Date Format Changing Program Not Working

[Ofnuts]

So below is the program that I am using to change files with European (DD-MM-YYYY) style date to American (MM-DD-YYYY) Style Date:

#! python3
   # renameDates.py - Renames filenames with American MM-DD-YYYY date format
   # to European DD-MM-YYYY.

Not very clear... changing European to American or vice-versa? Your regexp implies that your input is American (month first).

Several remarks:

1. (0|1|2|3) is best replaced by [0123] or even [0-3]
   
2. you can use re.sub (and back-references) to replace just the part that matches so you regexp doesn't have to deal with the bits outside the date part:
   

datePattern = re.compile(r'([01]?\d)-([0-3]?\d)-((19|20)\d\d)')

nameOut=re.sub(datePattern,r'\2-\1-\3',nameIn)

If nameOut==nameIn, the regexp was unmatched... or you where dealing with a file dated Jan 1st, Feb 2nd, etc... In any case you don't need to rename the file.