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

From: Atharva Raykar <raykar.ath@gmail.com>
Date: Mar 29, 2021, 08:12 UTC
Message-ID: <562DCDA0-EAE6-408F-97D7-127689DE5559@gmail.com>
In-Reply-To: <xmqqft0fm9uu.fsf@gitster.g>

On 28-Mar-2021, at 23:36, Junio C Hamano <gitster@pobox.com> wrote:

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> 
> Atharva Raykar <raykar.ath@gmail.com> writes:
> 
>>>> +         "(\\.|[^][)(\\}\\{ ])+"),
>>> 
>>> 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.

I have refined the regex, and now it is much simpler and does all of what I want it to:

	"([^][)(}{[:space:]])+"

I did not have to escape the various parentheses, so I avoided the need to handle backslashes separately. The "\\t" was causing problems as well because it took it as a '\' followed by a 't' (Thanks to j416 on #git-devel for helping me out on this).

Show 6 quoted lines

>> 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 ;-).

Yes, that should make it a lot more clear.