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-revision 459 by ph10,
Sun Oct  4 09:21:39 2009 UTC
+revision 513 by ph10,
Mon May  3 11:13:37 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 83 
 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 217 
 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
  .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, \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
+ "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set.
+ Outside a character class, these sequences have different meanings
  .\" HTML <a href="#uniextseq">
  .\" </a>
  (see below).
-Line 333 
 syntax for referencing a subpattern as a
+Line 334 
 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 468 
 one of the following sequences:
+Line 469 
 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
-Line 478 
 convention, for example, a pattern can s
+Line 479 
 convention, for example, a pattern can s
  .sp
    (*ANY)(*BSR_ANYCRLF)
  .sp
- Inside a character class, \eR matches the letter "R".
+ 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 513 
 Those that are not part of an identified
+Line 516 
 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 535 
 Cypriot,
  Cyrillic,
  Deseret,
  Devanagari,
+ Egyptian_Hieroglyphs,
  Ethiopic,
  Georgian,
  Glagolitic,
-Line 540 
 Hangul,
+Line 548 
 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 576 
 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 604 
 Thai,
  Tibetan,
  Tifinagh,
  Ugaritic,
+ Vai,
  Yi.
  .P
  Each character has exactly one general category property, specified by a
-Line 711 
 For example, when the pattern
+Line 740 
 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 768 
 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
  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
-Line 876 
 the lookbehind.
+Line 912 
 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 1163 
 stored.
+Line 1199 
 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)/
-Line 1204 
 In PCRE, a subpattern can be named in on
+Line 1240 
 In PCRE, a subpattern can be named in on
  parentheses from other parts of the pattern, such as
  .\" HTML <a href="#backreferences">
  .\" </a>
- backreferences,
+ back references,
  .\"
  .\" HTML <a href="#recursion">
  .\" </a>
-Line 1225 
 is also a convenience function for extra
+Line 1261 
 is also a convenience function for extra
  .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. (Duplicate
  names are also always permitted for subpatterns with the same number, set up as
  described in the previous section.) Duplicate names can be useful for patterns
  where only one instance of the named parentheses can match. Suppose you want to
  match the name of a weekday, either as a 3-letter abbreviation or as the full
-Line 1244 
 subpattern, as described in the previous
+Line 1280 
 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.
  .P
- If you make a backreference to a non-unique named subpattern from elsewhere in
+ If you make a back reference to a non-unique named subpattern from elsewhere in
  the pattern, the one that corresponds to the first occurrence of the name is
  used. In the absence of duplicate numbers (see the previous section) this is
  the one with the lowest number. If you use a named reference in a condition
-Line 1256 
 test (see the
+Line 1292 
 test (see the
  .\" </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
-Line 1288 
 items:
+Line 1324 
 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 1399 
 worth setting PCRE_DOTALL in order to ob
+Line 1435 
 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 1614 
 references to it always fail by default.
+Line 1650 
 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 1628 
 whitespace. Otherwise, the \eg{ syntax o
+Line 1664 
 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 1642 
 to the previous iteration. In order for
+Line 1681 
 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 1737 
 In some cases, the Perl 5.10 escape sequ
+Line 1785 
 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 1755 
 different numbers of bytes, are also not
+Line 1803 
 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 1828 
 characters that are not "999".
+Line 1876 
 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 1846 
 recursion, a pseudo-condition called DEF
+Line 1894 
 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 1899 
 Rewriting the above example to use a nam
+Line 1947 
 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"
-Line 1915 
 letter R, for example:
+Line 1963 
 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. 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>
  The syntax for recursive patterns
-Line 1933 
 If the condition is the string (DEFINE),
+Line 1981 
 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 2010 
 this kind of recursion was subsequently
+Line 2058 
 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 2026 
 PCRE_EXTENDED option is set so that whit
+Line 2074 
 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 2074 
 the match runs for a very long time inde
+Line 2122 
 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 2085 
 documentation). If the pattern above is
+Line 2132 
 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 2117 
 is the actual recursive call.
+Line 2161 
 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 2143 
 alternatives in the other order, things
+Line 2187 
 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 2207 
 matches "sense and sensibility" and "res
+Line 2251 
 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 2279 
 description of the interface to the call
+Line 2324 
 description of the interface to the call
  documentation.
  .
  .
+ .\" HTML <a name="backtrackcontrol"></a>
  .SH "BACKTRACKING CONTROL"
  .rs
  .sp
-Line 2294 
 a backtracking algorithm. With the excep
+Line 2340 
 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 2319 
 captured. (This feature was added to PCR
+Line 2377 
 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 2335 
 callout feature, as for example in this
+Line 2393 
 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 2382 
 effect as this example; although it woul
+Line 2538 
 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
+ 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
-Line 2393 
 for a pattern-based if-then-else block:
+Line 2559 
 for a pattern-based if-then-else block:
  .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).
  .
  .
  .SH "SEE ALSO"
  .rs
  .sp
  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
  \fBpcresyntax\fP(3), \fBpcre\fP(3).
  .
  .
-Line 2418 
 Cambridge CB2 3QH, England.
+Line 2586 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 04 October 2009
+ Last updated: 03 May 2010
- Copyright (c) 1997-2009 University of Cambridge.
+ Copyright (c) 1997-2010 University of Cambridge.
  .fi

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