Re: :alnum: broken?

[Paul Eggert]

On 2/23/20 2:21 AM, Mattias Engdegård wrote:
> > GNU grep treats a bracket expression like '[:space:]' as an error, since it's 
> > inevitably a typo....
> Such a check is obviously unsound, strictly speaking, which may be a reason for 
> objecting to it.

I understand the objection, but the check is not unsound. The syntax of regexps 
was not carved in stone by God. It is something that we decide, and we can 
change our minds if the change would be an overall win, as it would be if Emacs 
behaved like Grep.

When the [[:alnum:]] syntax was added to regular expressions many years ago, it 
invalidated some astronomically-unlikely but formerly-valid expressions. For 
example, "[[:x:]" formerly was a valid regexp (with the same meaning as "[:[x]") 
but it is now invalid. So there is precedent for invalidating some 
astronomically-unlikely regexps when the overall change is a net benefit, as it 
would be if Emacs behaved like Grep.

The byte-compiler could warn about some of these blunders and if someone wants 
to change the byte-compiler to do that, it would be an improvement. However, 
this would necessarily either cry wolf or let blunders through, because the 
byte-compiler cannot reliably determine whether a string will be used as a 
regular expression.