Question

What am i doing wrong in the below regular expression matching

>>> import re
>>> d="30-12-2001"
>>> re.findall(r"\b[1-31][/-:][1-12][/-:][1981-2011]\b",d)
[]

Answer 1

[1-31] matches 1-3 and 1 which is basically 1, 2 or 3. You cannot match a number rage unless it’s a subset of 0-9. Same applies to [1981-2011] which matches exactly one character that is 0, 1, 2, 8 or 9.

The best solution is simply matching any number and then checking the numbers later using python itself. A date such as 31-02-2012 would not make any sense – and making your regex check that would be hard. Making it also handle leap years properly would make it even harder or impossible. Here’s a regex matching anything that looks like a dd-mm-yyyy date: \b\d{1,2}[-/:]\d{1,2}[-/:]\d{4}\b

However, I would highly suggest not allowing any of -, : and / as : is usually used for times, / usually for the US way of writing a date (mm/dd/yyyy) and - for the ISO way (yyyy-mm-dd). The EU dd.mm.yyyy syntax is not handled at all.

If the string does not contain anything but the date, you don’t need a regex at all – use strptime() instead.

All in all, tell the user what date format you expect and parse that one, rejecting anything else. Otherwise you’ll get ambiguous cases such as 04/05/2012 (is it april 5th or may 4th?).