Control of ID username field

currenlty anyone could type anything in these fields. What would it take to add some smarts that prevents progress to the next page unless the field contains only a numerical value of 6 digits? Could we also rule out certain numbers patterns ie. (123456) (000000) (666666) ?

Currently, there is no validation done on the ID field. I think the only way to add this would be to add a page before the test that prompts for the user id, validates it and then passes it into the test as a parameter where it is valid.