Caesar encryption in Ruby

caesar.rb

ALPHABET = /^[A-Z]+$/
COMMANDS = ['encrypt', 'decrypt']

command = ARGV[0]
key = ARGV[1].to_i
text = ARGV[2]

if COMMANDS.include? command
  if ALPHABET =~ text
    encrypted = text.chars.map do |character| 
      (65 + (character.ord - 65 + key) % 26).chr
    end.join
    puts encrypted
  else
    puts "Error: Input does not match allowed alphabet (A-Z)"
  end
else
  puts "Error: command must be one of #{COMMANDS.join(', ')}"
end