Matching an optional substring in a lookbehind is apparently invalid lookbehind syntax...

I am trying to match the word `static`, but only when preceded by `url(` (either with or without the optional single quote)

```
(?<=url\([']?)static

```

should match both:

url('**static** with quotes') 

url(**static** without quotes) 

i resorted to doing 2 lookbehinds, but it seems unnecessary

```
(?:(?<=url\()|(?<=url\('))static

```

Is there a regex trick i can leverage here?

Here is a link to it in rubular: <http://rubular.com/r/iJ3ifrOmfZ>