- (?:(?:[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
- (?:-(?:\d{1,2}|\#))? # - nn possibly (eg G8BPQ-8) or -# (an RBN spot)
- (?:/[0-9A-Z]{1,7})? # / another prefix, callsign or special label (including /MM, /P as well as /EURO or /LGT) possibly
+ (?:\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{1,2}|\#))? # - nn possibly (eg G8BPQ-8) or -# (an RBN spot)
+ (?:/[0-9A-Z]{1,7})? # / another prefix, callsign or special label (including /MM, /P as well as /EURO or /LGT) possibly