string result=Regex.Replace(imgHtml,@"<img([^<]*)src=\"(?!(http))(.*?)\"([^<]*)>",@"<img$1src=\"http://www.xxx.com/$3\"$4>");
Regex r = new Regex("src=\"");
r.Replace(@"<img src=""logo.jpg"">", "src=\"http://www.xxx.com/");
Ctrl+H 查找替换-_-