"+" does not match?

[Thomas Thiele]

>>> import regex 

>>> s1 = "c++test/"
>>> s2 = "c++test/Derivetest.c++"
>>> regex.match(s1,s2)
-1

but if I substitude the "+" I'll get the correct behaviour.

>>> s1 = "c11test/"
>>> s2 = "c11test/Derivetest.c++"
>>> regex.match(s1,s2)
8

Why? Can anybody explain me that?