Best How To :

This is a job for Regix. The following should do what you need:

        string message1 = "Google.Apis.Requests.RequestError.Entity already exists. "
                          +"[409]. Errors [. Message[Entity already exists.] Location [ - ] Reason [duplicate] Domain [global] .]";
        string message2 = "Google.Apis.Requests.RequestError.Domain user limit reached. "
                          +"[412]. Errors [. Message[Domain user limit reached.] Location [If-Match - header] Reason[limitExceeded] Domain[global] .]";

        string pattern = @"Message\[((\w+\s){2,}(\w+\s?)*)\.\]";

        Regex regex = new Regex(pattern);

        Match m = regex.Match(message1); //or regex.Match(message2)
        if (m.Success)
        {
            Group g = m.Groups[1]; //m.Groups[0] will Match 'Message[.....]'
            CaptureCollection cc = g.Captures;

            for (int i = 0; i < cc.Count; i++)
            {
                Capture c = cc[i];
                Console.WriteLine("Message: {0}", c);
            }
        }

        Console.ReadLine();