Relaxed validation for format filters so that any arbitrary field can be used

README.md

@@ -767,7 +767,7 @@ Also filtering work for comparisons `=` (equals), `^=` (starts with), `$=` (ends
 
 Any string comparison may be prefixed with negation `!` in order to produce an opposite comparison, e.g. `!*=` (does not contain).
 
-Note that none of the aforementioned meta fields are guaranteed to be present since this solely depends on the metadata obtained by particular extractor, i.e. the metadata offered by the video hoster.
+Note that none of the aforementioned meta fields are guaranteed to be present since this solely depends on the metadata obtained by particular extractor, i.e. the metadata offered by the video hoster. Any other field made available by the extractor can also be used for filtering.
 
 Formats for which the value is not known are excluded unless you put a question mark (`?`) after the operator. You can combine format filters, so `-f "[height <=? 720][tbr>500]"` selects up to 720p videos (or videos where the height is not known) with a bitrate of at least 500 KBit/s.
 

youtube_dlc/YoutubeDL.py

@@ -1142,7 +1142,7 @@ class YoutubeDL(object):
                 '*=': lambda attr, value: value in attr,
             }
             str_operator_rex = re.compile(r'''(?x)
-                \s*(?P<key>ext|acodec|vcodec|container|protocol|format_id)
+                \s*(?P<key>[a-zA-Z0-9._-]+)
                 \s*(?P<negation>!\s*)?(?P<op>%s)(?P<none_inclusive>\s*\?)?
                 \s*(?P<value>[a-zA-Z0-9._-]+)
                 \s*$