regexfail.py

import re
pattern = re.compile('^.*?((?<!\.[a-z0-9]{8})png|jpg|gif|swf|ico|flv|zip)$')
# I want the first two to match, but not the second two
file_list = [
    'main.ie6.zip',
    'main.zip',
    'main.cb1c4aa1.zip',
    'main.cb1c4aa1.cb1c4aa1.zip'
]

for f in file_list:
    print f
    if pattern.match(f):
        print pattern.match(f).groups()

The lack of a dot before the extension and the fact that the lookbehind is in one of the multiple options in the final matching set is, I think, what causes the failure. This works for me:

pattern = re.compile('^.*?(?<!\.[a-z0-9]{8}).(png|jpg|gif|swf|ico|flv|zip)$')

Yeah that's what Norm came up with too:

pattern = re.compile('^.*?(?<!\.[a-z0-9]{8})\.(png|jpg|gif|swf|ico|flv|zip)$')