Hi. I'm new to this game, so please bear with me. I have a bunch of CDs that I'd like to tag correctly. The file is called 08-sabaton the valley of death , and I'd like it to be tagged 08-sabaton-the_valley_of_death , so I don't have to change it manually. Thanks for the tips.
"Tagging" means to add metadata inside your tracks.
Such metadata can be read and displayed by playing software.
Examples are TITLE, ALBUM, ARTIST and many more.
What you describe is some kind of renaming (of filenames).
So what exactly do you want to do?
I want to rename files. I just don't want to have to do it manually.
You want to replace the spaces in the trackname with underscores?
And add a hyphen between the artist and the trackname?
I'm not sure if Mp3tag is the preferred way to do that.
I'm just curious:
Did you find this forum as an answer in a AI or Google?
Try Convert>Tag-Tag for _FILENAME
Format string: $replace($regexp(%_filename%,(.*?) (.*),$1-$2), ,_)
But as @LyricsLover has said: this has nothing to do with tagging.
It is fiddling with the filename which is no tag.
ohrenkino, big thanks 
