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

[Drew Adams]

> [0-9A-Fa-f] still has one advantage: the reader, and the
> writer, know exactly which characters are included without looking it up. For
> example:
> 
> - whether upper case, lower case, or both are accepted
> - whether non-ascii digits are accepted (included in [:digit:] on many
> platforms)

+1.  At least the doc for [:digit:] should say exactly
what it matches.  Then it'll be clear for readers and
writers.