Custom pattern exclusion

I'm trying to create a custom pattern for highlighting disposable objects without the using keyword, more specifically on the form of 

$disp$ $var$ = $exp$;

where disp is of type System.IDisposable, example

var result = httpClient.PostAsync(...);

It works fine at finding them, but I haven't been able to make it so that it excludes the ones that already have using in front of them, that is 

using var result = httpClient.PostAsync(...);

should not get targetted.

Is there a way to exclude the ones that already have the using keyword?