Question

I am writing a program to validate and correct a given date as a string. Lets take 04121987 as the date in the format ddmmyyyy. A regular expression for such a date:

(0[1-9]|[12][0-9]|3[01])(0[1-9]|1[012])(19\d\d|20\d\d)

If I match my string with the regular expression it works well. In Python:

>>> regex = re.compile(r'(0[1-9]|[12][0-9]|3[01])(0[1-9]|1[012])(19\d\d|20\d\d)')
>>> regex.findall('04121987')
[('04', '12', '1987')]

If I have a string 04721987 one can clearly see 72 is not a valid month and thus the string will not match the regex.

>>> regex.findall('04721987')
[]

What I would like to find out is the character which causes the regex to fail and its position. In this case it is 7. How could I do this in Python?

Answer 1

I believe what you want isn’t possible, because _sre module is implemented in C ;(.

You could try using this package instead (by monkey patching sre_compile, modifying the path and importing your new _srefirst, etc.) but I don’t think it worths it. It is an implementation of the _sre package fully written in Python, so you’ll be able to see the source code, edit it, and do something right when the next character doesn’t match.

You could do a similar thing by either:

• Splitting the date string in 3 (day, month and year) and matching the regular expressions independently
• validating the date time using another way that doesn’t involve regex

Perhaps you don’t obtain the exact digit where the error is, but I don’t think it makes too much sense in this scenario, as long as you tell the user what is wrong (day, month or year).