Fix problem with regexp in getattr.filter

Current regexp causes exception: "Uncaught SyntaxError: Failed to execute query: ''#Shr6qrjpz1'' is not a valid selector.". It happening because `"url('#Shr6qrjpz1')".match(/^\s*url\((.+)\)/)[1]` returns "'#Shr6qrjpz1'" instead of "#Shr6qrjpz1".