Validating textbox input based off selection from dropdown on web form - C#

I need to require a user to first input their ID Type (in a dropdown list) and then input their ID in a text box. What is the best way to go about validating the user ID if the rules for validation will be different based off of what type is selected.  I have seen ways with javascript and customvalidator(s) and jquery, and others. I cannot seem to find a way that I can get to function.
<!-- My Dropdown Example --><asp:DropDownList ID="IDType" CssClass="dropboxtext" runat="server" AutoPostBack="true">                     <asp:L ...