From: Stefan Monnier <monnier@iro.umontreal.ca>
To: help-gnu-emacs@gnu.org
Subject: Re: will we ever have zero width assertions in regexps?
Date: Fri, 28 Jan 2011 21:51:55 -0500 [thread overview]
Message-ID: <jwvzkqke8n4.fsf-monnier+gnu.emacs.help@gnu.org> (raw)
In-Reply-To: slrnik6lh5.rji.nospam-abuse@powdermilk.math.berkeley.edu
>> To get rid of the occasional pathological case where matching takes
>> forever and Emacs appears to be frozen. Programmers who are used to
>> backtracking matchers will usually intuitively stay away from regexps
>> that can show such behaviors, but not all programmers do, and even if
>> you're careful there are cases that are hard to avoid.
> Did you try it with Perl recently (last 10 years or so)?
No, but neither have I bumped into pathological cases in Perl before
that (when I did use it).
> As I said, I put some optimizations which in most (AFAIK) practical
> senses remove such pathologies. (The underlying problems remain; the
> optimizations are only "heuristic"; but one needs to be extra
> inventive to circumvent the optimizations.)
A typical case could look something like "foo *(.*?) *bar". when
matching "foo ..<many space>.. baZ".
Emacs's regexp engine is not very clever and doesn't do much in terms of
avoiding backtracking (it mostly takes care of <foo>*<bar> when <foo>
can only match a single char and <bar> can only start with a char that's
not matched by <foo>), but I can't think of too many ways to handle the
above one efficiently within a "backtracking regexp matcher" framework.
>> Another minor reason is that it can be handy to have an incremental
>> matching primitive, so you can match over a long string one chunk at
>> a time. I'm not sure how often this would be useful, but I've come
>> across a few cases where it seemed like it could be put to good use
>> (tho, for lack of experience with it, I can't sweat that it would turn
>> out to be a good idea).
> Do not know what you mean by this...
Basically, provide a primitive like (match-string RE STRING LIMIT) that
can not only say "matched between START and END", but also "reached
LIMIT within yet finding a match, here's the suspended SEARCH-STATE at
LIMIT", so you can later resume the search starting at LIMIT by passing
that state.
Stefan
next prev parent reply other threads:[~2011-01-29 2:51 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.1.1296054361.23496.help-gnu-emacs@gnu.org>
2011-01-26 15:58 ` will we ever have zero width assertions in regexps? Stefan Monnier
2011-01-27 1:45 ` Le Wang
[not found] ` <mailman.6.1296092730.6982.help-gnu-emacs@gnu.org>
2011-01-27 2:21 ` Stefan Monnier
2011-01-27 6:34 ` Ilya Zakharevich
2011-01-27 16:10 ` Stefan Monnier
2011-01-28 23:49 ` Ilya Zakharevich
2011-01-29 2:51 ` Stefan Monnier [this message]
2011-01-29 22:28 ` Ilya Zakharevich
2011-01-31 16:08 ` Stefan Monnier
2011-01-31 17:10 ` Ilya Zakharevich
2011-01-31 21:29 ` Stefan Monnier
2011-02-02 15:09 ` Ilya Zakharevich
2011-02-07 20:30 ` Stefan Monnier
2011-02-08 22:41 ` Ilya Zakharevich
2011-01-26 14:55 Le Wang
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