Help creating a pattern using the re module

[zulu_likuum]

I have written a python3 script to rename files based on a pattern input by the user. It has worked well, but I now am trying to create a pattern, and I am running into problems.

I have four types of filenames:
filename 1 - flubber.MP4
filename 2.MP4
filename 3 - flubber.mp4
filename 4.mp4

I am trying to create a pattern that will work on all these file names and rename them all as:
filename1.mp4
filename2.mp4
filename3.mp4
filename4.mp4 (This one of course would not be renamed)

I can easily accomplish this in two passes, but I would like to accomplish it in one pass. Help?