List<string> strlist = new List<string>();

            strlist.Add("1asdf");
            strlist.Add("2asdf");
            strlist.Add("easdf");
            strlist.Add("tasdf");

            var list = from l in strlist
                       where ASCIIEncoding.ASCII.GetBytes(l.ToString().Substring(0,1))[0] >= 48 && ASCIIEncoding.ASCII.GetBytes(l.ToString().Substring(0,1))[0] <= 57
                       select l;

            foreach(string str in list)
            {
                Console.WriteLine(str);
            }