jeanluca, we can better serve you if you tell us why you want to do this. Perhaps substr is the better solution, perhaps a regex is. Perhaps just plain old concatenation is the answer. Won't you take a little time to tell us why you need to do what you need to do instead of how to solve what you think you need to solve? Thanks.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)