Thanks for your suggestions! Will take a closer look at each of them.

In fact, I'd definitely not want to take the exakt number of character repetitions into account. My example simplifies the sort of strings I really have to deal with but think of S as a sample of input strings already received and of R as the "types" of input strings we can (most likely) ever expect. So it would be completely fine to categorize into a's, b's, and c's.