Well, to make it simpler, what I'd want is already done by the "Autonumbering Wizard" button (with "Reset counter for each directory" option ticked).
I just would like to format the tracknumer tag with an action (so it's part my action group, and I don't forget to do it).
Basically i just need to know what is the RegEx formula of the "Autonumbering Wizard" button ?