Using Xpath to find a user defined keyword

I am using the following code to find a user defined Keyword anywhere in the entire document . 

 XPathDocument dx = new XPathDocument(XMLURL);
 XPathNavigator nav = dx.CreateNavigator();

nav.Select("//"+keyword+"*")  // "keyword" is the user-define keyword

But it's not working, is there something wrong with above Xpath expression?