import re
except_contexts = ['cat','hat','mat','ate']
print(re.sub(r'({})|a'.format("|".join(except_contexts)), lambda x: x.group(1) if x.group(1) else '', 'acatbatmate'))
aW1wb3J0IHJlCmV4Y2VwdF9jb250ZXh0cyA9IFsnY2F0JywnaGF0JywnbWF0JywnYXRlJ10KcHJpbnQocmUuc3ViKHInKHt9KXxhJy5mb3JtYXQoInwiLmpvaW4oZXhjZXB0X2NvbnRleHRzKSksIGxhbWJkYSB4OiB4Lmdyb3VwKDEpIGlmIHguZ3JvdXAoMSkgZWxzZSAnJywgJ2FjYXRiYXRtYXRlJykp