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    ^(...)*(X|O)\2\2|(X|O)(.{2,3}\3)\4|^..(X|O).\5.\5

#!/usr/bin/perl -w
# Test regular expression on Tic Tac Toe boards.
...
sub is_win {
  return shift =~ m/^(...)*(X|O)\2\2|(X|O)(.{2,3}\3)\4|^..(X|O).\5.\5/
+;
}

 ^(...)*(\w)\2\2|^..(\w).\3.\3|(\w)..\4..\4|(\w)...\5...\5

  (\w)(..(\1|.\1.)..\1|.\1.\1..$|\1\1(...)*$)