Laravel whereRaw regex search with parameters returns null

This seems like a simple syntax question, but nothing I have read online seems to help. All I want to do is use whereRaw to perform a regex search.

$regex = $request->regex;
$tags = Unknown_Tag::whereRaw("url REGEX ?", array('%'.$regex.'%'));

this always returns nothing, no matter what is entered in the text field. No errors are thrown, but nothing comes up in the search results.

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