OK, I'm not really answering your question (I'm not using a regexp), and this is a method you have probably tried before, but if it's speed you want, how about simply converting the string to an array and looping once over its elements?

use strict;
my $string="    aaaa    bbbbccccccccbbbb   aaaabbbbbcddddddddddddddddd
+dddd";
my @listedstring= split//,$string;
my $prev=shift @listedstring;
my $currstart=my $index=0;
for (@listedstring) {
  if ($_ eq $prev) {
    $index++;
  } else {
  print "('$prev',$currstart,".($index-$currstart+1).")\n";
  $currstart=++$index;
  $prev=$_;
  }
}
print "('$prev',$currstart,".($index-$currstart+1).")\n";
[download]

Interesting problem, BrowserUk!

CU
Robartes-