Though, note that that regular expression in my comment above is pretty inefficient as it looks for the longest match at every character instead of skipping chunks of the same character once the match fails at the character starting it (the regular expression in the OP is much better in that regard).

We can use (*SKIP) to avoid that: 

my $len = 0;
my $best = "";
while ($string =~ /((.)(?:(*SKIP)\2){$len,})/g) {
    $len = length $1;
    $best = $1
}

print "best: $best\n"

[download]

But that is still not completely efficient: the regexp is recompiled at every loop iteration because of $len, so maybe the following simpler code could be faster: 

my $best = "";
while ($string =~ /((.)\2+)/g) {
    $best = $1 if length $1 > length $best
}

print "best: $best\n"

[download]

Or maybe this more convoluted variation: 

my $best = "";
$best = $1 while $string =~ /((.)\2*)(*SKIP)(?(?{length $^N <= length 
+$best})(*FAIL))/g;

print "best: $best\n"

[download]

In reply to Re^2: substrings that consist of repeating characters by salva
in thread substrings that consist of repeating characters by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.