callsigns.rb

# https://en.wikipedia.org/wiki/Call_signs_in_the_United_Kingdom#Call_sign_assignments_for_amateur_radio

# This appears to work:
/
(?<reg>[UDJIMWPTHNSC]?)
(?<callsign>M\g<reg>[01356][A-Z]{3})|
(?<callsign>(G\g<reg>[123468][A-Z]{2,3})|
(G\g<reg>[07][A-Z]{3})|(G\g<reg>5[A-Z]{2}))|
(?<callsign>2[EUDJIMW][01][A-Z]{3})
/

# This doesn't:

/
(?<reg>[UDJIMWPTHNSC]?)(?<m_callsign>M\g<reg>[01356][A-Z]{3})|
(?<g_callsign>(G\g<reg>[123468][A-Z]{2,3})
(G\g<reg>[07][A-Z]{3})|(G\g<reg>5[A-Z]{2}))
(?<two_callsign>2[EUDJIMW][01][A-Z]{3})
(\k<m_callsign>|\k<g_callsign>|\k<two_callsign>)
/