>perl -wMstrict -le
"my $s = join '', qw(ABCbb bCBAd ddCAC DDDAC ABBBC ABCDA ABCCC CCCAB);
 print qq{'$s'};
 ;;
 my $row_width  = 5;
 my $contiguous = 3;
 $contiguous <= $row_width or die 'nonsense';
 $contiguous >  0          or die 'ridiculous';
 ;;
 my $pre_max = $row_width - $contiguous;
 my $post    = $contiguous - 1;
 ;;
 use re 'eval';
 my $mod;
 my @reps =
   grep { $mod = ! $mod }
   $s =~ m{
     \G (?: .{$row_width}){0,}? .{0,$pre_max}? ((.)\2{$post})
     (?(?{ pos($s) % $row_width }) .){$pre_max}
     }xmsg;
 ;;
 printf qq{'$_'  } for @reps;
"
'ABCbbbCBAdddCACDDDACABBBCABCDAABCCCCCCAB'
'DDD'  'BBB'  'CCC'  'CCC'