use strict;
use warnings;

my $string = 'ABCDEEFGHIJJJKLMNOOOOPQRSTUVWXXYZ';
print "matching: $string\n";

while( $string =~ m/((\w)\2[^\2])/g )
{
    print $1, "\n";
}

=pod
matching: ABCDEEFGHIJJJKLMNOOOOPQRSTUVWXXYZ
EEF
JJJ
OOO
XXY
=cut