static void Main(string[] args)
        {
            /*
             * 命题：计算 加 减 乘 除，不含括号
             */
            var str = "8*8*8-100*2";
            //计算乘法
            str = Calc(str, '*');
            str = Calc(str, '/');
            str = Calc(str, '+');
            str = Calc(str, '-');
            Console.WriteLine(str);
            Console.Read();
        }
        private static string Calc(string exp,char sign)
        {
            var r = new Regex(string.Format(@"(\d+)\{0}(\d+)", sign));
            var exps = r.Matches(exp);
            if (exps.Count == 0) return exp;
            exp = exps.Cast<Match>().Aggregate(exp, (current, p) => current.Replace(p.ToString(), Operate(p.ToString(), sign).ToString()));
            exp = Calc(exp, sign);
            return exp;
        }
        private static double Operate(string exp,char sign)
        {
            var arr = exp.Split(sign);
            double result = 0;
            var numA = NumberHelper.FormatDouble(arr[0]);
            var numB = NumberHelper.FormatDouble(arr[1]);
            switch (sign)
            {
                case '+':
                    result = numA + numB;
                    break;
                case '-':
                    result = numA - numB;
                    break;
                case '*':
                    result = numA * numB;
                    break;
                case '/':
                    result = numB != 0 ? numA/numB : 0;
                    break;
            }
            return result;
        }