Extensions » Modules » Form Builder
Form Tools allows you to add field validation through the Edit Form » Fields tab. There, click the edit icon for a row, and in the dialog window that appears, select the third "Validation" tab. There, you can specify whatever validation you want for the field. The available list of rules depends on the field type.
The Form Builder automatically recognizes whatever validation rules you have defined for your fields and adds them to your published forms.
Fields that are marked as "required" are special: an asterix appears next to the field label to indicate to the user that they must be entered. Other validation types, like "valid email", "digits only" don't show the asterix and only get executed if the fields have a value in them.
There are currently two limitations/weaknesses to the field validation.
1. Client- vs. Server-side validation
This is currently a limitation with the Form Tools Core and will be updated in a future version (no version or date known just yet).
2. Interface The interface for inputting and managing the field validation (in the Core) is pretty weak right now. Like the previous point, we intend to update this in a future version of the Core. Again, no version of date is known just yet.