React Router – how to constrain params in route matching?

React-router v4 now allows you to use regexes to match params — https://reacttraining.com/react-router/web/api/Route/path-string

const NumberRoute = () => <div>Number Route</div>;
const StringRoute = () => <div>String Route</div>;

<Router>
    <Switch>
        <Route exact path="/foo/:id(\\d+)" component={NumberRoute}/>
        <Route exact path="/foo/:path(\\w+)" component={StringRoute}/>
    </Switch>
</Router>

More info:
https://github.com/pillarjs/path-to-regexp/tree/v1.7.0#custom-match-parameters