padded.rb

#!/usr/bin/env ruby

def padded_tweet(message)
  if message.size >= 140
    return message[0..139]
  end
  dup = message.dup
  if %w{. ! ? …}.include? message[-1]
    return dup.ljust(140, message[-1])
  end
  return dup.ljust(140, '!')
end

puts padded_tweet(ARGV.first)