using System;
using System.Text.RegularExpressions;

namespace FilterOut
{
    class Program
    {
        static void Main(string[] args)
        {
            Validate("valid_strig_123");
            Validate("I think not!");
        }

        static void Validate(string s)
        {
            var re = new Regex(@"[^a-z0-9_]", RegexOptions.IgnoreCase);
            if (re.IsMatch(s))
                Console.WriteLine("Nope");
            else
                Console.WriteLine("OK");
        }
    }
}