RegEX for Usernames...

Trying to settle on a proper syntax to be used during user registration to insure only lowercase, no spaces, and at least 1 digit for the usernames,

I'm incorporating Moodle into my system and the Joomdle Integration for its important to insure this at registration. I know the expression is used in the workflow area, I just dont know how to rewrite the default to match the criteria I need.

Thanks!