Re: Problem with re-search-backward and "\\="

[Greg Hill <ghill@synergymicro.com>]

>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