Hash features of a natural language time declaration

reminder_time.rb

class String
  def scan_first(pattern)
    a = self.scan(pattern)
        a = a.first while a.is_a? Array unless a.empty?
    return a
  end
end

time_tokens = {}
given = ARGV[0]
months = [
  "January",
  "February",
  "March",
  "April",
  "May",
  "June",
  "July",
  "August",
  "September",
  "October",
  "November",
  "December"
]
regexps = {
           :time            => /\d?\d:\d\d/, 
           :meridian        => /[AaPp][Mm]/, 
           :tomorrow        => /tomorrow/, 
           :n_days_from_now => /(\d+) days from now/, 
           :in_n_days       => /in (\d+) days/
           }

regexps.each do |k, v|
  unless given.scan_first(v).empty?
    time_tokens[k] = given.scan_first(v)
    if given.scan_first(v) == 'tomorrow' then time_tokens[k] = 1 end
  end
end
unless time_tokens[:tomorrow] or time_tokens[:n_days_from_now] or time_tokens[:in_n_days]
  months.each do |m|
    if given.include?(m)
      time_tokens[:month] = m 
      unless given.scan_first(/#{m} (\d+)/).empty?
        time_tokens[:day_of_month] = given.scan_first(/#{m} (\d+)/)
      else raise "You must enter a date with a month"
      end
    end
  end
else raise "You can't use a month with 'N days from now'" end

# Clean up a bit
    [:n_days_from_now, :in_n_days].each do |x|
      if time_tokens[x]
        raise "Only use one 'n days' form" if time_tokens[:n_days]
        time_tokens[:n_days] = time_tokens[x] 
        time_tokens.delete(x)
      end
    end
puts time_tokens