EXTREME TIMTOWTDI the other way around :)

#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=551038

use strict;
use warnings;

$_ = 'ABDBACCBBBCBDDBCDCBCCDBABCBABBBBBADACDABACC';

print "longest run: ", length eval "'".s/(.)\1*\K/'|'/gr."'";
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