Ok, my apologies - dyslexia has indeed caused me to make some mistakes in my desired output, as some have suspected.
Also, a few words of further explanation: I decided to use Roman Numerals because (a) it made for much shorter examples than my "real" data, and (b) I thought the "order" would be implicit (M = 1000 and comes "first", D = 500 and comes "second", etc.) I should definitely have explcitly stated why this is the "order" in which characters should be considered.
I am taking another look at my "desired" output to insure the dyslexia demons have been banished. Then I shall take a look at the proposed solutions and report back.
Thanks again for your wisdom and, of course, your patience, kind Monks!