Python 2.4.3 (#2, Oct  6 2006, 07:52:30)
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2

Tried:

>>> re.findall(r'(?!def)\b(\S+)\(', "def bla(): dof blu()")

>>> re.findall(r'(?:def){0}\b(\S+)\(', "def bla(): dof blu()")

Result:

['bla', 'blu']

Expected:

['blu']


Why doesn't (?!) work like it does here?:

>>> re.findall(r'\b(\S+): (?!bad)', "bob: bad; suzy: good")
['suzy']


Wouldn't it be nice if (^) worked?

>>> re.findall(r'\b(\S+): (^bad)', "bob: bad; suzy: good")
[]

[^()] does, sorta. Also not before a capture group:

>>> re.findall(r'\b(\S+): [^(bad)]', "bob: bad; suzy: good")
['suzy']
>>> re.findall(r'[^(def)]\b(\S+)\(', "def bla(): dof blu()")
['bla', 'blu']
>>> re.findall(r'[^(def)] (\S+)\(', "def bla(): dof blu()")
[]
>>> re.findall(r'(^def) (\S+)\(', "def bla(): dof blu()")
[('def', 'bla')]

What you want is
>>> re.findall(r'(?<!def)\s(\S+)\(', "def bla(): dof blu()")

\b doesn't match a space, it matches an end of word. And to do a lookbehind assertion, use (?<[!]...)

Why on earth do you expect that (^...) works?
[^(bad)] is something entirely different, it's a character class excluding (, b, a, d and ).

I tried (^ because [^ works. (^ doesn't seem to do anything. To match ^ inside () you need to use (\^), anyway.

Btw, thanks for the explanation, and I think you meant (?<[!=]

My final pattern:
>>> re.findall(r'(?<!def)[\W]+([.\w]+)\(', "def bla(): [ryawry.aet().blu()]")
['ryawry.aet', 'blu']