enumerable_with_context.rb

module Enumerable
  def each_with_context(&block)
    block.call([nil, self[0], self[1]])
    self.each_cons(3, &block)
    block.call([self[self.size - 2], self[self.size - 1], nil])
    self
  end
  def enum_with_context
    Enumerable::Enumerator.new(self, :each_with_context)
  end
end

example.rb

require 'rubygems'
require 'activesupport'

class User < Struct.new(:id, :name)
  def self.find(n); self.new(n, "Matt"); end
end
class Post < Struct.new(:id, :name)
  def self.find(n); self.new(n, "HooHah"); end
end
class Comment < Struct.new(:id, :name)
  def self.find(n); self.new(n, "Sweet"); end
end

url = "users/1/posts/2/comments/3"

url.split('/').each_with_context do |(prev, element, _)|
  
  case element
    
  when /^\d+$/
    p [:element, prev.singularize.camelize.constantize.find(element).name]
    
  else
    p [:element, element]
    
  end
  
end