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

From: Marcin Borkowski
Subject: bug#19873: Ill-formed regular expression is constructed in forward-paragraph.
Date: Sun, 26 Feb 2017 17:44:51 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

On 2015-02-15, at 10:31, Alan Mackenzie <address@hidden> 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

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


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


Marcin Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

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