Prevent users from entering in special characters (e.g. &,*,%, -, etc.)

Hello, The form that I’m creating will interface with a back end system. That back end system doesn’t allow certain characters (i.e &, *, -, etc.). How can I stop a user from entering such characters in First name and Last Name fields?

You would need to use some JavaScript to prevent their entry on the front end, or, you can strip them out or return a validation error after the information is submitted if you like, using the gform_field_validation filter: