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