Documentation update
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@ -4112,8 +4112,8 @@ DIFFERENCES BETWEEN PCRE2 AND PERL
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3. Capturing subpatterns that occur inside negative lookaround asser-
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tions are counted, but their entries in the offsets vector are set only
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if the assertion is a condition. Perl has changed its behaviour in this
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regard from time to time.
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when a negative assertion is a condition that has a matching branch
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(that is, the condition is false).
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4. The following Perl escape sequences are not supported: \l, \u, \L,
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\U, and \N when followed by a character name or Unicode value. (\N on
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@ -4266,7 +4266,7 @@ AUTHOR
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REVISION
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Last updated: 29 March 2017
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Last updated: 03 April 2017
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Copyright (c) 1997-2017 University of Cambridge.
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------------------------------------------------------------------------------
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@ -7319,16 +7319,26 @@ ASSERTIONS
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More complicated assertions are coded as subpatterns. There are two
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kinds: those that look ahead of the current position in the subject
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string, and those that look behind it. An assertion subpattern is
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matched in the normal way, except that it does not cause the current
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matching position to be changed.
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string, and those that look behind it, and in each case an assertion
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may be positive (must succeed for matching to continue) or negative
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(must not succeed for matching to continue). An assertion subpattern is
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matched in the normal way, except that, when matching continues after-
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wards, the matching position in the subject string is as it was at the
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start of the assertion.
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Assertion subpatterns are not capturing subpatterns. If such an asser-
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tion contains capturing subpatterns within it, these are counted for
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the purposes of numbering the capturing subpatterns in the whole pat-
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tern. However, substring capturing is normally carried out only for
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positive assertions (but see the discussion of conditional subpatterns
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below).
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Assertion subpatterns are not capturing subpatterns. If an assertion
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contains capturing subpatterns within it, these are counted for the
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purposes of numbering the capturing subpatterns in the whole pattern.
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However, substring capturing is carried out only for positive asser-
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tions that succeed, that is, one of their branches matches, so matching
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continues after the assertion. If all branches of a positive assertion
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fail to match, nothing is captured, and control is passed to the previ-
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ous backtracking point.
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No capturing is done for a negative assertion unless it is being used
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as a condition in a conditional subpattern (see the discussion below).
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Matching continues after a non-conditional negative assertion only if
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all its branches fail to match.
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For compatibility with Perl, most assertion subpatterns may be
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repeated; though it makes no sense to assert the same thing several
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@ -7677,11 +7687,12 @@ CONDITIONAL SUBPATTERNS
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strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
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letters and dd are digits.
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For Perl compatibility, if an assertion that is a condition contains
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capturing subpatterns, any capturing that occurs is retained after-
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wards, for both positive and negative assertions. (Compare non-condi-
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tional assertions, when captures are retained only for positive asser-
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tions.)
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When an assertion that is a condition contains capturing subpatterns,
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any capturing that occurs in a matching branch is retained afterwards,
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for both positive and negative assertions, because matching always con-
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tinues after the assertion, whether it succeeds or fails. (Compare non-
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conditional assertions, when captures are retained only for positive
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assertions that succeed.)
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COMMENTS
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@ -8393,26 +8404,31 @@ BACKTRACKING CONTROL
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Backtracking verbs in assertions
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(*FAIL) in an assertion has its normal effect: it forces an immediate
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backtrack.
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(*FAIL) in any assertion has its normal effect: it forces an immediate
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backtrack. The behaviour of the other backtracking verbs depends on
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whether or not the assertion is standalone or acting as the condition
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in a conditional subpattern.
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(*ACCEPT) in a positive assertion causes the assertion to succeed with-
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out any further processing. In a negative assertion, (*ACCEPT) causes
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the assertion to fail without any further processing.
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(*ACCEPT) in a standalone positive assertion causes the assertion to
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succeed without any further processing; captured strings are retained.
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In a standalone negative assertion, (*ACCEPT) causes the assertion to
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fail without any further processing; captured substrings are discarded.
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If the assertion is a condition, (*ACCEPT) causes the condition to be
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true for a positive assertion and false for a negative one; captured
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substrings are retained in both cases.
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The effect of (*THEN) is not allowed to escape beyond an assertion. If
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there are no more branches to try, (*THEN) causes a positive assertion
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to be false, and a negative assertion to be true.
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The other backtracking verbs are not treated specially if they appear
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in a positive assertion. In particular, (*THEN) skips to the next
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alternative in the innermost enclosing group that has alternations,
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whether or not this is within the assertion.
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Negative assertions are, however, different, in order to ensure that
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changing a positive assertion into a negative assertion changes its
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result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg-
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ative assertion to be true, without considering any further alternative
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branches in the assertion. Backtracking into (*THEN) causes it to skip
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to the next enclosing alternative within the assertion (the normal be-
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haviour), but if the assertion does not have such an alternative,
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(*THEN) behaves like (*PRUNE).
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in a standalone positive assertion. In a conditional positive asser-
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tion, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the con-
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dition to be false. However, for both standalone and conditional nega-
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tive assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE)
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causes the assertion to be true, without considering any further alter-
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native branches.
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Backtracking verbs in subroutines
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@ -8449,7 +8465,7 @@ AUTHOR
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REVISION
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Last updated: 18 March 2017
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Last updated: 03 April 2017
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Copyright (c) 1997-2017 University of Cambridge.
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------------------------------------------------------------------------------
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@ -9225,9 +9241,6 @@ REVISION
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------------------------------------------------------------------------------
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------------------------------------------------------------------------------
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PCRE2SYNTAX(3) Library Functions Manual PCRE2SYNTAX(3)
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