Zabbix Trigger regex

[msth2]

Hello,
i try to extract an value from an log string.
The Item has the Type Character. And contains something like

2017-09-07 11:58:29
{"Time":"1970-01-03T01:35:51", "DS18B20":{"Temperature":21.8}, "TempUnit":"C"}

The Trigger Type regexp works, but i don't now how it works with regsub.

Here is the Regexp Trigger

{tmp-tws3-1:mqtt.tmp-tws3-1.temperature.regexp("[[+-]?([0-9]*[.])?[0-9]+]")}=1

I hope anyboby can help me

regards