Reply to Cucumber Transform comment http://is.gd/3AqhI

env.rb

def self.TokenTransform(token, pattern, &transform)
  unless Kernel.const_defined? token
    class << val = []
      def to_s
        "(#{Regexp.union(*self)})"
      end
    end
    Kernel.const_set token, val
  end

  no_captures = Regexp.new(pattern.source.gsub(/\^|\$/, '').gsub('(', '(?:'), 
                           pattern.options)
  Kernel.const_get(token) << no_captures

  Transform(pattern, &transform)
end

TokenTransform "PERSON", /^a young child$/ do
  ...code that creates a young child person and returns it...
end

TokenTransform "PERSON", /^an elderly person$/ do
  ...code that creates an elderly person and returns it...
end

TokenTransform "USER", /^a guest user$/ do
  ...code that creates a guest user and returns it...
end

TokenTransform "USER", /^a user with role (w+)$/ do | roleString |
  ...code that creates a user with a certain role and returns it...
end

tokens.rb

Given /^#{PERSON} added by #{USER}$/ do | person, user |
  ... (code that uses person and user as their corresponding types, already converted from a string)...
end