$ perl -wle'
    my %neg = ( o => "i", i => "o" );

    while (<>) {
        chomp;
        s/([io])(M*)(\.+)/ $1.$2.( $neg{$1} x length($3) ) /eg;
        s/(\.+)(?=M*([io]))/ $neg{$2} x length($1) /eg;
        print;
    }
'
....MMMMiiii
....MMMMoooo
iiiiMMMM....
ooooMMMM....

ooooMMMMiiii
iiiiMMMMoooo
iiiiMMMMoooo
ooooMMMMiiii

[download]

Simpler version that only works with Perl 5.10+: 

$ perl -wle'
    my %neg = ( o => "i", i => "o" );

    while (<>) {
        chomp;
        s/([io])M*\K(\.+)/   $neg{$1} x length($2) /eg;
        s/(\.+)(?=M*([io]))/ $neg{$2} x length($1) /eg;
        print;
    }
'
....MMMMiiii
....MMMMoooo
iiiiMMMM....
ooooMMMM....

ooooMMMMiiii
iiiiMMMMoooo
iiiiMMMMoooo
ooooMMMMiiii

[download]

I could have used -p above, but I wanted to make clear that %neg only needs to be initialized once without complicating the issue by adding a BEGIN. Here's the 5.10+ version using -p: 

perl -ple'
    BEGIN { %neg = qw( i o o i ) }
    s/([io])M*\K(\.+)/   $neg{$1} x length($2) /eg;
    s/(\.+)(?=M*([io]))/ $neg{$2} x length($1) /eg;
'

[download]

Update: Added \K version. \K rocks.
Update: Added explanation about -p.

In reply to Re^2: How to make these reg exp changes? by ikegami
in thread How to make these reg exp changes? by Anonymous Monk

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