Didn't notice the interesting bit :). Extended to handle AABCCCCAD:

use strict;
use warnings;

my $string = 'WXYYZAABCCCCADWWXYYYZAAbCCCCAd';

print $1, "\n" while $string =~
  m/(
    ([\w])(?=\2).(?!\2)(.) # Match AAB
    (?!\2|\3)(.)           # C
    \4{3}                  # CCC
    \2                     # A
    (?!\2|\3|\4).          # D
  )/xg;
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Prints:

AABCCCCAD
AAbCCCCAd
[download]