#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

for my $s (qw( xxabyycdzz axxbmmmcyyd xxyy exxyy xxyyf xxgyy mmm mmmennn )) {
    say for '---', $s;
    say $2 while $s =~ /(?|^((.))\2(?!\2)
                          |(?<=(.)(?!\1))(.)\2(?!\2))/gx;
}