First you'll want to be able to select the group or groups of files you want to operate on. Personally, I would use the Audio > Files view for this. You can choose a directory from the Location pane. You can even make it a parent directory if you have lots of these to do. Next, I'd use the File Type pane to remove all SACD files. Select the file type(s) that you want to rename and of course leave out SACD so it doesn't get selected.
As for actually re-writing the album tag, you can definitely use expressions. There are probably several ways to do it. Here's an expression I just tested to do what you're asking for:
if(compare(regex([Album], /#^D_(.+)$#/,0,0), =,1), DS_[R1], [Album])
This works on the [Album] tag. If you want to apply this to a different tag, you'll need to change the expression appropriately.
To test this out, make a new expression column and paste this expression into it. Then review it's values for the albums you are talking about. Make sure it looks correct. To apply this to the [Album] tag here's the procedure:
1. Select some files you want to change. Start with a small test group. If it doesn't work, use undo to reverse the changes right away.
2. Press Alt-enter, or right click to select "Tag". The tagging pane will appear on the left.
3. Find the Album tag and click in it to highlight it. You are now editing that tag for all selected files.
4. Paste in the expression with an = sign in front of it. Like this: =if(compare(regex([Album], /#^D_(.+)$#/,0,0), =,1), DS_[R1], [Album])
5. Press enter to make the changes.
6. Review to make sure the changes worked. If not press control-Z to undo. Or Edit > Undo
You can work on a larger group, or the whole thing after you've done a test or two.
NOTE: This changes the [Album] tag in all of the files you are working on. It does NOT change the filename. To change the file name, you would need to use the Rename, Move, and Copy tool *after* you make these changes to the Album tag. We can help you with that also if necessary.
Good luck. Let us know how you do and/or if you need any help tweaking this to work for you.
Brian.