route defined with negative lookaround matches the route despite the negative assertion

  php, regex, symfony, symfony-routing, symfony4

We want to open a DELETE endpoint that allows calls to all positive integers, but not id 1 (aka element 1 cannot be deleted)

Usually to open an endpoint that allows positive integers I configure the route like this

delete_elements:
 path: /elements/{id}
 methods: ["DELETE"]
 controller: app.elements_delete
 requirements:
  id: 'd+'

For this case I tried to change the regex to one that does not allow number 1 either

delete_elements:
 path: /elements/{id}
 methods: ["DELETE"]
 controller: app.elements_delete
 requirements:
  id: /^(?!(?:1)$)d+/

But when I modify the requirement and I call the endpoint the response is that the endpoint does not exist.

"No route found for "DELETE /elements/59": Method Not Allowed (Allow: GET, PUT)"

What is wrong with that regex? What would be the right way to forbid certain values?

Source: Symfony Questions

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