#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11140181
use warnings;

my %h = qw( bU i aU o bD o aD i );

while( <DATA> )
  {
  print "\n$_" . <DATA>;
  /U|D/ or tr/s/i/;
  s/(s*)(U|D)(s*)/$h{"b$2"} x length($1) . 'M' . $h{"a$2"}x length($3)
+ /ge;
  print;
  }

__DATA__
sssss
iiiii
sssUUss
iiiMMoo
sssDDss
oooMMii
sssssDDDDDDDssssUUUss
oooooMMMMMMMiiiiMMMoo
sssssUUUUUUUssssDDDss
iiiiiMMMMMMMooooMMMii
ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss
+sssssssssssssssssssssssssssssssssssssssssssssssss
???
ssDDDDDDDDDDDDDDDDDDDDDDssssssssDDDDDDDDDDssssssssDDDDDssssssssssssssD
+DDDDDDDDDDDDDDDsDDDDDDDDDssssssssssssssUUUUUUUUUs
???
[download]

Outputs (input/expected/actual):

sssss
iiiii
iiiii

sssUUss
iiiMMoo
iiiMMoo

sssDDss
oooMMii
oooMMii

sssssDDDDDDDssssUUUss
oooooMMMMMMMiiiiMMMoo
oooooMMMMMMMiiiiMMMoo

sssssUUUUUUUssssDDDss
iiiiiMMMMMMMooooMMMii
iiiiiMMMMMMMooooMMMii

ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss
+sssssssssssssssssssssssssssssssssssssssssssssssss
???
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
+iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

ssDDDDDDDDDDDDDDDDDDDDDDssssssssDDDDDDDDDDssssssssDDDDDssssssssssssssD
+DDDDDDDDDDDDDDDsDDDDDDDDDssssssssssssssUUUUUUUUUs
???
ooMMMMMMMMMMMMMMMMMMMMMMiiiiiiiiMMMMMMMMMMiiiiiiiiMMMMMiiiiiiiiiiiiiiM
+MMMMMMMMMMMMMMMiMMMMMMMMMiiiiiiiiiiiiiiMMMMMMMMMo
[download]

??? where no expected output was provided.