* Re: Problem with re-search-backward and "\\="
[not found] <mailman.313.1063738732.21628.help-gnu-emacs@gnu.org>
@ 2003-09-17 8:13 ` Alan Mackenzie
2003-09-17 17:39 ` Greg Hill
0 siblings, 1 reply; 4+ messages in thread
From: Alan Mackenzie @ 2003-09-17 8:13 UTC (permalink / raw)
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").
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Problem with re-search-backward and "\\="
2003-09-17 8:13 ` Problem with re-search-backward and "\\=" Alan Mackenzie
@ 2003-09-17 17:39 ` Greg Hill
0 siblings, 0 replies; 4+ messages in thread
From: Greg Hill @ 2003-09-17 17:39 UTC (permalink / raw)
At 8:13 AM +0000 9/17/03, Alan Mackenzie wrote:
>OK, I think I've got it now. It finds the _minimum_ match it can.
Well, not exactly. It finds the longest "rightmost" match it can.
Searching backward, once it has found a character that can serve as
the beginning of a complete match, it then still finds the longest
match possible starting from that point. For example, starting with
(point) just in front of the "b" in "fooooo gooo bar",
(re-search-backward "[fg]o*" nil t) will find and return "gooo", not
just "go", as the string returned by (match-string 0).
>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.
It sounds to me like the functions skip-chars-forward,
skip-chars-backward, skip-syntax-forward and skip-syntax-backward
will probably serve you well.
--Greg
^ permalink raw reply [flat|nested] 4+ messages in thread
* 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 17:39 ` Greg Hill
2003-09-16 18:57 Greg Hill
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