use strict;
use warnings;
use vars qw/ $N /;

my $string = "God dog";

my $re = qr/^
            (..+)
            (??{
              local $N = lc $^N;
              $N =~ tr!\t \f!!d;
              (
                $N eq reverse $N
              ) ? ''
              :   '(?!)'
            })
          $/ix;

print "$string is",
      ( $string =~ $re )
      ? ""
      : "n't"
      , " a palindrome\n";