Processing selected text in an asp:textbox control

In a windows forms application I used txtExpression.SelectionStartand txtExpression.SelectedText etc. methods of a typical TextBox control.  In my ASP.NET <asp:textbox /> control those methods do not exist.  I've tried using javascript and jquery in Visual Studio 2012 Ultimate to no avail (yet).  My questions, why is this so difficult and how do I get/set just the selected text as well as programatically set the selected text in an ASP.NET environment?  I mean its obviously possible but I have yet to come accross any web forum documentation on how this is done and it is not obvious (to me).