My example will give you the start and end positions without using regex. I did some testing, and it appears that this solution runs much slower than the regex solution. But it's just to show that there is more than one way to do it :

#!/usr/bin/perl

use strict;
use warnings;

my @STARTPOS = ();
my @ENDPOS = ();
my $str =   "BaaaaBBBBBBaaaaaaaBBBBBBBBBBBBBBBBBBBBaaaBBBBBBBBBBBxB";
print "$str\n\n";

my $i = 0;
while (($i = index($str, 'B', $i)) >= 0)
{
  push(@STARTPOS, $i++);
  while (vec($str, $i++, 8) == 66) {}
  push(@ENDPOS, $i-1);
}


# Display results:
for (my $i = 0; $i < @STARTPOS; $i++)
{
  print "\nstart: ", $STARTPOS[$i], "\t->  end: ", $ENDPOS[$i];
}
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