code: def _make_c_string_check(string):
      
      if isinstance(string, bytes):
          if b"\x00" in string:
              raise InvalidDocument("BSON keys / regex patterns must not "
                                    "contain a NUL character")
          try:
              _utf_8_decode(string, None, True)
              return string + b"\x00"
          except UnicodeError:
              raise InvalidStringData("strings in documents must be valid "
                                      "UTF-8: %r" % string)
      else:
          if "\x00" in string:
              raise InvalidDocument("BSON keys / regex patterns must not "
                                    "contain a NUL character")
          return _utf_8_encode(string)[0] + b"\x00"
docs: Make a 'C' string, checking for embedded NUL characters.