How to hide the server version?

[Thomas]

Using the following directive:
---
  ## Deny certain Referers (case insensitive)
  if $http_referer ~* (poker|sex|girl|grapx|allegro\.pl) {
    return 444;
  }
---

I get the following error message:
---
[emerg] 2854#0: invalid condition "$http_referer"


The spelling is correct, what did I make wrong?