It's simpler than that, just use regex greediness. You were very close. 

my $re = qr/((.)\2+)/;

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Here's an example, 
$ perl -e'my $re = qr/((.)\2+)/; $_="aabccccdddeffff"; while (m/$re/g)
+ { printf "\"%s\" x %d\n", $2, length($1) }'
"a" x 2
"c" x 4
"d" x 3
"f" x 4
$

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That skips capturing lone characters as a sequence of one - change the '+' quantifier to '*' to get them, too. There is no practical limit on the length of the match.

I didn't address picking out the maximum length substring captured. There are lots of ways to do that.

Update: Ok, here's an easy way to get the max length as the search is done, using the (?{}) construct. 

use re 'eval';

my $re = qr/((.)\2+)/;

my ($maxlen, $maxchr, $maxloc);
$_="aabccccdddeffff";

1 while m/$re(?{
    $maxlen = length($^N),
    $maxchr = substr($^N,0,1),
    $maxloc = pos() - $maxlen
        if length($^N) > $maxlen;
})/g;

print $maxchr, ' x ', $maxlen, ' at ', $maxloc, $/;

__END__
c x 4 at 3

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Access to the original matching chunk of the string is given by substr($_, $maxloc, $maxlen).

After Compline,
Zaxo

In reply to Re: Longest possible run of a single character by Zaxo
in thread Longest possible run of a single character by srdst13

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