Re: Progressively slow pattern match

[David Kastrup]

Ralf Angeli <address@hidden> writes:

> In AUCTeX there is a regexp used with `looking-at' where pattern
> matching seems to progressively get slower the longer a part of the
> (possible) match gets.  I reduced the regexp to a bare minimum for
> testing and the code now looks something like this:
>
> (looking-at "\\(%+\\)*foo")
>
> The problem occurs if this is used against a line with only %
> characters in it.  The more of these characters there are the slower
> it gets.  I checked the time one call of `looking-at' takes with
> (abs (- (float-time) (progn (looking-at "\\(%+\\)*foo") (float-time))))
> and got the following results (in seconds):
>
> %%%%%%%%%%                 0.0006
> %%%%%%%%%%%%%%%            0.0154
> %%%%%%%%%%%%%%%%%%%%       0.5132
> %%%%%%%%%%%%%%%%%%%%%%%%%  7.8058
>
> The regexp is used with `looking-at' for checking if there are LaTeX
> macros which have to be treated specially during paragraph movement.
> As paragraph movement is used quite extensively when a region is to be
> filled, users might get the notion that they are experiencing a hang
> if they have such line for visually separating parts in the file.
>
> Is this a deficiency in Emacs?  Is there a way matching can be sped up
> with this or maybe another, equivalent regexp?

Uh, "\\(%+\\)?foo" maybe?

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum