$seq=UUUUUUUUIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO

##</code><code>##

if($seq=~/^(U+)([I|O|P|B|M])/) 
{
 $part_to_change1=$1;
 $len1=length($part_to_change1);
 $char1=$2;
 substr($top, 0, $len1, ($char1 x $len1));			 
 }

##</code><code>##

$seq=IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOUUUUUUUU

##</code><code>##

if($seq=~/.*?([I|O])(U+)$/) 
{ 
  $char2=$1;
  $part_to_change2=$2;
  $len2=length($part_to_change2);
  substr($top, -$len2, $len2, ($char2 x $len2));			 
}

##</code><code>##

* $seq=IIIIIIIIIIIIIIIMMMMMMMMMMMUUUUUUUUMMMMMMMMMMMMMOOOO
* $seq=IIIIIIIUUUUUIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUUUUUUOO
* $seq=IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUOO