#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11146755
use warnings;

while( <DATA> )
  {
  chomp;
  print "\n$_\n";

  s/^(U+)/ 'I' x length $1 /e;
  s/(U+)$/ 'O' x length $1 /e;
  s/(?<=([^U]))(U+)(?=\1)/ $1 x length $2 /ge;

  print "$_\n";
  }

__DATA__
UUUUUUUUIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO
IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOUUUUUUUU
IIIIIIIIIIIIIIIMMMMMMMMMMMUUUUUUUUMMMMMMMMMMMMMOOOO
IIIIIIIUUUUUIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUUUUUUOO
IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUOO
IUIUIUIUIUIUIUIUI
[download]

Outputs:

UUUUUUUUIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO
IIIIIIIIIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO

IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOUUUUUUUU
IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOOOOOOOOO

IIIIIIIIIIIIIIIMMMMMMMMMMMUUUUUUUUMMMMMMMMMMMMMOOOO
IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMOOOO

IIIIIIIUUUUUIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUUUUUUOO
IIIIIIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOOOOOOO

IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUOO
IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOO

IUIUIUIUIUIUIUIUI
IIIIIIIIIIIIIIIII
[download]