIP) was encountered. (*SKIP)
+ signifies that whatever text was matched leading up to it cannot be part of a
+ successful match. Consider:
+ .sp
+   a+(*SKIP)b
+ .sp
+ 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 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".
+ .sp
+   (*SKIP:NAME)
+ .sp
+ When (*SKIP) has an associated name, its behaviour is modified. If the
+ following pattern fails to match, the previous path through the pattern is
+ searched for the most recent (*MARK) that has the same name. If one is found,
+ 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 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. The
+ 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).
+ .
+ .
  .SH "SEE ALSO"
  .rs
  .sp
- \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
+ \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
+ \fBpcresyntax\fP(3), \fBpcre\fP(3).
  .
  .
  .SH AUTHOR
-Line 1976 
 Cambridge CB2 3QH, England.
+Line 2597 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 19 June 2007
+ Last updated: 03 May 2010
- Copyright (c) 1997-2007 University of Cambridge.
+ Copyright (c) 1997-2010 University of Cambridge.
  .fi

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