perl -wle'
    my %neg = ( O => "I", I => "O" );

    $_ = "III...MMMMMOOOO....MMMIIII";
    print;
    s/([IO])(M+)(\.+)/ $1.$2.( $neg{$1} x length($3) ) /eg;
    s/(\.+)(?=M+([IO]))/ $neg{$2} x length($1) /eg;
    print;
'