string a = "<a   id=\"t1\"   class=link_blue   href=\"http://article.ednchina.com/Commu/20071219071733.htm\"> Infonetics：WiMAX设备销售额上升势头增强 </a> ";
            string reValue = "1212";
            string reValue1 = "CSDN";

            string regex1 = "(?<=<a[\\s\\S]*?href=\")[^\"']*?(?=\"[^>]*?>)";
            string regex2 = "(?<=<a[^>]+?>)[^<]*?(?=\\</a>)";
            string result = Regex.Replace(a, regex1,reValue);
            result = Regex.Replace(result, regex2, reValue1);

            MessageBox.Show(result);

------解决方案--------------------

给要捕获的组命名就是了：

C# code

result = Regex.Replace(result, @"(?<text1><a(?=[^<>]*?id=""t1"").*?href="")[^""]*(?<text2>""[^>]*>)[\s\S]*?(?=</a>)", string.Format("${text1}{0}${text2}{1}", textBox12.Text, textBox11.Text), RegexOptions.IgnoreCase);