What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Unfortunately there is no such functionality for a field on the form. The most that you can do for validation is to use the Input Mask option of the field.
I can see on your form https://www.jotform.com/70044448241651 that you have already done this.
There is though another field that have validation feature. It is the Email Validator widget
Hope this help. Let us know how we can be of further help.