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Regular Expressions are a broad topic and cannot be covered in full here. We This guide will demonstrate the use of HiveMQ’s pattern matching capabilities by creating a Trace Recording, matching all clients with a clientId beginning with publisher0, followed by a single character. Further the special case of creating a recording for topic tree patterns with use of the + character will be shown.

Prerequisites

In order to verify, that our messages are being delivered as we expect we will subscribe to a test topic pattern.

In our example we will capture messages by clients including their identifiers in the topic structure. This is a common pattern in production environments.

HiveMQ’s MQTT CLI lets us do subscribe to all of these this in one line like socommand by using the wildcard operator:

mqtt sub -t 'our/+/topic' -h localhost -i subscriber00

To have an easier time differentiating our clients, we gave this one the clientId identifier subscriber00.

Let this session remain active throughout the test.

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First, choose a new, unique name. This need not match a topic name and only serves to help identify the recording we are about to create.

Client Filter

Next, we have to define a Client Filter. Here we will make use of the wildcard character . that acts as a quantifier for “any single character”.

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After clicking Add Topic Client Filter it should become visible as an existing filter:

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Filtering for topics using the wildcard operator

In order for our recording to match the subscription of subscriber00, which uses MQTT’s single level wildcard character, a Regular Expression will allow us to do so. Here foreslashes need to be escaped with \.

our\/.+\/topic

Message Type

From here we can select the types of messages we are interested in. For this example, we are only interested in PUBLISH messages.

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for i in `seq 5 15`; do sleep 1; mqtt pub -t '"our/publisher0$i/topic'" -h 127.0.0.1 -m "message by publisher0$i" -i publisher0$i; done

Once $i reaches 10, the clientID will contain a character more than we specified in our filter. We expect to not to see any messages from publisher010 to publisher015 in our recording.

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2020-04-09 15:52:43,526 - [publisher05] - Received PUBLISH message (topic: our/publisher05/topic, QoS: 0, retain: false, duplicate delivery: false)
2020-04-09 15:52:45,297 - [publisher06] - Received PUBLISH message (topic: our/publisher06/topic, QoS: 0, retain: false, duplicate delivery: false)
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2020-04-09 15:53:16,852 - [publisher09] - Received PUBLISH message (topic: our/publisher09/topic, QoS: 0, retain: false, duplicate delivery: false)

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