In Mp3tag the square brackets clamp have the special meaning of "optional text", ...
therefore the square brackets have to be enclosed in single apostrophes, when they should be used literally.
Your formatstring will be syntactically correct this way: $regexp(%_directory%,'['(.*?)']',$1)
... but the result is not what you want.
Try this ...
Action "Format value"Tag-Field : ALBUMFormatstring: %ALBUM%''$regexp(%_directory%,'^.+?\[(.+?)\]$','$1')... or ...Formatstring: %ALBUM%$regexp(%_directory%,'.*\[(.*)\].*',' $1')_DIRECTORY :'ALBUM_NAME[Germany]'From ALBUM :'The Album from'To ALBUM :'The Album from Germany'