constraints

### Parameter Restriction Allows you to constrain the nested routes based on a set of rules. For instance, in order to change the routes to allow for a dot character in the `id` parameter:

constraints(id: /\d+\.\d+/) do
  resources :posts
end

Now routes such as `/posts/1` will no longer be valid, but `/posts/1.1` will be. The `id` parameter must match the constraint passed in for this example.

You may use this to also restrict other parameters:

resources :posts do
  constraints(post_id: /\d+\.\d+/) do
    resources :comments
  end
end

### Restricting based on IP

Routes can also be constrained to an IP or a certain range of IP addresses:

constraints(ip: /192\.168\.\d+\.\d+/) do
  resources :posts
end

Any user connecting from the 192.168.* range will be able to see this resource, where as any user connecting outside of this range will be told there is no such route.

### Dynamic request matching

Requests to routes can be constrained based on specific criteria:

constraints(-> (req) { /iPhone/.match?(req.env["HTTP_USER_AGENT"]) }) do
  resources :iphones
end

You are able to move this logic out into a class if it is too complex for routes. This class must have a `matches?` method defined on it which either returns `true` if the user should be given access to that route, or `false` if the user should not.

class Iphone
  def self.matches?(request)
    /iPhone/.match?(request.env["HTTP_USER_AGENT"])
  end
end

An expected place for this code would be `lib/constraints`.

This class is then used like this:

constraints(Iphone) do
  resources :iphones
end

# File actionpack/lib/action_dispatch/routing/mapper.rb, line 1046
        def constraints(constraints = {}, &block)
          scope(constraints: constraints, &block)
        end