namespace ---> System.Text.RegularExpressions.

        static void Main(string[] args)
        {
            // if (IsInputMatchesNumber())
            if (IsInputMatchesNumberByRegx())
            {
                Console.WriteLine("Input charectors are all numbers.");
            }
            else
            {
                Console.WriteLine("Input charectors are not pure numbers.");
            }
        }

        //Common way to judge whether a string is pure numbers or not
        static bool IsInputMatchesNumber()
        {
            Console.Write("Please input your password: ");
            string str = Console.ReadLine();
            bool isMatch = true;
            for (int i = 0; i < str.Length; i++)
            {
                if (str[i] < ‘0‘ || str[i] > ‘9‘)
                {
                    isMatch = false;
                    break;
                }
            }
            return isMatch;
        }


        //Use regular expressions to judge, result is the same as above 
        static bool IsInputMatchesNumberByRegx()
        {
            Console.Write("Please input your password: ");
            string str = Console.ReadLine();
            //Regular expression always come with @
            // @  means "do not convert \ in string"
            // ^  means "start from"
            // $  means "end at"
            // *  means "has any"
            // \d means "number"
            string pattern = @"^\d*$";
            return Regex.IsMatch(str, pattern);
        }

C#正则表达式基础