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#!/usr/bin/perl

...
  print " $2" while /(?=(?:^|(.)(?!\1))(([a-z])\3)(?!\3))/gi;
  print "\n";
  }

                   a -->
                  aa --> aa
...
   deefffgggghhiiiii --> ee hh
          aabbbccdee --> aa cc ee
            wwxxyyzz --> ww xx yy zz