Validating Field Content
You can provide runtime validation for user input into a text field and rule field via the validator element (which is a child of the field element. There are two types of built-in validators already provided: a NotEmptyValidator and a RegularExpressionValidator. You can also easily create your own validator. In all cases, if the chosen validator returns false, a messagebox will display the contents of the txt attribute and the user will be unable to continue to the next panel.