(?:\d?[A-Z]{1,2}\d*/)? # out of area prefix /
(?:\d?[A-Z]{1,2}\d+) # main prefix one (required)
[A-Z]{1,5} # callsign letters (required)
(?:\d?[A-Z]{1,2}\d*/)? # out of area prefix /
(?:\d?[A-Z]{1,2}\d+) # main prefix one (required)
[A-Z]{1,5} # callsign letters (required)