bug#64128: regexp parser zero-width assertion bugs

[Stefan Monnier]

> To make everything clear, we have to groups of zero-width assertions:
>
> Group A: ^ $ \` \' \b \B

IIRC `^` 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?

So the remaining problematic elements are \` \' \b and \B

I suspect if we don't want to signal errors, the next best thing is to
treat them like group B.


        Stefan