OK, I think this might be the last one I will find for the moment:

$ cat test.py
import re, regex

text = "test?"
regexp = "test\?"
sub_value = "result\?"
print repr(re.sub(regexp, sub_value, text))
print repr(regex.sub(regexp, sub_value, text))


$ python test.py
'result\\?'
'result?'