Thanks for your reply Kirk,
I'm working exclusively in the keywords field where I have reference numbers ("rin" numbers) for people in the respective images. When I began the system, I started with "rin1", "rin2" and so on, but now that the rin numbers have reached over the thousand mark I have problems with searching for individuals because "rin1" also matches "rin1234" etc. What I should have done was to start with "rin0001", "rin0002" and so on (isn't hindsight wonderful?). So, what I need to do is to add leading zeros to all rin numbers below 999 in about three thousand files and within subfolders. Any help with that would be very welcome!