Sure, if performance isn't an issue (e.g. it might need to be done per keystroke). Small amounts of logic duplication like that can be annoying though, esp. when inevitably they end up not agreeing and users don't understand why they're being told their input is invalid. One option I've used is to have the server precompute all possible outputs for all possible inputs, it can work pretty well even with 10s of 1000s of entries.