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From: "Peter Neilson" <neilson@windstream.net>
Subject: Re: Stackoverflow in regexp matcher
Date: Wed, 03 Feb 2016 06:44:17 -0500
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On Wed, 03 Feb 2016 06:19:43 -0500, Nicolas Goaziou  
<mail@nicolasgoaziou.fr> wrote:

> "Loris Bennett" <loris.bennett@fu-berlin.de> writes:
>
>>   re-search-forward("^[^%]*\\\\usepackage.*{biblatex}" nil t)
>
> This is a pathological regexp. [^%] is anything but a percent sign, so
> it can contain newline characters. Basically [^%]* can match an entire
> buffer if it doesn't contain any %.
>
> I think the regexp used in `reftex-using-biblatex-p' should be
>
>   "^[^%\n]*\\\\usepackage.*{biblatex}"

Here's a link about the extreme difficulty of detecting pathological  
regular expressions:

https://mail.python.org/pipermail/python-dev/2003-May/035916.html

The author analyses the exponential not-found situation for (x+x+)+y and  
then suggests that for larger expressions, "... no more than 1 programmer  
in 1000 has even a vague idea how to start looking for such problems."