Custom validation question for a text entry box

I have a custom validation question for a text entry box.

We expect to have thousands of user’s access this survey and we are wanting to validate that they are inputting their 20 character ID properly by matching it with a database or csv file that has existing User IDs. (e.g. We want to make sure the ID exists already in our database (sql) or csv file before the user may proceed.) What would be the easiest way to implement this validation?

Thanks in advance!