I suggested before that you should test your expressions with the Convert>Tag-Tag function.
The example that you give mixes IMHO the syntax for regular expressions and a normal replace. So it would be better to give an example that has been tested.
The field ARTISTS is a user-defined field and no standard field - this could be intentional or not, I don't know, it just strikes me.
I am not sure what you saw in the data folder but this is by far not the complete list of fields nor does it mean that _DIRECTORY is actually part of the tag data that goes with the file. As soon as you move the file, you get new data in this pseudo-field, something that should not happen to ordinary field data.