# ...

if($input =~ m/^(gi.*)([1-8])\/0\/([1-9]|[1-3][0-9]|4[0-8])$/i {
    my ($string, $digit, $number) = ($1, $2, $3); # e.g. "GigabitEthernet", "1" and "48"; $number will be at most 48

    # ...
}