Re: BUG: git grep behave oddly with alternatives

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

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> On Thu, Jan 12 2023, Jeff King wrote:
>
>> So it does seem like all bets are off for what people can and should
>> expect here. Which isn't to say we should make things worse. I mostly
>> wondered if REG_ENHANCED might take us closer to what glibc was doing by
>> default, but it doesn't seem like it.

I thought that René's "Use enhanced only when doing BRE" was fairly
focused, but I am very tempted to accept ...

> There's a couple of ways out of this that I don't see in this thread:
>
> - Declare it not a problem: We have -G, -E and -P to map to BRE, ERE and
>   PCRE. One view is to say the first two must match POSIX, another is
>   tha whatever the platform thinks they should do is how they should
>   act.

... this view.  The story "BRE and ERE work via what system
libraries provide, and 'git grep' matches what system grep' does" is
an easy to understand view.

> - Just mandate PCRE for Mac OS, then map -E to -P. We could do this with
>   the pcre2convert() API and PCRE2_CONVERT_POSIX_EXTENDED flag,
>   i.e. PCRE's own "translate this to ERE".

Presumably this is to ensure -E works identically everywhere?  If
so, then we should do that everywhere, not just on macOS.  But again
it makes "git grep -E" slightly incompatible with "grep" on systems
(including macOS), so...