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"));
}