With a route I can put a constraint:

For instance to allow only a-z letter in the book url.

Route::get ('book/{book}/view'), function ($book)
{
  return "book - view: $book";
})->where (array ('book' => [a-z]*));

Route::get ('book/{book}/edit'), function ($book)
{
  return "book - edit: $book";
})->where (array ('book' => [a-z]*));

I would like to do similar stuff for the Route::group command So I would assume the following is working, but not.

Route::group (array('prefix' => 'book/{book}'), function ($book)
{
  Route::get ('/view'), function ($book)
  {
    return "book - view: $book";
  });

  Route::get ('/edit'), function ($book)
  {
    return "book - edit: $book";
  });

})->where (array ('book' => [a-z]*));

I got Call to a member function where() on a non-object

This example works fine when the where function is removed, without the constraint...

Note: the example is just for illustration.

Any idea how to overcome?