Documentation update.
This commit is contained in:
Philip.Hazel
parent
c37995d2bd
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@ -37,9 +37,9 @@ for example, \b* (but not \b{3}), but these do not seem to have any use.
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</P>
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<P>
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3. Capturing subpatterns that occur inside negative lookaround assertions are
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counted, but their entries in the offsets vector are set only if the assertion
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is a condition. Perl has changed its behaviour in this regard from time to
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time.
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counted, but their entries in the offsets vector are set only when a negative
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assertion is a condition that has a matching branch (that is, the condition is
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false).
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</P>
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<P>
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4. The following Perl escape sequences are not supported: \l, \u, \L,
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@ -214,7 +214,7 @@ Cambridge, England.
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REVISION
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</b><br>
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<P>
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Last updated: 29 March 2017
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Last updated: 03 April 2017
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<br>
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Copyright © 1997-2017 University of Cambridge.
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<br>
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@ -2216,15 +2216,27 @@ coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
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<P>
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More complicated assertions are coded as subpatterns. There are two kinds:
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those that look ahead of the current position in the subject string, and those
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that look behind it. An assertion subpattern is matched in the normal way,
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except that it does not cause the current matching position to be changed.
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that look behind it, and in each case an assertion may be positive (must
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succeed for matching to continue) or negative (must not succeed for matching to
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continue). An assertion subpattern is matched in the normal way, except that,
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when matching continues afterwards, the matching position in the subject string
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is as it was at the start of the assertion.
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</P>
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<P>
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Assertion subpatterns are not capturing subpatterns. If such an assertion
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contains capturing subpatterns within it, these are counted for the purposes of
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Assertion subpatterns are not capturing subpatterns. If an assertion contains
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capturing subpatterns within it, these are counted for the purposes of
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numbering the capturing subpatterns in the whole pattern. However, substring
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capturing is normally carried out only for positive assertions (but see the
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discussion of conditional subpatterns below).
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capturing is carried out only for positive assertions that succeed, that is,
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one of their branches matches, so matching continues after the assertion. If
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all branches of a positive assertion fail to match, nothing is captured, and
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control is passed to the previous backtracking point.
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</P>
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<P>
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No capturing is done for a negative assertion unless it is being used as a
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condition in a
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<a href="#subpatternsassubroutines">conditional subpattern</a>
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(see the discussion below). Matching continues after a non-conditional negative
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assertion only if all its branches fail to match.
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</P>
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<P>
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For compatibility with Perl, most assertion subpatterns may be repeated; though
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@ -2604,10 +2616,11 @@ against the second. This pattern matches strings in one of the two forms
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dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
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</P>
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<P>
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For Perl compatibility, if an assertion that is a condition contains capturing
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subpatterns, any capturing that occurs is retained afterwards, for both
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positive and negative assertions. (Compare non-conditional assertions, when
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captures are retained only for positive assertions.)
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When an assertion that is a condition contains capturing subpatterns, any
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capturing that occurs in a matching branch is retained afterwards, for both
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positive and negative assertions, because matching always continues after the
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assertion, whether it succeeds or fails. (Compare non-conditional assertions,
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when captures are retained only for positive assertions that succeed.)
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<a name="comments"></a></P>
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<br><a name="SEC22" href="#TOC1">COMMENTS</a><br>
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<P>
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@ -3351,28 +3364,34 @@ in the second repeat of the group acts.
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Backtracking verbs in assertions
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</b><br>
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<P>
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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 whether or
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not the assertion is standalone or acting as the condition in a conditional
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subpattern.
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</P>
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<P>
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(*ACCEPT) in a positive assertion causes the assertion to succeed without any
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further processing. In a negative assertion, (*ACCEPT) causes the assertion to
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fail without any further processing.
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(*ACCEPT) in a standalone positive assertion causes the assertion to succeed
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without any further processing; captured strings are retained. In a standalone
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negative assertion, (*ACCEPT) causes the assertion to fail without any further
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processing; captured substrings are discarded.
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</P>
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<P>
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If the assertion is a condition, (*ACCEPT) causes the condition to be true for
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a positive assertion and false for a negative one; captured substrings are
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retained in both cases.
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</P>
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<P>
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The effect of (*THEN) is not allowed to escape beyond an assertion. If there
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are no more branches to try, (*THEN) causes a positive assertion to be false,
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and a negative assertion to be true.
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</P>
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<P>
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The other backtracking verbs are not treated specially if they appear in a
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positive assertion. In particular, (*THEN) skips to the next alternative in the
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innermost enclosing group that has alternations, whether or not this is within
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the assertion.
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</P>
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<P>
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Negative assertions are, however, different, in order to ensure that changing a
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positive assertion into a negative assertion changes its result. Backtracking
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into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
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without considering any further alternative branches in the assertion.
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Backtracking into (*THEN) causes it to skip to the next enclosing alternative
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within the assertion (the normal behaviour), but if the assertion does not have
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such an alternative, (*THEN) behaves like (*PRUNE).
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standalone positive assertion. In a conditional positive assertion,
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backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the condition to be
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false. However, for both standalone and conditional negative assertions,
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backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the assertion to be
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true, without considering any further alternative branches.
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<a name="btsub"></a></P>
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<br><b>
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Backtracking verbs in subroutines
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@ -3415,7 +3434,7 @@ Cambridge, England.
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</P>
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<br><a name="SEC30" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 18 March 2017
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Last updated: 03 April 2017
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<br>
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Copyright © 1997-2017 University of Cambridge.
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<br>
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@ -1,4 +1,4 @@
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.TH PCRE2COMPAT 3 "29 March 2017" "PCRE2 10.30"
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.TH PCRE2COMPAT 3 "03 April 2017" "PCRE2 10.30"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
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@ -24,9 +24,9 @@ assertion just once). Perl allows some repeat quantifiers on other assertions,
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for example, \eb* (but not \eb{3}), but these do not seem to have any use.
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.P
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3. Capturing subpatterns that occur inside negative lookaround assertions are
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counted, but their entries in the offsets vector are set only if the assertion
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is a condition. Perl has changed its behaviour in this regard from time to
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time.
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counted, but their entries in the offsets vector are set only when a negative
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assertion is a condition that has a matching branch (that is, the condition is
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false).
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.P
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4. The following Perl escape sequences are not supported: \el, \eu, \eL,
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\eU, and \eN when followed by a character name or Unicode value. (\eN on its
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@ -179,6 +179,6 @@ Cambridge, England.
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.rs
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.sp
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.nf
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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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.fi
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@ -1,4 +1,4 @@
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.TH PCRE2PATTERN 3 "18 March 2017" "PCRE2 10.30"
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.TH PCRE2PATTERN 3 "03 April 2017" "PCRE2 10.30"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH "PCRE2 REGULAR EXPRESSION DETAILS"
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@ -2225,14 +2225,28 @@ above.
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.P
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More complicated assertions are coded as subpatterns. There are two kinds:
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those that look ahead of the current position in the subject string, and those
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that look behind it. An assertion subpattern is matched in the normal way,
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except that it does not cause the current matching position to be changed.
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that look behind it, and in each case an assertion may be positive (must
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succeed for matching to continue) or negative (must not succeed for matching to
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continue). An assertion subpattern is matched in the normal way, except that,
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when matching continues afterwards, the matching position in the subject string
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is as it was at the start of the assertion.
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.P
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Assertion subpatterns are not capturing subpatterns. If such an assertion
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contains capturing subpatterns within it, these are counted for the purposes of
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Assertion subpatterns are not capturing subpatterns. If an assertion contains
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capturing subpatterns within it, these are counted for the purposes of
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numbering the capturing subpatterns in the whole pattern. However, substring
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capturing is normally carried out only for positive assertions (but see the
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discussion of conditional subpatterns below).
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capturing is carried out only for positive assertions that succeed, that is,
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one of their branches matches, so matching continues after the assertion. If
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all branches of a positive assertion fail to match, nothing is captured, and
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control is passed to the previous backtracking point.
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.P
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No capturing is done for a negative assertion unless it is being used as a
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condition in a
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.\" HTML <a href="#subpatternsassubroutines">
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.\" </a>
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conditional subpattern
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.\"
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(see the discussion below). Matching continues after a non-conditional negative
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assertion only if all its branches fail to match.
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.P
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For compatibility with Perl, most assertion subpatterns may be repeated; though
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it makes no sense to assert the same thing several times, the side effect of
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@ -2620,10 +2634,11 @@ subject is matched against the first alternative; otherwise it is matched
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against the second. This pattern matches strings in one of the two forms
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dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
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.P
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For Perl compatibility, if an assertion that is a condition contains capturing
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subpatterns, any capturing that occurs is retained afterwards, for both
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positive and negative assertions. (Compare non-conditional assertions, when
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captures are retained only for positive assertions.)
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When an assertion that is a condition contains capturing subpatterns, any
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capturing that occurs in a matching branch is retained afterwards, for both
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positive and negative assertions, because matching always continues after the
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assertion, whether it succeeds or fails. (Compare non-conditional assertions,
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when captures are retained only for positive assertions that succeed.)
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.
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.
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.\" HTML <a name="comments"></a>
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@ -3381,25 +3396,30 @@ in the second repeat of the group acts.
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.SS "Backtracking verbs in assertions"
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.rs
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.sp
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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 whether or
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not the assertion is standalone or acting as the condition in a conditional
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subpattern.
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.P
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(*ACCEPT) in a positive assertion causes the assertion to succeed without any
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further processing. In a negative assertion, (*ACCEPT) causes the assertion to
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fail without any further processing.
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(*ACCEPT) in a standalone positive assertion causes the assertion to succeed
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without any further processing; captured strings are retained. In a standalone
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negative assertion, (*ACCEPT) causes the assertion to fail without any further
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processing; captured substrings are discarded.
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.P
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If the assertion is a condition, (*ACCEPT) causes the condition to be true for
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a positive assertion and false for a negative one; captured substrings are
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retained in both cases.
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.P
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The effect of (*THEN) is not allowed to escape beyond an assertion. If there
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are no more branches to try, (*THEN) causes a positive assertion to be false,
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and a negative assertion to be true.
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.P
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The other backtracking verbs are not treated specially if they appear in a
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positive assertion. In particular, (*THEN) skips to the next alternative in the
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innermost enclosing group that has alternations, whether or not this is within
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the assertion.
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.P
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Negative assertions are, however, different, in order to ensure that changing a
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positive assertion into a negative assertion changes its result. Backtracking
|
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into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
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without considering any further alternative branches in the assertion.
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Backtracking into (*THEN) causes it to skip to the next enclosing alternative
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within the assertion (the normal behaviour), but if the assertion does not have
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such an alternative, (*THEN) behaves like (*PRUNE).
|
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standalone positive assertion. In a conditional positive assertion,
|
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backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the condition to be
|
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false. However, for both standalone and conditional negative assertions,
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backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the assertion to be
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true, without considering any further alternative branches.
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.
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.
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.\" HTML <a name="btsub"></a>
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@ -3445,6 +3465,6 @@ Cambridge, England.
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.rs
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.sp
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.nf
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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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.fi
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