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)(?<=<imgs+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)(?<=<imgs+src=(['""]?))(?!http://)(?=[^'""s>]+1)");
            string result = reg.Replace(test, "http://" + outhost + "/");