If you are dealing with large strings then the following may be a little quicker than the regex based techniques posted so far:

use warnings;
use strict;

my $str;
my $len = 0;

while ($len < 1000000) {
    my $runLen = int rand (50);
    $str .= chr (ord ('a') + int rand (26)) x $runLen;
    $len += $runLen;
}

my $sstr = substr ($str, 0, 1) . (substr ($str, 1) ^ $str);
my @bestRuns;
my $match = "\0";
my $bestRunLen = 2;
my $scan = 0;

while (-1 != (my $start = index $sstr, $match, $scan)) {
    my $runLen = length ((substr ($sstr, $start) =~ /(\0+)/)[0]) + 1;
    
    if ($runLen > $bestRunLen) {
        # new best match
        @bestRuns = $start - 1;
        $bestRunLen = $runLen;
        $match = "\0" x ($bestRunLen - 1);
    } else {
        # another best match
        push @bestRuns, $start - 1;
    }
    
    $scan = $start + $bestRunLen - 1;
}

for (@bestRuns) {
    print "Run of " . substr ($str, $_, 1) . " from $_ for $bestRunLen
+\n";
}

[download]

Prints:

Run of r from 766269 for 144

[download]

Note that this finds all the matches and their start indexes.

DWIM is Perl's answer to Gödel

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

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