using System.Text.RegularExpressions;
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"));