Route::get('/user/{name}', function ($name) {
    //
})->where('name', '[A-Za-z]+');

Route::get('/user/{id}', function ($id) {
    //
})->where('id', '[0-9]+');

Route::get('/user/{id}/{name}', function ($id, $name) {
    //
})->where(['id' => '[0-9]+', 'name' => '[a-z]+']);

reference : https://laravel.com/docs/8.x/routing#parameters-regular-expression-constraints