$pattern =~
    /
        ^
        _*                  # 0 or more underscores to start
        (
            (\d*|#*|\**)+   # Any combo of digits, #s, or *s
            (\[\d+\-\d\])?  # Maybe a [x-y]
        )
        (X*\.?\!?)          # Zero or more Xs, and maybe a . or !
        $
    /gx;

##</code><code>##

use strict;
use warnings;

use Test::More qw(no_plan);

my $regex = qr{ ... stuff ... }x;

#  Tests
like(  '100', $regex, 'Valid numeric' );
like( '_100', $regex, 'Valid numeric with leading underscore' );
# etc.

unlike( '', $regex, 'Invalid empty string' );
# etc.