Route Parameters Validation with RegExp

We can validate parameters directly in the route, with “where” parameter. A pretty typical case is to prefix your routes by language locale, like fr/blog and en/article/333. How do we ensure that those two first letters are not used for some other than language?

routes/web.php:

1Route::group([
2    'prefix' => '{locale}',
3    'where' => ['locale' => '[a-zA-Z]{2}']
4], function () {
5    Route::get('/', 'HomeController@index');
6    Route::get('article/{id}', 'ArticleController@show');
7});