Can I perform any regex action prior to import?

The images in a couple of feeds I need to import are small and getting a bigger image is just a matter of changing the suffix of the file name (like from file_s.jpg to file_m.jpg for example) or the path in the url (like /imgs/thumb/file.jpg to /imgs/big/file.jpg for example).

This obviously needs to be done prior to the image import.
Is there any plugin for Autoblog that does this?
If there isn't, can you recommend one of the existing ones as a starting point?