'bestvideo+bestaudio/best' was incorrectly interpreted as 'bestvideo+(bestaudio/best)', so it would fail if 'bestaudio' doesn't exist instead of falling back to 'best'.
Better to extract 'http://manifest.us.rtl.nl' from the json, I'd say. And I think it's better to use the default json-url to make it more futureproof.
Succesfully tested with tarball.
The spec string is processed using 'tokenize.tokenize' to split it in words and operators, the filters are still processed using regular expressions.
This should make easier to allow grouping operators with parens.