# Controller::Rewrite

The <code class="language-ruby">Controller::Rewrite</code> layer can match and rewrite requests before they processed. This allows you to handle URLs like `/post/15/view` or `/blog/123-pictures-of-my-cat` easily. The basic rewrite operation is to extract some part of the path and optionally executes a block. That means that the path is modified before being passed on to the next layer in the controller, and controller instance variables may be set.

## Regular Expressions

In your `controller.rb`:

```ruby
prepend Rewrite, Actions

rewrite.extract_prefix permalink: /(?<id>\d+)-(?<title>.*)/ do |request, path, match|
	# The rewrite matched, but there was no valid post, so we fail:
	fail! unless @post = Post.find(@permalink[:id])
	
	# If the path matched, but there was no suffix, we make it default to the post action:
	if match.post_match.empty?
		match.post_match.components << "post"
	end
end

on 'post' do
	# You can do further processing here.
	fail! unless @post.published?
	
	@comments = @post.comments.first(5)
end

on 'edit' do
	# You can do further processing here.
	fail! unless @current_user&.editor?
end
```

In your `post.xnode`, as an example:

```trenni
<page>
	<heading>Post #{attributes[:permalink][:id]} about #{attributes[:permalink][:title]}</heading>
	
	<p>#{attributes[:post].content}</p>
</page>
```

Keep in mind, that URLs like `/123-pictures-of-my-cat/edit` will work as expected, and hit the `edit` action of the controller.

## Restful Resources

Similar to the above, if we were solely interested in IDs, we could do the following:

```ruby
prepend Rewrite, Actions

rewrite.extract_prefix post_id: Integer do |request, path, match|
	# The rewrite matched, but there was no valid post, so we fail:
	fail! unless @post = Post.find(@post_id)
	
	# If the path matched, but there was no suffix, we make it default to the post action:
	if match.post_match.empty?
		match.post_match.components << "post"
	end
end
```

This will only match complete integers. Assuming this code is in `/blog/controller.rb`, it would match something like `/blog/123/view` and assign <code class="language-ruby">Integer("123")</code> to <code class="language-ruby">@post_id</code>.

### Matching.. other things

It's possible to match using <code class="language-ruby">Integer</code>, <code class="language-ruby">Float</code>, <code class="language-ruby">String</code>, and you can provide your own class which will be instantiated. If it doesn't match, raise an exception and the rewrite rule will fail.