CODEWITHSUNDEEP
pythonregex
Jack Smother
Jack Smother

Reputation: 207

regex with ? and { } quantifiers

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

12
24
28
16

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.

Upvotes: 1

Views: 2038

Answers (2)

Ry-
Ry-

Reputation: 225125

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.

Upvotes: 4

user2357112
user2357112

Reputation: 281683

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.

Upvotes: 3

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