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

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-revision 210 by ph10,
Mon Aug  6 15:23:29 2007 UTC
+revision 211 by ph10,
Thu Aug  9 09:52:43 2007 UTC
 Line 35 
 man page, in case the conversion went wr
  <li><a name="TOC20" href="#SEC20">RECURSIVE PATTERNS</a>
  <li><a name="TOC21" href="#SEC21">SUBPATTERNS AS SUBROUTINES</a>
  <li><a name="TOC22" href="#SEC22">CALLOUTS</a>
- <li><a name="TOC23" href="#SEC23">SEE ALSO</a>
+ <li><a name="TOC23" href="#SEC23">BACTRACKING CONTROL</a>
- <li><a name="TOC24" href="#SEC24">AUTHOR</a>
+ <li><a name="TOC24" href="#SEC24">SEE ALSO</a>
- <li><a name="TOC25" href="#SEC25">REVISION</a>
+ <li><a name="TOC25" href="#SEC25">AUTHOR</a>
+ <li><a name="TOC26" href="#SEC26">REVISION</a>
  </ul>
  <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
  <P>
-Line 195 
 Thus \cz becomes hex 1A, but \c{ becomes
+Line 196 
 Thus \cz becomes hex 1A, but \c{ becomes
  After \x, from zero to two hexadecimal digits are read (letters can be in
  upper or lower case). Any number of hexadecimal digits may appear between \x{
  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 \x{
+ 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 \x will be interpreted as a basic hexadecimal escape,
+ </P>
- with no following digits, giving a character whose value is zero.
+ <P>
+ If characters other than hexadecimal digits appear between \x{ and }, or if
+ there is no terminating }, this form of escape is not recognized. Instead, the
+ initial \x will be interpreted as a basic hexadecimal escape, with no
+ following digits, giving a character whose value is zero.
  </P>
  <P>
  Characters whose value is less than 256 can be defined by either of the two
-Line 546 
 the Lu, Ll, or Lt property, in other wor
+Line 551 
 the Lu, Ll, or Lt property, in other wor
  a modifier or "other".
  </P>
  <P>
+ The Cs (Surrogate) property applies only to characters in the range U+D800 to
+ U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
+ cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
+ (see the discussion of PCRE_NO_UTF8_CHECK in the
+ <a href="pcreapi.html"><b>pcreapi</b></a>
+ page).
+ </P>
+ <P>
  The long synonyms for these properties that Perl supports (such as \p{Letter})
  are not supported by PCRE, nor is it permitted to prefix any of these
  properties with "Is".
-Line 1983 
 description of the interface to the call
+Line 1996 
 description of the interface to the call
  <a href="pcrecallout.html"><b>pcrecallout</b></a>
  documentation.
  </P>
- <br><a name="SEC23" href="#TOC1">SEE ALSO</a><br>
+ <br><a name="SEC23" href="#TOC1">BACTRACKING CONTROL</a><br>
+ <P>
+ 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>
+ <P>
+ Since these verbs are specifically related to backtracking, they can be used
+ only when the pattern is to be matched using <b>pcre_exec()</b>, which uses a
+ backtracking algorithm. They cause an error if encountered by
+ <b>pcre_dfa_exec()</b>.
+ </P>
+ <P>
+ The new verbs make use of what was previously invalid syntax: an opening
+ parenthesis followed by an asterisk. In Perl, they are generally of the form
+ (*VERB:ARG) but PCRE does not support the use of arguments, so its general
+ form is just (*VERB). Any number of these verbs may occur in a pattern. There
+ are two kinds:
+ </P>
+ <br><b>
+ Verbs that act immediately
+ </b><br>
+ <P>
+ The following verbs act as soon as they are encountered:
+ <pre>
+    (*ACCEPT)
+ </pre>
+ This verb causes the match to end successfully, skipping the remainder of the
+ pattern. When inside a recursion, only the innermost pattern is ended
+ immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
+ capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
+ captured. For example:
+ <pre>
+   A(A|B(*ACCEPT)|C)D
+ </pre>
+ This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
+ captured.
+ <pre>
+   (*FAIL) or (*F)
+ </pre>
+ 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:
+ <pre>
+   a+(?C)(*FAIL)
+ </pre>
+ A match with the string "aaaa" always fails, but the callout is taken before
+ each backtrack happens (in this example, 10 times).
+ </P>
+ <br><b>
+ Verbs that act after backtracking
+ </b><br>
+ <P>
+ The following verbs do nothing when they are encountered. Matching continues
+ with what follows, but if there is no subsequent match, a failure is forced.
+ The verbs differ in exactly what kind of failure occurs.
+ <pre>
+   (*COMMIT)
+ </pre>
+ This verb 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 start point take place. Once (*COMMIT) has been
+ passed, <b>pcre_exec()</b> is committed to finding a match at the current
+ starting point, or not at all. For example:
+ <pre>
+   a+(*COMMIT)b
+ </pre>
+ This matches "xxaab" but not "aacaab". It can be thought of as a kind of
+ dynamic anchor, or "I've started, so I must finish."
+ <pre>
+   (*PRUNE)
+ </pre>
+ This verb causes the match to fail at the current position 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), 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.
+ <pre>
+   (*SKIP)
+ </pre>
+ This verb 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:
+ <pre>
+   a+(*SKIP)b
+ </pre>
+ If the subject is "aaaac...", after the first match attempt fails (starting at
+ the first character in the string), the starting point skips on to start the
+ next attempt at "c". Note that a possessive quantifer does not have the same
+ effect in this example; although it would suppress backtracking during the
+ first match attempt, the second attempt would start at the second character
+ instead of skipping on to "c".
+ <pre>
+   (*THEN)
+ </pre>
+ 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:
+ <pre>
+   ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
+ </pre>
+ If the COND1 pattern matches, FOO is tried (and possibly further items after
+ the end of the group if FOO succeeds); on failure the matcher skips to the
+ second alternative and tries COND2, without backtracking into COND1. If (*THEN)
+ is used outside of any alternation, it acts exactly like (*PRUNE).
+ </P>
+ <br><a name="SEC24" href="#TOC1">SEE ALSO</a><br>
  <P>
  <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
  </P>
- <br><a name="SEC24" href="#TOC1">AUTHOR</a><br>
+ <br><a name="SEC25" href="#TOC1">AUTHOR</a><br>
  <P>
  Philip Hazel
  <br>
-Line 1996 
 University Computing Service
+Line 2124 
 University Computing Service
  Cambridge CB2 3QH, England.
  <br>
  </P>
- <br><a name="SEC25" href="#TOC1">REVISION</a><br>
+ <br><a name="SEC26" href="#TOC1">REVISION</a><br>
  <P>
- Last updated: 06 August 2007
+ Last updated: 09 August 2007
  <br>
  Copyright &copy; 1997-2007 University of Cambridge.
  <br>

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