Question

I’m trying to use a modified preg format from preg_match: check birthday format (dd/mm/yyyy) to match credit card expiration dates (yyyy-MM formats)

    if (!preg_match('/([0-9]{4})\-([0-9]{2})/', $expirationDate, $matches)) {
        throw new Services_Payment_Exception('Card expiration date is invalid');
    }

For some reason, it also validate invalid values such as 20111-02 (invalid year).
What am I doing wrong here? I want to confirm the year is 4 digits and the month is 2 digits (01, 02.. 12)

Answer 1

Anchor your regexp:

preg_match('/^([0-9]{4})-([0-9]{2})$/', $expirationDate, $matches)

Your regexp didn’t do what you expected because it matches “0111-02” substring of “20111-02”.

Anchors ^ and $ match particular positions within the input string: ^ matches the beginning of the string and $ matches the end.

Note also that there is no need to escape the hyphen since it only has a special function in [].