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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _unpack_header(self, headers):
 """ Unpacks a jwt token from a request header """

        jwt_header = headers.get(self.header_name)
        MissingTokenHeader.require_condition(
            jwt_header is not None,
            "JWT token not found in headers under '{}'",
            self.header_name,
        )

        match = re.match(self.header_type + r'\s*([\w\.-]+)', jwt_header)
        InvalidTokenHeader.require_condition(
            match is not None,
            "JWT header structure is invalid",
        )
        token = match.group(1)
        return token