Please explain the router code.

I can't think of a case where this code would make sense.

PHP Code:

$replacekey = str_replace('/(.*)', '', $routeKey);
$handler    = preg_replace('#^' . $routeKey . '$#u', $handler, $uri);
$handler    = str_replace($replacekey, str_replace('/', '\\', $replacekey), $handler); 


PHP Code:

// for route 
$routes->get('path/(:segment)/(:any)', 'Controller::method/$1/$2');

// $routeKey = path/([^/]+)/(.*)
$replacekey = str_replace('/(.*)', '', $routeKey); // path/([^/]+)

$handler    = preg_replace('#^' . $routeKey . '$#u', $handler, $uri);  // \App\Controllers\Controller::method/1/2

$handler    = str_replace($replacekey, str_replace('/', '\\', $replacekey), $handler); // replacing "path/([^/]+)" with "path\([^\]+)" in "\App\Controllers\Controller::method/1/2"