StreamReader reader = new StreamReader("e:\\1.txt");
            string content = reader.ReadToEnd();
            System.Text.RegularExpressions.Regex reg = new System.Text.RegularExpressions.Regex(@"(?<=<a href=""[^""]*?)[^""]+(?="")");
            System.Text.RegularExpressions.MatchCollection ms = reg.Matches(content);
            foreach (System.Text.RegularExpressions.Match m in ms)
            {
                MessageBox.Show(m.Value);
            }

------解决方案--------------------

(?i)<a\b[^>]*href=(["'\s]?)(.*?)\1[^>]*>
取第二个分组，Groups[2].Value