How to define dynamic route path using regex in Nuxt's serverMiddleware?

I'm trying to define a regex based dynamic path in Nuxt's serverMiddleware, but I'm not able to figure out how to achieve it because in the doc also there is no any mention about dynamic route.

I want to achieve something like this,

// ./nuxt.config.js
export default {
    modules: [
      ..
    ],
    serverMiddleware: [
        {
            path: '/sitemap/:filename(.*xml.gz)',
            handler: '~/api/serveSitemap.js'
        }
    ]  
}

And then get :filename param inside the handler..

//./api/serveSitemap.js
export default function(req, res) {
    filename = req.params.filename // is 'req.params' available in nodejs?
    // some code
    res.end()
}

Just to inform, I'm using Nuxtjs in SSR mode with Nodejs as a rendering server, and not using Expressjs. And this is the link which I'm referring for using Nuxt serverMiddleware: https://nuxtjs.org/api/configuration-servermiddleware/#custom-server-middleware