using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string uri = @"Selection=(AreaId%3d*%27S0304%27%26PersonId%3d%27197803140412%27)&Selection=(AreaId%3D*'S0304')&SelectionPanelVisible=True&SelectionSubmitted=true";

        var regex = new Regex(Regex.Escape(@"Selection=(AreaId%3d*%27S0304%27%26PersonId%3d%27197803140412%27)"));

        uri = regex.Replace(uri, string.Empty, 1);

        Console.WriteLine(uri);

    }

}