I am trying to understand what this state means in ui-router:

```
$stateProvider.state('app', {
  abstract: true,
  url: '/{lang:(?:da|en)}',
  template: '<ui-view/>'
});
$stateProvider.state('app.home', {
  url: '',
    templateUrl: 'views/home-page.html',
});

```

Specifically what does {lang:(?:da|en)} do. I've not seen this construct before.