User Profile Field Validation

[bigrover]

I need to be able to validate a user profile field that has been defined as always required and editable only at registration. I want to verify the value is unique across users and that it does not match any other username. I also want the field to follow the minimum and maximum username length requirements. Lastly, I want the system to report the error and return to the registration for the user to correct the entry.

As an added plus, it would be nice if the client-side javascript would validate the field on the fly, just like the username field during registration.