Re: [PATCH] [Outreachy] git/userdiff.c fix regex pattern error

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 6, 2018, 23:41 UTC
Message-ID: <xmqq7eiuwsm7.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <CA+=o6KHxwYdYsvFDzBaG1q2jvgtN5f4LruD5k=dqei8workWfQ@mail.gmail.com>

Ananya Krishna Maram <ananyakittu1997@gmail.com> writes:

Show 8 quoted lines

>> But it does not need to be escaped, when you specify the regular
>> expression the way we do. And the way we specified it is really the
>> standard when specifying regular expressions in C code, i.e. *without* the
>> suggested backslash.
>
> Aha!. this makes total sense. I was thinking from a general regular expression
> point of view. But I should be thinking from C point of view and how C
> might interpret this newly submitted string.

If you were thinking from a general regex point of view, you would never have treated '/' as anything special, though.

Historically, some languages (like sed and perl) had an regexp match operator, which is denoted by enclosing a regexp inside a pair of slashes. In these languages, if you use /<regexp>/ operator, and if you want to match slash with the pattern, you somehow need a way to write an regexp that has slash in it. E.g. if you want a pattern that would match 'a' followed by '/' followed by 'b', your regexp would look like "a/b", but the regexp match operation you would write in these languages would be like /a\/b/, so that '/<regexp>/' parser can tell that the second slash is not a slash that signals the end of the match operator.

And then there is an unnamed misdesigned language that has a regmatch() function, which takes a string that contains a regular expression, but somehow requires that string to begin and end with a slash for no justifiable reason ;-). If you were thinking about regexp from that brain-dead languages' point of view, yes, you should unlearn it and what Dscho gave you would make sense.

C's regexp(3) library does not share such a misdesign and just takes a regular expression as a string. You would still need to follow the quoting rules of C string literals (e.g. write a literal double-quote or backslash after an escaping backslash), but of course slash is not special there.