string tempStr = @"<a href=""xw_read129""></a>";
                string pattern = @"(?i)(?<=<a[^>]*?href=(['""]?)[^'""]*?xw_read[^'""]*)(?=(['""]?)>)";
                string result = Regex.Replace(tempStr, pattern, ".html");//<a href=\"xw_read123.html\"></a>

------解决方案--------------------

C# code
html=Regex.Replace(html,@"(?i)(?<=<a\b[^>]*?href=(['""]?)xw_read\d+)(?=\1[^>]*?>[^<>]*</a>",".html");

------解决方案--------------------

C# code

        private string ReplaceStr(Match m)
        {
            return m.Value+".html";
        }
        private void button10_Click(object sender, EventArgs e)
        {
            StreamReader reader = new StreamReader("c:\\1.txt");
            string source = reader.ReadToEnd();
            Regex reg = new Regex(@"(?is)(?<=<a\s?href[^>].*?)xw_read[\d]+");
            source = reg.Replace(source, ReplaceStr);
        }

------解决方案--------------------
C# code
            string str = "<a href=\"xwread/xw_read1213\"></a>";
            string result = Regex.Replace(str, @"(?is)(?<=<a[^>]*?href=(['""\s]?).*?xw_read\d+)(?=\1)", ".html");
            Console.WriteLine(result);