Subject: bug#19873: Ill-formed regular expression is constructed in forward-paragraph.

From: Lars Ingebrigtsen <[email-masked]>
Date: Thu, 02 Dec 2021 11:39:51 +0100

Alan Mackenzie <[email-masked]> writes:

> I think this idea is workable, but you'll have to check for one or both
> of paragraph-s{tart,eparate} starting with "[ \t]+".  A good strategy
> here might be to begin the target regexp with "^[ \t]*", then begin one
> or both components with "[ \t]" (without the "*").
>
> There may be other gotchas which I haven't thought about yet.
>
> One needs a twisted mind to do this sort of thing properly, so I offer my
> services to review your upcoming patch.  ;-)

The problem seems rather intractable to me.  Is there really any way to
examine a regexp to determine "does this in practice match [ \t]*"?

I wonder whether instead of trying to construct a better overall regexp
could rewrite the loop.  That is, instead of searching for sp-parstart,
search for parstart "\\|" parsep, and then check whether
(match-beginning 0) of that comes after "^[ \t]*".  Or something along
those lines.

But I don't know whether that'd be any faster in practice.

Do you have a test case that demonstrates the slowness?  In that case I
could try to see whether there's any alternate approach here that's
faster.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no