yes, I'm guessing it filters by class first as well. but it would be interesting to find out if just updating all the timestamps would be as fast (or faster) than doing the secondary regex filter anyway? i'm not sure, i haven't run any benchmarks on it.
Or, another alternative, add a secondary class so that the overall markup is class="_timestamp _timestamp_group0", etc. Perhaps they tested this and found it to be a negligible improvement, but then you're only selecting on the one class, rather than selecting on class and filtering.
