How to Create This Route in CakePHP

Question

I want to create a route something like:

http://images.example.com/[models]/[sizes]_[filename].[extension]

[models] = (blog|event|user) [sizes] = (t|s|l|o) [filename] = example [extension] = (png|gif|jpg)

How do I create the route for this? I've tried the following w/ no success:

Router::connect('/:model/:size\_:filename.:extension', 
array('controller' => 'images', 'action' => 'server'),array( 
                'model'=>'(blog|event|users)+', 
                'size'=>'(t|m|l|o)', 
                'filename'=>'\w+', 
                'extension'=>'.(gif|png|jpg)', 
                ));