Extract track number from title and place it in track field

I have several files that with file names that contain the track number in the file name, but have no track number listed in the track field. I want to extract and write the track field.

current title
Die laute Lili (2/5): Pepipuppe - Betthupferl - Gute-Nacht-Geschichten für Kinder - BR - 03.12.2019

Track number should be

I came up with the following, but I'm only a regular expression beginner. So my solution seems rather clumsy to me and it only works for one digit track numbers

Copy the content of the title track field to the track field
Format value "TRACK": %title%

Delete all non number characters from the track field
Regular expression"TRACK": "[^\d]" -> ""

Keep the first number of the track field and delete all other numbers
Regular expression"TRACK": "^(\d)\d.*" -> "$1"

How can I improve this?

You could use an action of the type "Guess value" (import tag fields)
Source: %title%
Target: %dummy%(%track%/%dummy%

of if you have to do it with a regular expression:
Format value for TRACK
Format string: $regexp(%title%,.*(\d+)\/.*,$1)