Code:
            Regex regex = new Regex(@"^(\s)*(\d ){6}\d(\s)*$");//note there's a blank space after the first '\d'
           bool isValid = true;
            string[] ts = textBox1.Text.Split(new string[]{"\r\n"}, StringSplitOptions.RemoveEmptyEntries);
            if (ts == null || ts.Length < 1)
            {
                isValid = false;
            }
            else
            {
                foreach (string t in ts)
                {
                    if (regex.IsMatch(t) == false)
                    {
                        isValid = false;
                        break;
                    }
                }
            }
            if (isValid)
            {
                  //do sth.
            }