Regex Expressions will allow you to restrict the users to input any value that you don't want them to input.
For example, the email field has a regex expression that checks for the field to have a value/pattern that is actually an email.
You can use your own expressions with our extension for any form field.
To use an expression, you can use regex101.com where you can easily:
Create your expressions
Test your expressions
Access the library for quick use