How can I convert a Comment tag with content like
Track 07
into the track Tag ?
The format string
Track %track%
does not work ( syntax like scanf in C )!
You can use Convert Tag -> Tag (or press ALT + 5)
with the field TRACK and format string %comment%
BUT: I do not advice to fill text like "Track" into this field. TRACK should be in format xx/nn,
where xx is the actual tracknummer and nn is the total number of tracks.
A tracknumber like 07 is fine too.
The official standard says about TRCK:
The 'Track number/Position in set' frame is a numeric string
containing the order number of the audio-file on its original
recording. This may be extended with a "/" character and a numeric
string containing the total numer of tracks/elements on the original
recording. E.g. "4/9"
If you only want to fill 07 from the comment tag "Track 07" into the track field, use
the format string $repl(%comment%,Track ,)
This would replace the text "Track " from the comment field with nothing = delete it and then fill only the number 07 into the tag TRACK. (The comment tag itself remains untouched).
Thank You, it works fine.
Very helpful to use Functions / expressions in a format string ( even it is a liitle strange for an old C programmer like me 
If you do not like to enter functions in format strings, perhaps a different action, this time "Guess value" could relieve your troubled mind:
Source: %comment%
Target: %dummy% %track%
There are many ways to skin a cat in/with MP3tag.