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

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Glenn Morris <rgm@gnu.org>,  vmalloc@gmail.com,  Eli Zaretskii
>  <eliz@gnu.org>,  16577@debbugs.gnu.org
> Date: Mon, 21 Feb 2022 17:23:11 +0100
> 
> By the way, it'd be nice to have a zero-width regexp matcher for "start
> of file name component" (i.e., match after "/" or at the start of the
> string).  We don't have that already anywhere, by any chance?

Regular expressions don't understand file names, they are just strings
as far as regexps are concerned.