Re: [PATCH v2 2/4] diff.c: implement a sanity check for word regexes

[Junio C Hamano <gitster@pobox.com>]

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.