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Postings from September 2011
On Wed, 21 Sep 2011, Father Chrysostomos via RT wrote: > > A (B (*THEN) C) > > A (B (*THEN) C | D) > > > > In the first, Perl fails the match without any backtracking if C > > fails; > > in the second, it backtracks into A. Or that is what my experiments > > imply. > > Oddly, I don¢t find that counterintuitive at all. Do we need three > versions of prune/then? Aha! One person's intuition is always another's totally craziness. :-) I wonder what the percentages each way would be if we surveyed the general Perl-using population? At least one other person thinks as I do, because it was a bug report for PCRE - which was behaving more like Perl - that got me into this issue in the first place. Is there a forum where we could ask the following question? Folks, consider the pattern ^A(B(*THEN)C), where A, B, and C are complex patterns. If matching fails in C, do you expect that (a) the entire match should fail, or (b) the matching should backtrack into A? I will ask this question on the pcre-dev mailing list and see what answers (if any) I get. I might try Jeffrey Friedl as well. I will not be in the least offended if I am "outvoted". It seems that you intuitively think that a group without a | is a different kind of animal to a group that contains a |, whereas I don't. I just think that a group without a | has "one alternative", maybe better expressed as "one branch" (since "alternative" implies at least one of two). I can, however, understand your logic; I have to say that to me it seems rather mathematically pedantic (with respect :-). I'd rather not created yet another version of prune/then! I *thought* I understood these verbs. It seemed to me that they provide different "strengths" of pruning when backtracked onto, as follows (from weakest to strongest): *THEN fails the current alternation branch, and restarts at the next alternation in the current group, or fails the whole group if there are no more alternatives. *PRUNE fails the current match, but allows an advance to the next starting position (unless anchored). *SKIP is like *PRUNE, but can skip forward more than one character. *COMMIT fails the entire matching process, not allowing any further advance in the subject. That's fairly straightforward; the issue between us is what constitutes "the current group". Having two verbs (one for me, one for you) is just a recipe for even more confusion. Regards, Philip -- Philip HazelThread Previous | Thread Next