Quick and dirty pluralization in Ruby

my_pluralize.rb

# CAUTION, it opens String class
String.class_eval do
  # HACK: quick and dirty english pluralization.
  def to_plural(*arg)
    plural = lambda do |str|
      if str =~ /\s\z|men\z|people\z|[^aious]s\z/i # is a plural already
        str
      elsif str =~ /\Aman\z|woman\z/i # man & woman, except caiman, human...
        str.sub(/an\z/i, 'en')
      elsif str =~ /[^aeiou][aeiou]?(f|fe)\z/ # knife, half...
        str.sub(Regexp.new("#{Regexp.last_match(1)}\\z"), 'ves')
      elsif str =~ /[^o]y\z/ # berry, activity... except boy, joy, convoy...
        str.chop!
        str.concat 'ies'
      elsif str =~ /[sxz]\z|[cs]h\z/ # box, fish, pitch...
        str.concat 'es'
      else
        str.concat 's'
      end
    end

    if arg[0].nil? || arg[0] != 1
      plural.call self
    else
      self
    end
  end
end