In a previous post discussing Openhab JSONPATH transformations there was an issue when the same MQTT topic is used for several different messages. In the case of the LaCrosse Weather sensor one message contains temperature and humidity and the other message contains wind speed and direction.
The solution is to chain transformations. First use a REGEX to filter and then use the JSONPATH to extract the value. This is discussed in the Openhab forums. For the weather sensor the wind direction transformation looks like:
REGEX:(.*wind_dir_deg.*)?JSONPATH:$.wind_dir_degPay special attention to the union symbol ?. It is really what makes the magic. As explained in the forum post the parenthesis in the REGEX pass the entire message on to the next transform if there is a match.