def convert(chr)
  chr = chr.upcase
  # subtract "A"
  n = (chr.ord - 65) / 3
  # account for #7 & #9 which have 4 chars
  n -= 1 if chr == "S".freeze || chr == "V".freeze || chr >= "Y".freeze
  (n + 2).to_s
end

puts "180044STERN".gsub(/[a-z]/i) { |chr| convert(chr) }
puts "555AXYZ".gsub(/[a-z]/i) { |chr| convert(chr) }