StreamReader reader = new StreamReader("c:\\1.txt",Encoding.Default);
            string source = reader.ReadToEnd();
            Regex reg = new Regex(@"(?is)((?<=<a href=[^>]*?).*?(?=</a>))|((?<=<img src=[^>]*?).*?(?= />))");
            MatchCollection mc = reg.Matches(source);
            foreach (Match m in mc)
            {
                MessageBox.Show(m.Value);
            }

------解决方案--------------------
string str = "【共同社3月17日电】日本自民党总裁谷垣祯一在17日下午的记者会上表示，鉴于福岛第一核电站事故，今后核电站选址将变得非常困难，核能政策将难<span><a href=\"/UserDir/Documents/太阳能09台湾.pdf\">/UserDir/Documents/太阳能09台湾.pdf</a></span>实施。他同时暗示将修改“增建核电站”的党内方针。　　谷垣祯一在记者会上考虑到核能发电已成主要电力来源一事强调，<span><img src=\"http://localhost:12988/UserDir/Images/0865b5181b7beb3b35fa41a9.jpg\" /></span>";

       var match = Regex.Match(str, @"(?<=<a[^>]+href=[""'])([^""']+)(?=[""'])[\s\S]+(?<=<\s?img\s*src=[""'])([^""']+)");
       Response.WriteLine(match.Groups[1].Value);//    /UserDir/Documents/太阳能09台湾.pdf
       Response.WriteLine(match.Groups[2].Value);//    http://localhost:12988/UserDir/Images/0865b5181b7beb3b35fa41a9.jpg