string1 = '%(example_1).40s-a%(example-2)s_-%(example3)s_s1'

'-a', '_-', '_s1'

re.findall("[-_a-z0-9]+(?![^%]*s)", string1)

['example_1', '0s-a', 'example-', 's_-', 'example', 's_s1']

re.findall("[-_a-z0-9]+(?![^(]*))", string1)

['40s-a', 's_-', 's_s1']