正则替换内容中图片的src

string test = "<IMG src=\"http://www.baidu.com/upload/2009_11/09112110144808.jpg\" border=0><IMG src=\"upload/2009_11/09112110144808.jpg\" border=0><img alt=\"\" src=\"/upload/201510/28/201510281002087002.png\">";
            string outhost = "www.myname.com";
            Regex reg = new Regex(@"(?is)(?<=<img\s+alt=.*src=(['""]?))(?!http://)(?=[^'""\s>]+\1)");
            string result = reg.Replace(test, "http://" + outhost + "/");

   string test = "<IMG src=\"http://www.baidu.com/upload/2009_11/09112110144808.jpg\" border=0><IMG src=\"upload/2009_11/09112110144808.jpg\" border=0>";
            string outhost = "www.myname.com";
            Regex reg = new Regex(@"(?is)(?<=<img\s+src=(['""]?))(?!http://)(?=[^'""\s>]+\1)");
            string result = reg.Replace(test, "http://" + outhost + "/");