Regular expression for date validation. Just for fun.

regexp-date-validation.php

$pattern =
'/^
    # year
    (?!0000)\pN{2}(?:(
        (?:
            # leap year
            ([02468])|[13579])(?(2)[048]|[26]))(?(?<=00)(?(?<=[02468].{3})(?<=[048].{2})|(?<=[26].{2})))|
            # non-leap year
            \pN{2}
        )
    \.
    # month
    (?:(0)|1)(?(3)[1-9]|[0-2])
    \.
    # date
    (?!00)(?:(?<!02\.)(3)|[0-2])
        (?(4)
            # IV quarter
            (?(?<=(?:01|03|05|07|08|10|12).{2})[01]|0)|
            # I-III quarter
            (?(?<=02.{2})
                # February
                (?(?<=2)
                    # III quarter
                    (?(1)
                        # leap year
                        \pN|
                        # non-leap year
                        [0-8]
                    )|
                    # I-II quarter
                    \pN
                )|
                # not February
                \pN
            )
    )
$/ux';

var_dump(preg_match($pattern, date('Y.m.d'))); // 1