grepc: Consistently use Perl regex

grepc: Consistently use Perl regex

If we use different regex types within the program, then users
can't use regex patterns in the identifier due to
incompatibilities.  Let's use Perl regex everywhere we can, so
that users can consistently use Perl regex patterns in the
identifier.  Example:

$ grepc (vf|as)printf;

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>