$string='III...MMMMMOOOO....MMMIIIII';

##</code><code>##

III...MMMMMOOOO  --> IIIIIIMMMMMOOOO

OOOO....MMMIIIII --> OOOOOOOOMMMIIIII

##</code><code>##

$string=~s/\.+M+i+/oMi/g;

##</code><code>##

III...MMMMMOOOOoMi