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"`.