apt-file's -x, --regexp

[Marco Ippolito <maroloccio@gmail.com>]

I would find it more intuitive if the program used -P as the command
option to enable Perl-style regular expression matching.

-x is slightly surprising to me and --regexp isn't specific enough, in
my opinion. When reading apt-file --help:

    --regexp           -x               pattern is a regular expression

I am left wondering: "what can I feed to this regex engine"?

Maroloccio