using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Regex r = new Regex(@"^(?!Mozilla\s?\/)", RegexOptions.IgnoreCase | RegexOptions.Compiled);

        string aMozilla = "aMozilla/5.0 a b c";

        string mozilla = "Mozilla/5.0 a b c";

        Console.WriteLine("aMozilla match: " + r.IsMatch(aMozilla));

        Console.WriteLine("mozilla match: " + r.IsMatch(mozilla));

    }

}