bug#16577: Add regexp-based version of completion-ignored-extensions

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: monnier@iro.umontreal.ca,  rgm@gnu.org,  vmalloc@gmail.com,
>   16577@debbugs.gnu.org
> Date: Mon, 21 Feb 2022 18:01:03 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Regular expressions don't understand file names, they are just strings
> > as far as regexps are concerned.
> 
> Of course they're strings, but a regexp operator has semantics.

I thought you were asking about some builtin feature of regexp
matches, and tried to explain why that doesn't exist.

If you are talking about something we should concoct, then I don't
think I see a problem expressing "match after / or at the start of the
string" in the regexp notation.  So why were you asking about it?