How to use the h11._util.validate function in h11

def normalize_and_validate(headers, _parsed=False):
    new_headers = []
    saw_content_length = False
    saw_transfer_encoding = False
    for name, value in headers:
        # For headers coming out of the parser, we can safely skip some steps,
        # because it always returns bytes and has already run these regexes
        # over the data:
        if not _parsed:
            name = bytesify(name)
            value = bytesify(value)
            validate(_field_name_re, name, "Illegal header name {!r}", name)
            validate(_field_value_re, value, "Illegal header value {!r}", value)
        name = name.lower()
        if name == b"content-length":
            if saw_content_length:
                raise LocalProtocolError("multiple Content-Length headers")
            validate(_content_length_re, value, "bad Content-Length")
            saw_content_length = True
        if name == b"transfer-encoding":
            # "A server that receives a request message with a transfer coding
            # it does not understand SHOULD respond with 501 (Not
            # Implemented)."
            # https://tools.ietf.org/html/rfc7230#section-3.3.1
            if saw_transfer_encoding:
                raise LocalProtocolError(
                    "multiple Transfer-Encoding headers", error_status_hint=501
                )
            # "All transfer-coding names are case-insensitive"

saw_content_length = False
    saw_transfer_encoding = False
    for name, value in headers:
        # For headers coming out of the parser, we can safely skip some steps,
        # because it always returns bytes and has already run these regexes
        # over the data:
        if not _parsed:
            name = bytesify(name)
            value = bytesify(value)
            validate(_field_name_re, name, "Illegal header name {!r}", name)
            validate(_field_value_re, value, "Illegal header value {!r}", value)
        name = name.lower()
        if name == b"content-length":
            if saw_content_length:
                raise LocalProtocolError("multiple Content-Length headers")
            validate(_content_length_re, value, "bad Content-Length")
            saw_content_length = True
        if name == b"transfer-encoding":
            # "A server that receives a request message with a transfer coding
            # it does not understand SHOULD respond with 501 (Not
            # Implemented)."
            # https://tools.ietf.org/html/rfc7230#section-3.3.1
            if saw_transfer_encoding:
                raise LocalProtocolError(
                    "multiple Transfer-Encoding headers", error_status_hint=501
                )
            # "All transfer-coding names are case-insensitive"
            # -- https://tools.ietf.org/html/rfc7230#section-4
            value = value.lower()
            if value != b"chunked":
                raise LocalProtocolError(
                    "Only Transfer-Encoding: chunked is supported",

def _validate(self):
        # "A server MUST respond with a 400 (Bad Request) status code to any
        # HTTP/1.1 request message that lacks a Host header field and to any
        # request message that contains more than one Host header field or a
        # Host header field with an invalid field-value."
        # -- https://tools.ietf.org/html/rfc7230#section-5.4
        host_count = 0
        for name, value in self.headers:
            if name == b"host":
                host_count += 1
        if self.http_version == b"1.1" and host_count == 0:
            raise LocalProtocolError("Missing mandatory Host: header")
        if host_count > 1:
            raise LocalProtocolError("Found multiple Host: headers")

        validate(request_target_re, self.target, "Illegal target characters")

def _decode_header_lines(lines):
    for line in _obsolete_line_fold(lines):
        # _obsolete_line_fold yields either bytearray or bytes objects. On
        # Python 3, validate() takes either and returns matches as bytes. But
        # on Python 2, validate can return matches as bytearrays, so we have
        # to explicitly cast back.
        matches = validate(header_field_re, bytes(line))
        yield (matches["field_name"], matches["field_value"])

def maybe_read_from_SEND_RESPONSE_server(buf):
    lines = buf.maybe_extract_lines()
    if lines is None:
        return None
    if not lines:
        raise LocalProtocolError("no response line received")
    matches = validate(status_line_re, lines[0])
    # Tolerate missing reason phrases
    if matches["reason"] is None:
        matches["reason"] = b""
    status_code = matches["status_code"] = int(matches["status_code"])
    class_ = InformationalResponse if status_code < 200 else Response
    return class_(
        headers=list(_decode_header_lines(lines[1:])), _parsed=True, **matches
    )