You can apply validation rules inside your forms using external web pages.
The system can call an external web page and pass the action (preupdate, preinsert, predelete) and the list of the field name/ field value pairs.
This web page should be declared in the Options tab, inside the "Before Update Post Url" property. Absolute url is required.
The final Uri should be something like :
If you want to apply validation rules, you could define a predefined prefix (property Passed Validation Prefix) and inside your validation page, return a predefined string that begins with that prefix if no validation error occurs.
Otherwise, the system will cancel the Update and show the returned error message to the user.
NOTE: If the url is external and in a different domain with .xap file, you will have to place the clientaccesspolicy.xml file in site root folder as shown in the instructions Here.