Format value "_FILENAME": .\$regexp(%_filename%,'\[0','\[from TLIB-0')
performed on a file in the current directory should fail - but instead Mp3tag ignores the .\ !
This caused much damage here, by delaying my discovery that the Action's .\ was in error until Iit was performed on sub-folder files which it then messed-up.
(Thanks for Undo, which at least rectified some of the damage).
the question right hier to me looks like "Did it perform a format value or didn't it?".
I assume that the action was carried out only the result did not meet your expectations, right?
I doubt that an interpreter of the kind "do not as I say but guess what I have meant" is feasable.