% 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%   --