@PraveenkumarR3390

You should create an Edit validate rule that fits your requirements. You can use something like this:  

for (int pos = 0, len = 2; pos < len; ++pos) {
	if (! Character.isLetterOrDigit(theValue.charAt(pos)))
         {
		return false;
	}
}
return true;

Then you have to add your newly created Edit Validate rule to the property VehicleRegistrationNumber in Advance tab > Use validate.