welcome dear!!! keep it up... :)

This error occurred when your dropdown is already filled and if you try to assign (selects) that value which was selected previously. so can you provide your code?

Hi, you can use dropdown selected index changed event on dropdownlist. means if your regular expression validator id is regular1 then just false its visibility from server side as reqular1.Visible=false; try it. this can also done by Jquery.