It looks like you are using the V1 MQTT binding, rather than V2. Did you mean to do that? If you’re just starting you should use V2, as for configuration will then be valid for the upcoming openHAB 3.
Assuming you change to the V2 binding, you can chain two transformations together: REGEX first then JSONPATH. See: MQTT 2.5 M1+ How to implement the equivalent to MQTT1 REGEX filters