It might work to do a double back slash if you want to replace a single back slash. Try a few before you do very many.
Yeah, you might have to use some escape characters to get it to work. I'm
pretty sure I fixed an Imported Library via RMCF before from Windows to Mac with Find & Replace. But it has been a while.
In general, the forward slash in MC is an escape character, so if you want to do a / literal, you need to double it, like http://// (for example). You might need to do some trickery to get it to work, since it expects forward slashes in the Mac filename, but not backslashes. Maybe escape the backslashes with a forward slash or some nonsense?
Of course, it is also possible that this never worked, or worked in the past, but some "better" (in other ways) filename parsing added to RMCF more recently has broken it. One thing you could (maybe) do is try the "real" Find & Replace tool instead. This still works on [Filename], if you pick that field, and if all of the selected, current, filename links are broken, it'll act just like RMCF in
Update Database Only mode.
That's the way we used to have to do it before there was a Find & Replace template in RMCF. It might be less "smart" about the filename slashes and just do what you tell it to do, so that could be a way to go if you don't just go Brian's route.
EDIT: Another thing I just thought of before I sign off... If you still have the Windows machine, you might be able to use it to make a backup already "fixed" for the Mac. Clone the working Library over to a new one (so you don't mess up the current one). Then, do RMCF on it in Update Only mode, or use Find & Replace, to change the slashes on the cloned Library. It'll break all the links (turning them into the Mac valid filenames), which is exactly what you want. You might be able to get the forward slash to work
there by escaping it with the double slash method, where the Mac reverse the slashes logic might mess you up on the Mac.