I think I like LanX's grep/unpack approach best (Update: also the approaches of hdb here and kcott here), but here's another (no claims for efficiency): 

>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'

[download]

Update: If I correctly understand how all this works, the regex sub-expression
    (?(?{ pos($s) % $row_width }) .){$pre_max}
will continue to "loop" until the value of the  $pre_max quantifier is exhausted even though a row boundary has been reached. A way to "break out" of this loop once a boundary is reached (again, IIUC) is
    (?(?{ $+[1] % $row_width }) . | (*SKIP:AHEAD)){$pre_max} (*MARK:AHEAD)
($+[1] used in place of pos($s)). Again, no timings have been done to confirm this is actually beneficial. See Special Backtracking Control Verbs (Perl version 5.10+) in perlre and note that these verbs are marked "experimental". If you really want to walk on the wild side, try  (*ACCEPT) which is marked highly experimental:
    (?(?{ $+[1] % $row_width }) . | (*ACCEPT)){$pre_max}
and no  (*MARK) is needed. Both of these variations have been tested.

In reply to Re: Regex matching on grid alignment by AnomalousMonk
in thread Regex matching on grid alignment by Anonymous Monk

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