Python Regex Behaviour

Question

I'm trying to parse a text document with data in the following format: 24036 -977. I need to separate the numbers into separate values, and the way I've done that is with the following steps.

values = re.search("(.*?)\s(.*)")
x = values.group(1)
y = values.gropu(2)

This does the job, however I was curious about why using (.*?) in the second group causes the regex to fail? I tested it in the online regex tester(https://regex101.com/r/bM2nK1/1), and adding the ? in causes the second group to return nothing. Now as far as I know .*? means to take any value unlimited times, as few times as possible, and the .* is just the greedy version of that. What I'm confused about is why the non greedy version.*? takes that definition to mean capturing nothing?

@stribizhev also programmers.stackexchange.com/questions/113237/… — NDevox
– NDevox, Commented Jul 15, 2015 at 14:39
@JustinWang don't forget to mark the answer most suited to your problem as correct. — NDevox
– NDevox, Commented Jul 15, 2015 at 14:40

iobender · Accepted Answer · 2015-07-15 14:28:03Z

3

Because it means to match the previous token, the *, as few times as possible, which is 0 times. If you would it to extend to the end of the string, add a $, which matches the end of string. If you would like it to match at least one, use + instead of *.

The reason the first group .*? matches 24036 is because you have the \s token after it, so the fewest amount of characters the .*? could match and be followed by a \s is 24036.

answered Jul 15, 2015 at 14:28

iobender

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NDevox · Accepted Answer · 2015-07-15 14:28:19Z

1

@iobender has pointed out the answer to your question.

But I think it's worth mentioning that if the numbers are separated by space, you can just use split:

>>> '24036 -977'.split()
['24036', '-977']

This is simpler, easier to understand and often faster than regex.

answered Jul 15, 2015 at 14:28

NDevox

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2 Comments

Justin Wang Over a year ago

Yep, this is actually a better implementation. I've just mostly been doing text parsing in python, so I defaulted to regex, so it didn't occur to me to look for other things. Thanks

NDevox Over a year ago

@JustinWang, no problem - I know many people who default to regex for the simplest things, always worth trying another route first.

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