regex with ? and { } quantifiers

Question

I'm trying to create a regex to validate numbers that checks the year for leap year. here's a part of the code. for some reason this code would let number 4 8 24 28 as a valid regex.

(0{2}?)
([2468][480] | [13579][26])



pattern = re.compile (r"""

    ((0{2}?)([2468][480] | [13579][26]))

    """, re.X)

when I left out

(0{2}?)

24 12 and everything works..

I'm using verbose so spacing shouldn't matter..

Invalid

EDIT :: Actually all is invalid now..

i don't understand why 24 is invalid and 28 is invalid this doesn't make sense at all. I appreciate your guidance.

We have a problem. "Wait, I know regex." Now we have two problems. — g.d.d.c
– g.d.d.c, Commented Feb 9, 2014 at 5:16
Can you show us runnable code that demonstrates the problem, with actual syntax and actual spacing? — user2357112
– user2357112, Commented Feb 9, 2014 at 5:18

Ry- · Accepted Answer · 2018-03-09 22:37:37Z

4

When you write (0{2}?), that means “match two 0s here, but match as few as possible”. Non-greediness doesn’t really make sense for an {n} quantifier (it does for {n,}, and {m,n}) – did you mean (0{2})??

Oh, and do keep in mind that years divisible by 400 are leap years.

edited Mar 9, 2018 at 22:37

answered Feb 9, 2014 at 5:28

Ry-♦

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2 Comments

Jack Smother Over a year ago

Yes that's my mistake. Thank you! What kind of mistake what am I looking for and which documentation should I look at? Thanks so much.

Ry- Over a year ago

@LeonardLie: I don’t think this is specifically documented anywhere, but most quantifiers can have a ? appended to them to make them non-greedy. Just remember that a quantifier can’t directly follow a quantifier.

user2357112 · Accepted Answer · 2014-02-09 05:30:08Z

3

Using the re.DEBUG flag to show debug info about the expression, we get

>>> pattern = re.compile(r'0{2}?', re.DEBUG)
min_repeat 2 2
  literal 48

The min_repeat shows that 0{2}? isn't being interpreted as ? applied to 0{2}. It's being interpreted as a lazy quantifier, attempting to match 0 any number of times from 2 to 2, but as few as possible. This doesn't quite seem consistent with the documentation; the docs only show the {m,n}? form.

answered Feb 9, 2014 at 5:30

user2357112

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