I have tried this regEx.
Its working fine with all your test cases.
static void Main(string[] args)
{
string text = Console.ReadLine();
Match match = Regex.Match(text, "\\bGOBACK\\b[\\.\\s]?", RegexOptions.IgnoreCase);
if (match.Success)
{
Console.WriteLine("success");
}
else
{
Console.WriteLine("Failure");
}
Console.ReadLine();
}