You told us that if the "." is not the first character, one should look back. Are you saying that's wrong?

If you actually want to look to the other side of the adjacent Ms, 

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;
'

[download]

In reply to Re^5: How to make these reg exp changes? by ikegami
in thread How to make these reg exp changes? by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.