using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var regex = new Regex("^(?!0)\\d+$");

        Console.WriteLine("0: " + regex.IsMatch("0"));

        Console.WriteLine("00: " + regex.IsMatch("00"));

        Console.WriteLine("0100: " + regex.IsMatch("0100"));

        Console.WriteLine("100: " + regex.IsMatch("100"));

    }

}