读取txt正则匹配行写入txt

StreamReader sr = new StreamReader("C:\Users\Administrator\Desktop\blogbbs\dd.txt",Encoding.Default);
            StringBuilder sb = new StringBuilder();
            string s = "";
            string patt = "【(.+?)】";
            Regex r = new Regex(patt);
            while((s = sr.ReadLine()) != null)
            {
                if (r.IsMatch(s))
                {
                    Match m = r.Match(s);
                    sb.AppendLine(m.Groups[1].Value);
                }
            }
            File.AppendAllText("C:\Users\Administrator\Desktop\blogbbs\123.txt",sb.ToString(),Encoding.Default);