When using 'bestvideo/best,bestaudio', 'bestvideo/best' must be set as the current_selector (instead of appending it to the selectors), otherwise when it gets the ',' it would append 'None' to the selectors.
'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'.
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.
For some extractors that are hard to workout a good _VALID_URL we use very vague and unrestrictive ones,
e.g. just allowing anything after hostname and capturing part of URL as id.
If some of these extractors happen to have an video embed of some different hoster or platform
and this scenario was not handled in extractor itself we end up with inability to download this embed
until extractor is fixed to support embed of this kind.
Forcing downloader to use the generic extractor can be a neat temporary solution for this problem.
Example: FiveTV extractor with Tvigle embed - http://www.5-tv.ru/rabota/broadcasts/48/
Otherwise it's impossible to only download non-DASH formats, for example `best[height=?480]/best` would download a DASH video if it's the only one with height=480, instead for falling back to the second format specifier.
For audio only urls (soundcloud, bandcamp ...), the best audio will be downloaded as before.
It doesn't work well with 'bestvideo' and 'bestaudio' because they are usually before the max quality.
Format filters should be used instead, they are more flexible and don't require the requested quality to exist for each video.
Without the '--keep-video' option the two files would be downloaded again and even using the option, ffmpeg would be run again, which for some videos can take a long time.
We use a temporary file with ffmpeg so that the final file only exists if it success
Sergey M․
Jaime Marquínez Ferrándiz
Aurélio A. Heckert
Yen Chi Hsuan
Shrimadhav U K
zouhair