Maybe it's worth using this negative lookbehind prefix (instead of (?<!_)), to avoid matching /__tests__/ and /__init__.py (i.e., "__" is not valid after "/" or "_"):

(?<!_|/)

...and this suffix, for the same cases:

(?!/|\.\S)

(i.e., "__" is not valid before ".x" or "/").

Will update and send out a new diff once I am sure it works... I readily admit my regex foo is yellow belt at best. =D