Might improve user experience a bit, but your password hashing scheme needs to take much longer if an attacker knows he can rule out lower-case letters.
For example, 8-character alphanumeric passwords:
((26*2)+10)^8 / (26+10)^8 = 77.4
You have 77 times fewer passwords of length 8 (probably worse than that, most people skew more towards letters than numbers), so it should take 77 times longer to test one.
For example, 8-character alphanumeric passwords:
((26*2)+10)^8 / (26+10)^8 = 77.4
You have 77 times fewer passwords of length 8 (probably worse than that, most people skew more towards letters than numbers), so it should take 77 times longer to test one.