Re: :alnum: broken?

[Mattias Engdegård]

I just made this very mistake while adding a new regexp-error checking feature 
to xr. Needless to say I now am strongly in favour of turning it into a hard 
error.

Patch attached. It was written for master, but I would suggest it go in 
emacs-27.

The error message could be improved. For the benefit of isearch-forward-regexp, 
it's probably a good idea if it doesn't start or end in a square bracket.

0001-Signal-an-error-for-the-regexp-alnum.patch
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