bug#64351: probabe bug associated with “completion-regexp-list”

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Shynur Xie <one.last.kiss@outlook.com>,  64351@debbugs.gnu.org
> Date: Fri, 30 Jun 2023 14:33:49 -0400
> 
> >> Evaluate this:
> >>     (setq completion-regexp-list '("^[^-]"))
> 
> Bad idea.
> Admittedly, `completion-regexp-list` is documented poorly enough that
> it's not clear what usage is valid and what isn't, but a lot of
> completion code will be broken if you set it globally to anything else
> than nil.

Would you mind telling more about valid uses of this?  I'd like to
improve the documentation.