"custom" validation of form fields

Is there a way to use Javascript or something to create customized data validation of form entries?

I am sorry, but we do not allow embedding any JavaScript into our forms. In order to add special JavaScript into our forms, you would need to download the source code of your form and add your scripts into it manually.

If you are looking for a way to ensure that your users are entering a specifically crafted code, such as a license plate or a similar predictable combination of letters and numbers, you can use an Input Mask to control what can be entered:

I hope this helps. Please let us know if you need further assistance.