i was also going to suggest that maybe refactoring may be possible if more context is known. index or straight regex can be used in a loop to traverse a string, don't know if more efficient or not. using index function with each letter (alphabetically) and incrementing the start index, could also be worth a try, but only if you're ok to traverse the string this way. there's also databases with bitmap indexes, which would allow very fast access to (pre-fetched cache) of particular sequences. but all depends on the semantics of your larger goal. a question that comes to mind, why is each protein sequence processed 20 times?