How can I make it so that new users can include uppercase in their username?

I've received a couple of complaints from people trying to register, but they're being restricted in their username because it demands all lowercase letters. Is there a simple way to make this include uppercase as well?
Alternately, is there a way to include more specific instructions so that registrants know that lowercase is demanded? In these cases the registrants received the message that they couldn't include punctuation marks, but it doesn't say anything about upper/lowercase in the explanation. Seems like a good way to lose registrants.
Any help appreciated.