Another way to handle the enabling and disabling of controls is to add a record to the ControlCondition Table. Identify the dialog and control, set the action (Default - set as default control, Enable, Disable, Hide, Show), and then enter a conditional that describes how you want the control to behave.
You can easily eliminate blank entries in the conditional statement. Spaces are legitimate characters, so they will satisfy simple conditionals. For more complex testing, I suspect you would need to launch a custom action for field validation.
Please note that Leigh's comment applies here, too. The conditional is not evaluated until the edit control loses the focus, so you will need to have another control to accept the focus in order to get the 'disabled' control to be 'enabled'.