how to get some info inside string

hello assume that i have the following string :

string myString="<div id='mydiv' style='background-image:url('http://myhost.coom/uploads/paper.gif');'>some text bla bla  .... </div>";
and my target is to extract the background image url from that string , by server side (C#.net)  , then add runat='server' to that div to be as follow :

"<div runat='server' id='mydiv' style='background-image:url('http://myhost.coom/uploads/paper.gif');'>some text bla bla  .... </div>";

please note : background-image may be set to null , so i want to check if its contain url.
thanks