using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

         const string RegexSpec_LettersDigitsUnderscores = @"^[a-zA-Z_][a-zA-Z0-9_]$";

         var rgx = new Regex(RegexSpec_LettersDigitsUnderscores);

         Console.WriteLine(rgx.IsMatch("w%"));

         Console.WriteLine(rgx.IsMatch("%w"));

        Console.WriteLine(rgx.IsMatch("www22345"));

    }

}