From: Stefan Monnier <monnier@iro.umontreal.ca>
To: "Andreas Röhler" <andreas.roehler@online.de>
Cc: Emacs developers <emacs-devel@gnu.org>
Subject: Re: Possible problem with looking-back function
Date: Thu, 19 Aug 2010 10:01:53 +0200 [thread overview]
Message-ID: <jwvmxsj11kk.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <4C6CCA8B.5060106@online.de> ("Andreas Röhler"'s message of "Thu, 19 Aug 2010 08:09:15 +0200")
>>> Shouldn't it return 1?
Not according to the docstring:
If GREEDY is non-nil, extend the match backwards as far as
possible, stopping when a single additional previous character
cannot be part of a match for REGEXP. When the match is
extended, its starting position is allowed to occur before
LIMIT.
>> The algorithm searches backward until it finds a position from which there
>> is a match that extends to point. Doing more would be quadratic
Actually our regexp matcher (i.e. "looking-at") is already much worse
than quadratic in worst-case complexity.
> AFAIK exists a bug, resp. missing implementation in re-search-backward.
Ignoring back-references, our regexps are pretty much within the
"regular language" domain, so in theory matching and searching can be
performed in linear time. But our regexp engine uses a backtracking
implementation, so it's not even close to linear. And the "backward"
matcher is not implemented, so we "simulate" it with a forward matcher,
which is why looking-back is slow and doesn't behave like we want.
Stefan
next prev parent reply other threads:[~2010-08-19 8:01 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-19 1:15 Possible problem with looking-back function Vinicius Jose Latorre
2010-08-19 2:43 ` Davis Herring
2010-08-19 6:09 ` Andreas Röhler
2010-08-19 8:01 ` Stefan Monnier [this message]
2010-08-19 9:02 ` Stephen J. Turnbull
2010-08-19 9:32 ` Andreas Röhler
2010-08-20 2:11 ` Vinicius Jose Latorre
2010-08-20 13:07 ` Stefan Monnier
2010-08-21 0:08 ` Vinicius Jose Latorre
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