title: Symfony route parameter requirement restriction
body: <p>How do I enforce a requirement that a paramater in a route be a string?</p>
  
  <p>Given the route</p>
  
  <p>my_foobar_route:
    url: /example/routing/:s1/:id
    requirements: { id: \d+ }</p>
  
  <p>Can anyone remind me of how to force param s1 to be a string?</p>