3

I have a bit of problem with python regular expression first

import re

then I run a few test

>>> match = re.search(r'\w*i', "piiiiiiiiiiiiiiiiiiiip")
>>> print match.group()
>>>piiiiiiiiiiiiiiiiiiii


>>> match = re.search(r'i*i', "piiiiiiiiiiiiiiiiiiiip")
>>> print match.group()
>>>iiiiiiiiiiiiiiiiiiii

>>> match = re.search(r'i*', "iiiiiiiiiiiiiiiiiiiip")
>>> print match.group()
>>>iiiiiiiiiiiiiiiiiiii

>>>match = re.search(r'i*', "piiiiiiiiiiiiiiiiiiiig")
>>>print match.group()

>>> and got nothing

Do you guys know why the last one got nothing? I was expecting iiiiiiiiiiiiiiiiiiii as well.

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2 Answers 2

Reset to default
5

Because * is zero or more of the preceding element.

match = re.search(r'i+', "piiiiiiiiiiiiiiiiiiiig")
match.group()

Output:

'iiiiiiiiiiiiiiiiiiii'

Update

* is the equivalent of {0,}. When it find p, it satisfies requirement 0 i.

So it returns null character.

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1 Comment

Sorry, I still don't really get it. 0 ~ max?
3

Despide regexes being "greedy" unless there is a ? at the right place, i* matches at the start - before the p - as there are 0 is. As this counts as a match, the search isn't continued.

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1 Comment

Thanks for the explanation of why greediness doesn't affect the behavior here.

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