Formatting outgoing command on TCP/UDP binding

Platform information:
Hardware: Raspberry Pi 4
OS: Raspbian GNU/Linux 10
Java Runtime Environment: build 11.0.6+10-post-Raspbian-1deb10u1
openHAB version: 2.5.3

I have a very fundamental question for which I, for a long time, fail to find an answer.

Background: I’m working on a custom hardware. I’m writing a UDP bridge application that communicates with this hardware. For this, a UDP packet must contain identification of the item generating the packet and then the value passed to the item.

Problem: I cannot figure out how to format the outgoing command, i.e. pre-pend or post-pend it with some custom information. When I use the item definition (1), a value (e.g. “85”) is sent to the UDP, so far so good. I would expect the definition (2) to prepend "test " to the value and send it out, i.e. “test 85”. Unfortunately, this is not the case and I cannot find any way to do it.

(1) Number Test_Slider “test” { udp=">[localhost:8150:]" }
(2) Number Test_Slider “test” { udp=">[localhost:8150:‘test %d’]" }

Number Test_Slider “test” { udp=">[localhost:8150:‘REGEX(s/(.+)/pre-pend text $1 post-pend text/g)’]" }

(enable regex in services/addons.cfg: find line with “transformation”, uncomment and change to “transformation = regex”, restart the OpenHAB2 service)