example.html.erb

<%= url('/:post.year/:post.month/:post.slug', post: post) %>

url.rb

  def url(path, records)
    records = records.with_indifferent_access
    regex = /(:([a-z]+)\.([a-z]+))/i
    path.gsub regex do |match|
      records[$2].send($3)
    end
  end