Catch-all route in Symfony 3

I have a catch-all fallback route in Symfony2 that I couldn't get to work in Symfony3. I tried this exact syntax (a verbatim copy of my Symfony2 route) and that didn't work.

fallback:
    path:     /{req}
    defaults: { _controller: MyBundle:Default:catchAll }
    requirements:
        req: ".+"

How can I get this working in Symfony3? (It's literally the only thing holding me back from using Symfony3 and keeping me at v2.8)

I found the current accepted answer almost useful for Symfony 4, so I'm going to add my solution:

This is what I did to get it working in Symfony 4+:

• In /src/Controller/DefaultController.php make sure there is a function called index()

  • It's not required to add the Request $request as first param as some comment suggest.
  • This is the method that will handle all urls caught by the routes.yaml

• Open /config/routes.yaml, add this:

    yourRouteNameHere:
      path: /{req}
      defaults: { _controller: 'App\Controller\DefaultController::index' }
      requirements: #            the controller --^     the method --^
        req: ".*"`  # not ".+"