How to find overlapping matches with a regexp?

python regex overlapping
futurenext110 picture futurenext110 · Jul 11, 2012 · Viewed 34.6k times · Source 
>>> match = re.findall(r'\w\w', 'hello')
>>> print match
['he', 'll']

Since \w\w 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

Otto Allmendinger picture Otto Allmendinger · Jul 11, 2012

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

>>> re.findall(r'(?=(\w\w))', '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'.

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