diff --git a/cookbook/security/custom_authentication_provider.rst b/cookbook/security/custom_authentication_provider.rst
index 0410519eae5..f79f8156c73 100644
--- a/cookbook/security/custom_authentication_provider.rst
+++ b/cookbook/security/custom_authentication_provider.rst
@@ -135,7 +135,7 @@ set an authenticated token in the token storage if successful.
         {
             $request = $event->getRequest();
 
-            $wsseRegex = '/UsernameToken Username="([^"]+)", PasswordDigest="([^"]+)", Nonce="([a-zA-Z0-9+/]+={0,2})", Created="([^"]+)"/';
+            $wsseRegex = '/UsernameToken Username="([^"]+)", PasswordDigest="([^"]+)", Nonce="([a-zA-Z0-9+\/]+={0,2})", Created="([^"]+)"/';
             if (!$request->headers->has('x-wsse') || 1 !== preg_match($wsseRegex, $request->headers->get('x-wsse'), $matches)) {
                 return;
             }