I don't know if I'm doing something wrong in my quest, but I'm trying to work around some registration page behavior.

You can of course establish limits to what is entered into a given field. For example, you can limit a username to be between 6 and 16 characters, and contain no spaces.

If I have a registration form with multiple fields, and an individual enters an invalid username for example, when he or she submits the form, they are warned that their entry is invalid, but everything else that they entered in the registration form is lost, and they have to start again.

Is there a way for an error check before the form is closed so that individual fields can be corrected by the user before the full form is submitted for processing?