asp.net字符串转换 123456789转换为12-345-6789的3种方法

string a = "123456789";
a = int.Parse(a).ToString("##-###-####");
Console.Write(a);

string a = "123456789";
a = a.Insert(5, "-").Insert(2, "-");
Console.Write(a);

using System;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;
namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            string a = "123456789"; // www.jb200.com

            Regex reg = new Regex(@"^(d{2})(d{3})(d{4})$");
            a = reg.Replace(a, "$1-$2-$3");
            Console.Write(a);
        }       
    }
}