Regex to capture possible US date entries

Question

The Problem

I'm relying on plaintext entry to capture dates.

End users are urged to enter as MM-DD-YY, but it'll be great to account for as many possibilities.

US Date Format is assumed

I.e. MM-DD-YYYY, M-D-YYYY,M-D-YY, MM/DD/YYYY, MM/DD/YY, etc..

Current Code

//'1 or 2 digits...and then a single hyphen or forward slash..(REPEAT LAST PATTERN) followed by 2 or 4 digits
//MOD named group stands for 'Month or Day'

/(?<MOD>\d{1,2}[-\/])(?&MOD)(?:\d{4}|\d{2})/ 

You can run this with some unit tests I created too on https://regex101.com/r/LzoqRa/4

Questions

1. Anything wrong with regex?
2. Any way to simplify regex?
3. Any other edge cases I should be thinking about?
4. Is it worth it to capture MOD and recurse? Or should I just copy the pattern?
5. What's the difference between regex match and recurse (relating to doing this on a previous pattern)?

Answer 1

Your ~(?<MOD>\d{1,2}[-/])(?&MOD)(?:\d{4}|\d{2})~ pattern takes 204 steps on your sample string.

The equivalent ~(?:\d{1,2}[-/]){2}(?:\d{4}|\d{2})~ pattern takes 188steps on your sample string.

The step count is a loose metric available to help you understand the efficiency of your pattern. The step count indicates that the recursion is doing more work than necessary.

But consider the string 5/6-1999, neither of the above patterns are preventing mixed delimiters.

Using a capture group on the first delimiter and then matching that specific delimiting character as the second delimiter seems critical if you are not going to pre-sanitize the input string. ~\d{1,2}([-/])\d{1,2}\1(?:\d{4}|\d{2})~ This further reduces the step count to 161, so it not only improves accuracy, it improves efficiency, but at a trivial cost to pattern brevity.

What @KIKOSoftware mentions as a comment under the question is also important. Your list of valid formats is not world-ready.

---

So, what is your goal here? If you are trying to validate a date looking "kinda" like a date, regex is fine. If you are actually trying to validate a date as a real date, then regex is certainly the wrong tool for the job.

To validate the string, use a loose pattern to isolate the components of your string, then feed them to the datetime class for parsing. If the string is parsed as a valid date that matches the sanitized input, then it is a good date. (Demo)

function isValidDate(string $date, string $format = 'm/d/Y', string $yearPadding = '19'): bool {
    $date = preg_replace_callback(
        '~^(\d+)(\D)(\d+)\2(\d{2}|\d{4})$~',
        fn($m) => sprintf('%02d/%02d/%d', $m[1], $m[3], str_pad($m[4], 4, $yearPadding, STR_PAD_LEFT)),
        $date
    );  // sanitize
    $d = DateTime::createFromFormat($format, $date);  // attempt to parse
    return $d && $d->format($format) === $date;  // check if formatted string is same as sanitized string
}

---

To validate the string and return it if it is deemed to be valid, you can modify the above like this: (Demo)

function getFormattedDate(string $date, string $format = 'm/d/Y', string $yearPadding = '19'): string {
    $sanitizedDate = preg_replace_callback(
        '~^(\d+)(\D)(\d+)\2(\d{2}|\d{4})$~',
        fn($m) => sprintf('%02d/%02d/%d', $m[1], $m[3], str_pad($m[4], 4, $yearPadding, STR_PAD_LEFT)),
        $date
    );
    $d = DateTime::createFromFormat($format, $sanitizedDate);
    if (!$d) {
        throw new Exception("Could not parse date: $date");
    }
    $formattedDate = $d->format($format);
    if ($formattedDate !== $sanitizedDate) {
        throw new Exception("Date not in a desired American format: $date");
    }
    return $date;
}

foreach ($tests as $test) {
    try {
        echo getFormattedDate($test);
    } catch (Exception $e) {
        echo $e->getMessage();
    }
    echo "\n";
}