Thank you so much for posting this.
A question if I may and I apologize if my post is bumping an old thread, but this is really a a good place to finish what I am trying to do.
The files I am trying to joinn are named in a pattern xxx_yyyy_zzzz and sometimes xxxx_yyyy_zzzz.
All I want to do is except of having the full name is adding the xxx or xxxx before the first underscore. I am aware that all I have to do is change the expression. But I am such a novice and I really appreciate any help.