> Not that you cannot see s+D+s+D+s+ sequence, or s+U+s+U+s+. D and U alternate.

If you mean "Note" by "Not", than your sample input is invalid, as both the sequences contain sDsDs.

Also, I think the following works correctly even for the non-alternating sequences in the way you originally showed: 

my %before = (U => 'I', D => 'O');
my %after; @after{ keys %before } = reverse values %before;
sub subst {
    local ($_) = @_;
    1 while s/(s+)(?=([DU]))/$before{$2} x length $1/e
          | s/(?<=([DU]))(s+)/$after{$1} x length $2/e;

    tr/DU/MM/ or tr/s/I/;
    return $_
}

[download]
It uses the bitwise or that doesn't short circuit to try both the substitutions every time while there's anything to replace.

Tested against:

use Test::More;

my %simple = (ss => 'II',
              ssUUss => 'IIMMOO',
              ssDDss => 'OOMMII',
              ssDDssUUss => 'OOMMIIMMOO',
              ssUUssDDss => 'IIMMOOMMII');

for my $input (sort keys %simple) {
    is subst($input), $simple{$input}, $input;
}

[download]

Might need some tweaking if the specification changes.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re^5: Pattern matching simultaneous substitution by choroba
in thread Pattern matching simultaneous substitution by Anonymous Monk

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