Bronkhorst Flowmeters use a somewhat unique protocol for communication. In the ascii protocol the resonse is transmitted as an unsigned integer coded as 4 hexadecimal chiffres. The values are normalized so that 32000 (7D00h) is 100% of the capacitiy of the device.

For my case, valid Values range from 35C0h to 3D3Fh which this RegEx captures.