You could use a change node with jsonata expression
$match(payload,/name=\"(.*?)\".*externalId=\"(.*?)\"/)
output (see groups array):
{"match":"name=\"controller.door.motorlock.locked\" type=\"controller\" timestamp=\"1612423924000\" domain=\"4\" domainName=\"Great Domain\"><argument value=\"LCU\" type=\"controller\" id=\"11\"/><argument value=\"Kundentré\" type=\"dac\" id=\"20\" externalId=\"ID:20_20181002_150003\"","index":13,"groups":["controller.door.motorlock.locked","ID:20_20181002_150003"]}
Example flow:
[{"id":"5906ce56.3e41a8","type":"inject","z":"b7a67819.1a6dd","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"id=\"1509194\" name=\"controller.door.motorlock.locked\" type=\"controller\" timestamp=\"1612423924000\" domain=\"4\" domainName=\"Great Domain\"><argument value=\"LCU\" type=\"controller\" id=\"11\"/><argument value=\"Kundentré\" type=\"dac\" id=\"20\" externalId=\"ID:20_20181002_150003\"><![CDATA[address=1]]></argument><argument value=\"Kundentré\" type=\"door\" id=\"27\"/></event></batch><batch name=\"controller.door.motorlock.locked.batch\">","payloadType":"str","x":336,"y":240,"wires":[["cea5f0bf.edb7c"]]},{"id":"cea5f0bf.edb7c","type":"change","z":"b7a67819.1a6dd","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$match(payload,/name=\\\"(.*?)\\\".*externalId=\\\"(.*?)\\\"/)","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":512,"y":240,"wires":[["d291ac93.9cf6a"]]},{"id":"d291ac93.9cf6a","type":"debug","z":"b7a67819.1a6dd","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":718,"y":240,"wires":[]}]