Can I use Regexp in Route::group without unnecessary parameters? Laravel5

Can I use Regexp in Route::group without unnecessary parameters?
I want to separate routing, whether url contains subdomain.

Using following code, When I access 'http://ift.tt/1Kob2pC', output 'test'.

# app/Http/routes.php

Route::group(array('domain' => "{subdomain}.main.com"),function()
{
  Route::get('/site/{id}', 'SiteController@show');
}

,

# app/Http/Controller/SiteController.php

public function show($id)
{               
  // When I access 'http://ift.tt/1Kob2pC', I check 'test'.
  var_dump($id);
}

If I code following , I can get 12. But $subdomain is unnecessary parameter.

# app/Http/Controller/SiteController.php

public function show($subdomain, $id)
{               
  // When I access 'http://ift.tt/1Kob2pC', I check '12'.
  var_dump($id);
}

Appreciate any suggestions you can make.

via Chebli Mohamed