Re: [GSOC][PATCH] userdiff: add support for Scheme

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 28, 2021, 18:06 UTC
Message-ID: <xmqqft0fm9uu.fsf@gitster.g>
In-Reply-To: <EBC020E6-BE8B-4332-8225-A988CB7CFA69@gmail.com>

Atharva Raykar <raykar.ath@gmail.com> writes:

Show 14 quoted lines

>>> +         "(\\.|[^][)(\\}\\{ ])+"),
>> 
>> One or more "dot or anything other than SP or parentheses"?  But
>> a dot "." is neither a space or any {bra-ce} letter, so would the
>> above be equivalent to
>> 
>> 	"[^][()\\{\\} \t]+"
>> 
>> I wonder...
>
> A backslash is allowed in scheme identifiers, and I erroneously thought that
> the first part handles the case for identifiers such as `component\new` or 
> `\"id-with-quotes\"`. (I tested it with a regex engine that behaves differently
> than the one git is using, my bad.)

Ah, perhaps you didn't have enough backslashes. A half of the doubled one before the dot is eaten by the C compiler, so the regexp engine is seeing only a single backslash before the dot, which means "literally a single dot". If you meant "literally a single backslash, followed by any single char", you probably would write 4 backslashes and a dot---half of the backslashes would be eaten by the compiler, so you'd be passing two backslashes and a dot, which is probably what you meant.

Having said that, two further points.

 - the "anything but whitespaces and various forms of parentheses"
   set would include backslash, so 'component\new' would be taken as
   a single word with "[^][()\\{\\} \t]+", wouldn't it?

 - how common is the use of backslashes in identifiers?  I am trying
   to see if the additional complexity needed to support them is
   worth the benefit.

Show 6 quoted lines

> But somehow, the regexp you suggested, ie:
>
> 	"[^][()\\{\\} \t]+"
>
> does not handle the case of make\foo -> make\bar (it will only diff on foo).
> I am not too sure why it treats backslashes as delimiters.

Perhaps because you have included two backslashes inside [] to say "backslash is not a word character" in the original, and I blindly copied that? IOW, do you need to quote {} inside []?

> Yes, this is exactly what I was trying to express. All words should be
> delimited by either whitespace or a parenthesis, and all other special
> characters should be accepted as part of the word.

That sentence after "All words should be..." would be a good comment to replace what you wrote in the original, then ;-).