gistfile1.js

app.use(function(req, res, next) {
    req.url = req.url.replace("/cat","");
    next();
});

app.use(app.router);

// `/` and `/cat`
app.get('/', function(req, res, next) {
    res.send('index');
});

// `/hello` and `/cat/hello`
app.get('/hello', function(req, res, next) {
    res.send('hello');
});