Matching dates with regular expressions in Python?

Question

I know that there are similar questions to mine that have been answered, but after reading through them I still don\'t have the solution I\'m looking for.

Using Pyth

Answer 1

Here's one way to make a regular expression that will match any date of your desired format (though you could obviously tweak whether commas are optional, add month abbreviations, and so on):

years = r'((?:19|20)\d\d)'
pattern = r'(%%s) +(%%s), *%s' % years

thirties = pattern % (
     "September|April|June|November",
     r'0?[1-9]|[12]\d|30')

thirtyones = pattern % (
     "January|March|May|July|August|October|December",
     r'0?[1-9]|[12]\d|3[01]')

fours = '(?:%s)' % '|'.join('%02d' % x for x in range(4, 100, 4))

feb = r'(February) +(?:%s|%s)' % (
     r'(?:(0?[1-9]|1\d|2[0-8])), *%s' % years, # 1-28 any year
     r'(?:(29), *((?:(?:19|20)%s)|2000))' % fours)  # 29 leap years only

result = '|'.join('(?:%s)' % x for x in (thirties, thirtyones, feb))
r = re.compile(result)
print result

Then we have:

>>> r.match('January 30, 2001') is not None
True
>>> r.match('January 31, 2001') is not None
True
>>> r.match('January 32, 2001') is not None
False
>>> r.match('February 32, 2001') is not None
False
>>> r.match('February 29, 2001') is not None
False
>>> r.match('February 28, 2001') is not None
True
>>> r.match('February 29, 2000') is not None
True
>>> r.match('April 30, 1908') is not None
True
>>> r.match('April 31, 1908') is not None
False

And what is this glorious regexp, you may ask?

>>> print result
(?:(September|April|June|November) +(0?[1-9]|[12]\d|30), *((?:19|20)\d\d))|(?:(January|March|May|July|August|October|December) +(0?[1-9]|[12]\d|3[01]), *((?:19|20)\d\d))|(?:February +(?:(?:(0?[1-9]|1\d|2[0-8]), *((?:19|20)\d\d))|(?:(29), *((?:(?:19|20)(?:04|08|12|16|20|24|28|32|36|40|44|48|52|56|60|64|68|72|76|80|84|88|92|96))|2000))))

(I initially intended to do a tongue-in-cheek enumeration of the possible dates, but I basically ended up hand-writing that whole gross thing except for the multiples of four, anyway.)