private void WriteBson(BsonWriter writer, Regex regex)
        {
            // Regular expression - The first cstring is the regex pattern, the second
            // is the regex options string. Options are identified by characters, which 
            // must be stored in alphabetical order. Valid options are 'i' for case 
            // insensitive matching, 'm' for multiline matching, 'x' for verbose mode, 
            // 'l' to make \w, \W, etc. locale dependent, 's' for dotall mode 
            // ('.' matches everything), and 'u' to make \w, \W, etc. match unicode.

            string options = null;

            if (HasFlag(regex.Options, RegexOptions.IgnoreCase))
                options += "i";

            if (HasFlag(regex.Options, RegexOptions.Multiline))
                options += "m";

            if (HasFlag(regex.Options, RegexOptions.Singleline))
                options += "s";

            options += "u";

            if (HasFlag(regex.Options, RegexOptions.ExplicitCapture))
                options += "x";

            writer.WriteRegex(regex.ToString(), options);
        }
        public void WriteRegexPlusContent()
        {
            MemoryStream ms = new MemoryStream();
            BsonWriter writer = new BsonWriter(ms);

            writer.WriteStartObject();
            writer.WritePropertyName("regex");
            writer.WriteRegex("abc", "i");
            writer.WritePropertyName("test");
            writer.WriteRegex(string.Empty, null);
            writer.WriteEndObject();

            byte[] expected = HexToBytes("1A-00-00-00-0B-72-65-67-65-78-00-61-62-63-00-69-00-0B-74-65-73-74-00-00-00-00");

            Assert.Equal(expected, ms.ToArray());
        }