# before.pl
use strict;
use warnings;

use Benchmark 'cmpthese';

my $sz      = ( shift || 10 ) - 4;
my $X = 'N' . 'x' x $sz . '000';
my $Y = 'N' . 'x' x $sz . '001';

print '# input length: ', length( $X ), $/;

cmpthese( -1,
          {
           '?!X'  => '$X =~ /^N(?!.*00$).*$/',
           '?!Y'  => '$Y =~ /^N(?!.*00$).*$/',
           '?<!X' => '$X =~ /^N.*(?<!00$)$/',
           '?<!Y' => '$Y =~ /^N.*(?<!00$)$/',
           }
        );
__END__


# after.pl
use strict;
use warnings;

use Benchmark 'cmpthese';

my $sz      = ( shift || 10 ) - 4;
my $X = 'N' . 'x' x $sz . '001';
my $Y = 'N' . 'x' x $sz . '000';

print '# input length: ', length( $X ), $/;

cmpthese( -1,
          {
           '?!X'  => '$X =~ /^N(?!.*00$).*$/',
           '?!Y'  => '$Y =~ /^N(?!.*00$).*$/',
           '?<!X' => '$X =~ /^N.*(?<!00$)$/',
           '?<!Y' => '$Y =~ /^N.*(?<!00$)$/',
           }
        );
__END__

##</code><code>##

% perl before.pl 7
# input length: 7
          Rate ?<!Y  ?!Y ?<!X  ?!X
?<!Y 1553555/s   -- -70% -75% -77%
?!Y  5222982/s 236%   -- -17% -22%
?<!X 6285324/s 305%  20%   --  -6%
?!X  6718816/s 332%  29%   7%   --
% perl after.pl
# input length: 7
          Rate ?<!X  ?!Y ?<!Y  ?!X
?<!X 2414483/s   --  -1%  -4% -19%
?!Y  2434774/s   1%   --  -3% -18%
?<!Y 2506841/s   4%   3%   -- -16%
?!X  2970871/s  23%  22%  19%   --