Can you give me an example what you want to achieve?

@HLeithner In my case i want to override the Router::createUri, in order to encode the special characters included in the url's query vars.

But i think that can be used in a more broad way e.g. set validation and escape rules.

Maybe I'm wrong but wouldn't it be easier to add a preprocessor?

@HLeithner Thanks for the suggestion.
Any idea on how this can be implemented?

Create a plugin and add a buildRule in the onAfterInitialise event. somehing like this:

class plgSystemMyRouterExtgension extends JPlugin {

public function onAfterInitialise() {
  $app = JFactory::getApplication();
  if ($app->isSite()) {
    $router = $app->getRouter();
    // attach build rules for translation on SEF
    $router->attachBuildRule(array($this, 'buildRule'), JRouter::PROCESS_BEFORE);
  }
}

public function buildRule(&$router, &$uri) {
  /* modify $uri here */
}

}

@HLeithner Thank you!
The actual problem is in the Uri class, hence it requires more than adding rules.

In any case i think that providing the ability to set our own Router could add extra flexibility.

Can you send me the code you want to use in your router if this variable is not private?

I am overwriting the Router::createUri by calling my own Uri class within that fn.

@sakiss if you're essentially overriding the Uri class probably you should consider doing it like the example here: #19842

@dgrammatiko Valid. The only reason i don't like that, is that is sets an object in the container which will be used globally. I prefer having something with limited scope. But i understand the point that the same class should be used on input and output (build/parse).