From: Junio C Hamano <gitster@pobox.com>
Date: Sat, 18 Dec 2010 21:00:32 GMT
Subject: Re: [PATCH v2 2/4] diff.c: implement a sanity check for word regexes
Message-ID: <7vvd2qg5jj.fsf@alter.siamese.dyndns.org>
In-Reply-To: <ee3026bd997fc6d8508b8e5617e572f99c8bf3d6.1292688058.git.trast@student.ethz.ch>

Thomas Rast <trast@student.ethz.ch> writes:

> Word regexes are a bit of a dangerous beast, since it is easily
> possible to not match a non-space part, which is subsequently ignored
> for the purposes of emitting the word diff.  This was clearly stated
> in the docs, but users still tripped over it.
>
> Implement a safeguard that verifies two basic sanity assumptions:
>
> * The word regex matches anything that is !isspace().
>
> * The word regex does not match '\n'.  (This case is not very harmful,
>   but we used to silently cut off at the '\n' which may go against
>   user expectations.)
>
> This is configurable via 'diff.wordRegexCheck', and defaults to
> 'warn'.

How expensive to run this check twice, every time word_regex finds a
match?

As this is about making sure that we got a sane regex from the user (or a
builtin pattern), I wonder if we can make it not depend on the payload we
are matching the regex against.  Then before using a word_regex that we
have not checked, we check if that regex is sane, mark it checked, and do
not have to do the check over and over again.