Field validation before submitting

Hi. I am building a website which is accessible only for members. The users has to register first to acccess the content and they also need to fill up a field in the registration form, which checks for user id’s in a database. I need a form field with validation (the user has to enter a valid user id. If he enters a wrong value, he/she can’t submit the registration form). Is this possible using Pie Register? Thanks!