r =  _bson_append (bson,
        5, // Number of data elements to add
        (1 + key_length + 1 + regex_len + options_sorted->len), // bytes to add
        1,          // length of first element (BSON type, int8)
        &type,      // the BSON type
        key_length, // the length of the field name
        key,        // the field name (not 0-termined)
        1,          // the length of the ending 0 byte
        &gZero,     // the null 0 byte
        regex_len,  // the length of the regular expression (including 0 byte)
        regex,      // the regular expression with 0 byte
        options_sorted->len + 1, // the length of the sorted options, with 0 byte
        options_sorted->str);    // the sorted options, with 0 byte