Extract attributes from a MQTT topic in ThingsBoard

[ziotibia81]

I have an electrical measuring device that exports a list of measurements via MQTT. The topic is formatted as follows: measuring/[site]/[param], and the data is published in plain text.

For example:

measuring/site1/L1-Voltage -> 230.0
measuring/site1/Freq -> 50.0
measuring/site1/L1-Current -> 123.4

I'm using ThingsBoard IoT Gateway (version 3.7.0) with ThingsBoard version 3.9.1. I've configured an MQTT connector that connects to the broker of the measuring device.

In the "Data Mapping" section, I used a topic filter like measuring/site1/+ to subscribe to all parameters for site1. I set the Payload type to Bytes and the value expression to [:]. However, I can't figure out how to extract the key (e.g., L1-Voltage, Freq) from the MQTT topic using a regular expression or placeholder. If I enter a regular expression into the key field, the telemetry data arrives in ThingsBoard with the literal regex string as the key name, not the extracted parameter name.