bug#36167: [PATCH] Replace manually crafted hex regexes with [[:xdigit:]]

[Paul Eggert]

If we're going to make such a change we should be systematic about it. 
Unfortunately the currently-proposed patches don't handle examples like 
these:

[A-Fa-f0-9] (in dnd.el)

[0-9a-fA-F.] (in org/org-table.el)

[0-9a-fA-FxXzZ_] (in progmodes/vera-mode.el)

(skip-chars-backward "0-9a-fA-F'") (in progmodes/cc-mode.el)

I will cc this message to Mattias Engdegård, as he has a good scanner 
for regular expressions and patterns. Mattias, the bug-report thread is 
here:

https://debbugs.gnu.org/36167