python - How to find overlapping matches with a regexp?

Question

>>> match = re.findall(r'ww', 'hello')
>>> print match
['he', 'll']

Since ww means two characters, 'he' and 'll' are expected. But why do 'el' and 'lo' not match the regex?

>>> match1 = re.findall(r'el', 'hello')
>>> print match1
['el']
>>>

Answer 1

findall doesn't yield overlapping matches by default. This expression does however:

>>> re.findall(r'(?=(ww))', 'hello')
['he', 'el', 'll', 'lo']

Here (?=...) is a lookahead assertion:

(?=...) matches if ... matches next, but doesn’t consume any of the string. This is called a lookahead assertion. For example, Isaac (?=Asimov) will match 'Isaac ' only if it’s followed by 'Asimov'.