Node.js - Express.js URL parameters validation

I'm a node.js/express.js newbie. How can I validate :id parameters? I would like to pass only numbers into :id parameters. If :id is a string or contain once of them, I would to display a 404 error like a zend framework routing http://framework.zend.com/manual/current/en/user-guide/routing-and-controllers.html

/routes/users.js

var express = require('express');
var router = express.Router();

/* GET users listing. */
router.get('/:id?/', function(req, res, next) {
  var id = req.params.id;
  if(id){
    res.render('single-users', { title: 'Single ' + id });
  }else {
    res.render('users', { title: 'All users' });
  }

});

module.exports = router;

I tried to change

router.get('/:id?/', function(req, res, next)

to

router.get('/[0-9]+?/', function(req, res, next)

but

localhost:3000/users/ab/

works and display single-users page and I want it..

SOLUTION SUGGESTED BY LUCAS COSTA

var express = require('express');
var router = express.Router();

/* GET users listing. */
router.get('/:id(\\d+)?/', function(req, res) {
  var id = req.params.id;

  if(id){
    res.render('single-users', { title: 'Single ' + id });
  }else {
    res.render('users', { title: 'All users' });
  }

});

module.exports = router;

or

router.get('/:id([0-9]+)?/', function(req, res)