Revision: 1293
http://vcs.pcre.org/viewvc?view=rev&revision=1293
Author: ph10
Date: 2013-03-17 17:13:14 +0000 (Sun, 17 Mar 2013)
Log Message:
-----------
Document new multiple backtracking verb behaviour.
Modified Paths:
--------------
code/trunk/doc/pcrecompat.3
code/trunk/doc/pcrepattern.3
Modified: code/trunk/doc/pcrecompat.3
===================================================================
--- code/trunk/doc/pcrecompat.3 2013-03-17 16:51:40 UTC (rev 1292)
+++ code/trunk/doc/pcrecompat.3 2013-03-17 17:13:14 UTC (rev 1293)
@@ -100,11 +100,17 @@
match succeeds (compare capturing parentheses in assertions). Note that such
subpatterns are processed as anchored at the point where they are tested.
.P
-11. There are some differences that are concerned with the settings of captured
+11. If a pattern contains more than one backtracking control verb, the first
+one that is backtracked onto acts. For example, in the pattern
+A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
+triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
+same as PCRE, but there are examples where it differs.
+.P
+12. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
.P
-12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
+13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
names is not as general as Perl's. This is a consequence of the fact the PCRE
works internally just with numbers, using an external table to translate
between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
@@ -114,18 +120,18 @@
names map to capturing subpattern number 1. To avoid this confusing situation,
an error is given at compile time.
.P
-13. Perl recognizes comments in some places that PCRE does not, for example,
+14. Perl recognizes comments in some places that PCRE does not, for example,
between the ( and ? at the start of a subpattern. If the /x modifier is set,
Perl allows white space between ( and ? but PCRE never does, even if the
PCRE_EXTENDED option is set.
.P
-14. In PCRE, the upper/lower case character properties Lu and Ll are not
+15. In PCRE, the upper/lower case character properties Lu and Ll are not
affected when case-independent matching is specified. For example, \ep{Lu}
always matches an upper case letter. I think Perl has changed in this respect;
in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all
letters, regardless of case, when case independence is specified.
.P
-15. PCRE provides some extensions to the Perl regular expression facilities.
+16. PCRE provides some extensions to the Perl regular expression facilities.
Perl 5.10 includes new features that are not in earlier versions of Perl, some
of which (such as named parentheses) have been in PCRE for some time. This list
is with respect to Perl 5.10:
Modified: code/trunk/doc/pcrepattern.3
===================================================================
--- code/trunk/doc/pcrepattern.3 2013-03-17 16:51:40 UTC (rev 1292)
+++ code/trunk/doc/pcrepattern.3 2013-03-17 17:13:14 UTC (rev 1293)
@@ -2973,16 +2973,16 @@
than one character. (*COMMIT) is the strongest, causing the entire match to
fail.
.P
-If more than one such verb is present in a pattern, the "strongest" one wins.
-For example, consider this pattern, where A, B, etc. are complex pattern
-fragments:
+If more than one such verb is present in a pattern, the one that is backtracked
+onto first acts. For example, consider this pattern, where A, B, etc. are
+complex pattern fragments:
.sp
- (A(*COMMIT)B(*THEN)C|D)
+ (A(*COMMIT)B(*THEN)C|ABD)
.sp
-Once A has matched, PCRE is committed to this match, at the current starting
-position. If subsequently B matches, but C does not, the normal (*THEN) action
-of trying the next alternative (that is, D) does not happen because (*COMMIT)
-overrides.
+If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
+fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
+the next alternative (ABD) to be tried. This behaviour is consistent, but is
+not always the same as Perl's.
.
.
.SH "SEE ALSO"
