正则表达式取<img src="">src中包含特定字符的地址

string str = @"<img src=""http://img.cn.com/a/latest.gif"" /> <img src=""http://img.cn.com/topics/latest.gif"" /> <img src=""http://img.cn.com/b/free/latest.gif"" /> <img src=""http://img.cn.com/topics/latest.gif"" /> <img src=""http://img.cn.com/main/c/latest.gif"" />"; Regex reg = new Regex(@"(?is)<img[^>]*?src=(['""\s]?)((?:(?!topics)[^'""\s])*)\1[^>]*?>");foreach (Match m in reg.Matches(str)) {     Response.Write(m.Groups[2].Value + "<br/>"); }/* http://img.cn.com/a/latest.gif http://img.cn.com/b/free/latest.gifhttp://img.cn.com/main/c/latest.gif */