bug#64128: regexp parser zero-width assertion bugs

[Stefan Monnier]

I wish there was a way to emit warnings about oddball constructs
(starting with the "* is literal when encountered at the beginning of
a regexp").


        Stefan


Mattias Engdegård [2023-06-19 10:44:04] wrote:

> 19 juni 2023 kl. 05.04 skrev Stefan Monnier <monnier@iro.umontreal.ca>:
>
>> `^` is only special if it's at the beginning of a group, so `^*` will
>> always treat this * as a literal, right?
>> "Similarly" `$` is only special if it's at the end of a group, so `$*` will
>> always be a repetition of the $ character no?
>
> Yes, ^ and $ have additional rules for when they are plain literals and not
> subject to these bugs at all.
>
> The literal-splitting powers of ^ have now (075e77ac44) been removed.
>
>> So the remaining problematic elements are \` \' \b and \B
>
> \`* has been observed, so we probably need to keep that working as well.
>
>> I suspect if we don't want to signal errors, the next best thing is to
>> treat them like group B.
>
> Yes, maybe; they are less likely to be followed by an operator-literal, but
> it would also be good to have all zero-width assertions work the same way.
> On the other hand, it can't be worse than we have now, as long as we get rid
> of the "quack,\\b*" semantics.