#!/usr/bin/perl
use warnings;
use strict;

use Test::More;
my $r = qr/^-?           # Can start with a minus.
            (:?          # Non capturing group.
               [0-9]+    # Digits. Does not match Unicode digits as \d does.
               (?:       # Another group, this one will be optional.
                  \.     # The dot. Backslashed to lose its special meaning.
                  [0-9]+ # Digits again.
               )?        # End of the inner group, the ? makes it optional.
            )            # End of the outer group.
            $            # And nothing more.
            /x;

like  ($_, $r) for qw/ 4   -7 0.656 -67.35555 /;
unlike($_, $r) for qw/ 5. 56F   .32      -.04 /;

done_testing();