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

[Marcin Borkowski]

On 2015-02-15, at 10:31, Alan Mackenzie <acm@muc.de> wrote:

> Hello, Emacs!
>
> In forward-paragraph, L37, a regular expression is constructed as
> follows:
>
> (let* ...
>  (sp-parstart (concat "^[ \t]*\\(?:" parstart "\\|" parsep "\\)"))
>  ...)
>
> .  Here parstart and parsep are, more or less,
> paragraph-{start,separate}.
>
> The problem is that parstart and parsep themselves are likely to begin
> with "[ \t]*" (the default values certainly do), so we have two
> consecutive matchers for an arbitrary amount of whitespace.  This causes
> the regexp engine to run very slowly when a line starts with lots of WS
> but doesn't match.
>
> This problem seems to be the cause of bug # 19846 (where holding down the
> spacebar inside a C comment causes Emacs to seize up when auto-fill mode
> is enabled).

Hi Alan, hi all,

I put this bug on my todo-list some time ago and decided now to revisit
it.

I'm wondering what could be done about it.  First of all, my Emacs has
this as paragraph-start:

"\\|[  ]*$"

and this as paragraph-separate:

"[      ]*$"

and frankly speaking, I'm not sure why they differ at all (by default).
Also, even though forward-paragraph checks for "^" at their beginning,
they actually don't begin with that character (again, by default).

My first thought is to add a check whether paragraph-start and
paragraph-sep match something like

"^\\^?\\[[[:space:]]+\\][+*]?"

and if yes, make parstart/parsep equal to them, but without the matching
part.

WDYT?

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University