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Re: Problem with re-search-backward and "\\="
From: |
Alan Mackenzie |
Subject: |
Re: Problem with re-search-backward and "\\=" |
Date: |
Wed, 17 Sep 2003 08:13:55 +0000 |
User-agent: |
tin/1.4.5-20010409 ("One More Nightmare") (UNIX) (Linux/2.0.35 (i686)) |
Greg Hill <ghill@synergymicro.com> wrote on Tue, 16 Sep 2003 11:57:03
-0700:
>>GNU Emacs 21.1
>>Suppose I have this in a buffer:
>>foo bar
>>With point immediately before bar (re-search-backward " *\\=") fails.
>>I would have expected this search to have succeeded, leaving point just
>>after foo.
>>Is this a bug, or have I misunderstood something?
> Alan,
> You have misunderstood the nature of the "greediness" of the '*' --
> and probably also the '+' -- postfix operator when applied to
> backward searches. It is not "symmetrical" with its effect on
> forward searches. I suggest you do some experimentation with these
> operators, never minding the "\\=" for the moment, to better
> understand the way these postfix operators work.
OK, I think I've got it now. It finds the _minimum_ match it can. With
my expression, it matches on zero spaces. It's even documented in the
elisp info pages: "A true mirror-image of `re-search-forward' would
require a special feature for matching regular expressions from end to
beginning. It's not worth the trouble of implementing that." ;-(
> I may not have this completely right, but this is the way I
> conceptualize it. When searching forward, the match-beginning
> advances forward from point until the first possible match is found;
> then match-beginning is fixed and match-end advances until going any
> farther would break the rule or exceed the specified limit. In
> searching backward, the match-beginning moves backward until the
> first possible match is found; then match-beginning is fixed and the
> match-end advances forward until going any farther would break the
> rule, using the initial value of point as the limit to how far the
> match-end is allowed to advance.
> I have never experimented with the "non-greedy" postfix operators
> '*?' '+?' and '??', so I can't tell you how using them effects the
> conceptualization described above.
I wasn't aware these existed. Thanks for the tip!
> I hope this helps.
Very much so. For what I actually need to do (moving back any
combination of whitespace and a few things like "//." as the first
non-space stuff on a line), I can do it easily enough by hand.
Many thanks.
> --Greg
--
Alan Mackenzie (Munich, Germany)
Email: aacm@muuc.dee; to decode, wherever there is a repeated letter
(like "aa"), remove half of them (leaving, say, "a").