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* Re: Problem with re-search-backward and "\\="
@ 2003-09-16 18:57 Greg Hill
  0 siblings, 0 replies; 4+ messages in thread
From: Greg Hill @ 2003-09-16 18:57 UTC (permalink / raw)


>GNU Emacs 21.1
>
>Suppose I have this in a buffer:
>
>foo     bar
>
>If point is immediately after foo, then (re-search-forward "\\= *")
>leaves point immediately before bar, as one would expect.
>
>However, with point immediately before bar (re-search-backward " *\\=")
>fails.  I would have expected this search to have succeeded, leaving
>point just after foo.
>
>The definition of `\=' in the elisp info page "Regexp Backslash" is:
>
>>  matches the empty string, but only at point.  (This construct is not
>>  defined when matching against a string.)
>
>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.

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-beninning 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 hope this helps.

--Greg

^ permalink raw reply	[flat|nested] 4+ messages in thread
* Problem with re-search-backward and "\\="
@ 2003-09-16 16:44 Alan Mackenzie
  0 siblings, 0 replies; 4+ messages in thread
From: Alan Mackenzie @ 2003-09-16 16:44 UTC (permalink / raw)


GNU Emacs 21.1

Suppose I have this in a buffer:

foo     bar

If point is immediately after foo, then (re-search-forward "\\= *")
leaves point immediately before bar, as one would expect.

However, with point immediately before bar (re-search-backward " *\\=")
fails.  I would have expected this search to have succeeded, leaving
point just after foo.

The definition of `\=' in the elisp info page "Regexp Backslash" is:

> matches the empty string, but only at point.  (This construct is not
> defined when matching against a string.)

Is this a bug, or have I misunderstood something?

-- 
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").

^ permalink raw reply	[flat|nested] 4+ messages in thread

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2003-09-17  8:13 ` Problem with re-search-backward and "\\=" Alan Mackenzie
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2003-09-16 18:57 Greg Hill
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2003-09-16 16:44 Alan Mackenzie

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