lookahead & lookbehind in python

lookahead.py

>>> import re
>>> msg1 = "aaabbbccc"
>>> msg2 = "dddbbbeee"
>>> re.findall("bbb(?=ccc)", msg1) # lookahead
['bbb']
>>> re.findall("bbb(?=ccc)", msg2)
[]
>>> re.findall("bbb(?!ccc)", msg1) # negative lookahead
[]
>>> re.findall("bbb(?!ccc)", msg2)
['bbb']
>>> re.findall("(?<=aaa)bbb", msg1) # lookbehind
['bbb']
>>> re.findall("(?<=aaa)bbb", msg2)
[]
>>> re.findall("(?<!aaa)bbb", msg1) # negative lookbehind
[]
>>> re.findall("(?<!aaa)bbb", msg2)
['bbb']