bug#36644: Git log search

[Eli Zaretskii <eliz@gnu.org>]

> Cc: rpluim@gmail.com, 36644@debbugs.gnu.org, juri@linkov.net
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 25 Jul 2019 15:36:54 +0300
> 
> > I also see your point: it would be nice to be able to document the
> > semantics of PATTERN in a backend-independent way.  But I think this
> > is next to impossible in this case, both because of significant
> > differences in the backend capabilities (e.g., bzr doesn't have the
> > equivalent of Git's --fixed-strings, AFAICT), and because some backend
> > allow great flexibility in interpreting PATTERN, under control of
> > optional switches passed to the backend.
> 
> The other option is to standardize on basic or extended regexp, and 
> simply give up for backends that can't support that.

We could simply say "regular expression" and leave the details
unspecified.  But I think Juri said that fixed strings was the lowest
common denominator, which is why I proposed a slightly more vague doc
string.  Juri, which backends don't support regular expressions?  And
Robert, why did you insist on saying STRING?

> Git supports all kinds of regexps. 'hg grep' uses Perl-compatible ones 
> (meaning extended regexps are supported, at least). I'm not sure which 
> regular expressions are expected by 'bzr log -match', but if it doesn't 
> support the extended ones, *shrug*.

I didn't dig deep enough, but since bzr is written in Python, I'd bet
it supports whatever Python supports natively.

> Anyway, if people disagree, I'm not going to press the issue.

If all the backends either support regular expressions or don't
support this feature at all, then we had better mentioned regular
expressions in the doc string.

> > So I think we should treat this as we do in "M-x grep": leave the
> > semantics of PATTERN backend-dependent, and rely on the user to quote
> > some characters in it as needed.  Admittedly, 'grep' is lower-level
> > than 'vc-log-search', but at least we have a precedent.
> 
> The difference is, 'M-x grep' doesn't use different backends.

It actually leaves that to the user: you can invoke any program you
want.  E.g., I invoke 'fgrep' this way very frequently.