Subject: Re: [PATCH/RFC v2 0/2] add regex match flags to git describe

From: Junio C Hamano <[email-masked]>
Date: Mon, 04 Jan 2016 09:46:42 -0800

Mostyn Bramley-Moore <[email-masked]> writes:

> On 12/31/2015 01:23 AM, Junio C Hamano wrote:
> ...
>> Swapping the option key and value may not be a bad idea, but one
>> problem that the above does not solve, which I outlined in the
>> message you are responding to, is that "match-pattern-type" does not
>> give any hint that this is about affecting the match that is done to
>> "refs", e.g. you cannot tell in
>>
>>    $ git mgrep --match-pattern-type=perl-regexp -e foo --refs 'release_*'
>>
>> if the perl-regexp is to be used for matching branch names or for
>> matching the strings the command looks for in the trees of the
>> matching branches.
>
> There is a hint: --foo-pattern-type applies only to --foo.

Hmph.

> In your mgrep example --match-pattern-type would apply to --match
> patterns only, and if we choose to implement it then
> --refs-pattern-type would apply to --refs patterns only.
> eg:
> $ git mgrep --match '^foo' --match-pattern-type=perl-regexp --refs
> release_*' --refs-pattern-type=glob

Most likely the hypothetical "mgrep" would not use "--match" but use
"-e" to retain similarity to "grep", and "--e-pattern-type" would be
confusing.  But I agree that "--refs-pattern-type" uniformly used
where we take pattern parameter on the command line to match with
refs may make it clear that you are only affecting the matches
against refs.

>> Magic pattern annotation like we do for pathspecs Duy raised may not
>> be a bad idea, either, and would probably be easier to teach people.
>> Just like in Perl "(?i)$any_pattern" is a way to introduce the case
>> insensitive match with $any_pattern, we may be able to pick an
>> extensible magic syntax and decorate the pattern you would specify
>> for matching refnames to tell Git what kind of pattern it is, e.g.
>>
>>    $ git mgrep -P -e foo --refs '/?glob/release_*'
>>
>> I am not suggesting that we must use /?<pattern type name>/ prefix
>> as the "extensible magic syntax" here--I am just illustrating what
>> I mean by "extensible magic syntax".
>
> I hadn't seen the pathspec magic patterns before- interesting.  We
> could possibly share syntax with pathspecs, ie
> :(?pattern-options...)pattern

Even though we have DWIM between revisions and paths on the command
line when the user omits "--" for disambiguation, I do not think we
look at the shape of the string to DWIM/decide that it is a pattern,
so as long as the magic syntax cannot be a valid pattern to match
against refs right now (and your ":(?...)"  qualifies as such, as a
refname would not contain a component that begins with a colon), it
would be possible to introduce it as the magic syntax for matching
refs.

Or did you mean to use this syntax also for patterns that match
strings in contents, e.g.

    $ git grep -e ':(?perl-regexp)...'

I am not bold enough to say that it would be a good idea, but I
offhand do not think of a reason why we shouldn't go that route,
either.

> Would this be confusing for commands that already have --perl-regexp
> etc?  What should happen if you specify both --perl-regexp and and a
> different type of pattern like :(glob)foo (error out, I suppose)?

If we were to go that route, ideally, I would say that

    $ git grep --perl-regexp -e 'A' -e ':(?basic-regexp)B' -e ':(?fixed-string)C'

should match with A as pcre, B as BRE and C as a fixed string.

I do not offhand remember if we built the underlying grep machinery
in such a way that it is easy to extend it to allow such mixture,
though.