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

##</code><code>##

#!/usr/bin/perl -w
# Test regular expression on Tic Tac Toe boards.
my @X_wins = ( 
              'XXX......' ,
              '...XXX...' ,
              '......XXX' ,
              'X..X..X..' ,
              '.X..X..X.' ,
              '..X..X..X' ,
              'X...X...X' ,
              '..X.X.X..' ,
             );
my @O_wins = @X_wins;
s/X/O/g for @O_wins;
my @noone_wins = (
             'XO-------' ,
             'XX-XOO---' ,
             '-XX-XX-O-' ,
             'OXO--XXO-' ,
             'OOXXXO---' ,
             'OXXX-XOO-' ,
             'OOXXX----' ,
             'XOOOXOOO-' ,
             'OXOXOX---' ,
                 );

foreach my $board (@X_wins, @O_wins, @noone_wins){
  print "$board " . ( is_win($board) ? "is" : "is not") . " a win.\n";
}       
sub is_win {
  return shift =~ m/^(...)*(X|O)\2\2|(X|O)(.{2,3}\3)\4|^..(X|O).\5.\5/;
}

##</code><code>##

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

##</code><code>##

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