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

[Alan Mackenzie]

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).