docs: def _encode_regex(name, value, dummy0, dummy1):
      """Encode a python regex or bson.regex.Regex."""
      flags = value.flags
      # Python 2 common case
      if flags == 0:
          return b"\x0B" + name + _make_c_string_check(value.pattern) + b"\x00"
      # Python 3 common case
      elif flags == re.UNICODE:
          return b"\x0B" + name + _make_c_string_check(value.pattern) + b"u\x00"
      else:
          sflags = b""
          if flags & re.IGNORECASE:
              sflags += b"i"
          if flags & re.LOCALE:
              sflags += b"l"
          if flags & re.MULTILINE:
              sflags += b"m"
          if flags & re.DOTALL:
              sflags += b"s"
          if flags & re.UNICODE:
              sflags += b"u"
          if flags & re.VERBOSE:
              sflags += b"x"
          sflags += b"\x00"
          return b"\x0B" + name + _make_c_string_check(value.pattern) + sflags