GNU bug report logs - #19873
Ill-formed regular expression is constructed in forward-paragraph.

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From: Alan Mackenzie <acm <at> muc.de>
To: 19873 <at> debbugs.gnu.org
Subject: bug#19873: Ill-formed regular expression is constructed in forward-paragraph.
Date: Sun, 15 Feb 2015 10:31:22 +0000

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).

-- 
Alan Mackenzie (Nuremberg, Germany).

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