You might want to check www.regexlib.com

You could also look into DateTime.TryParseExact. It will try to parse a
date in a specific format, and tells you if it was a valid date or not.

something like this

Regex r = new Regex(@"(\d{1,2})/(\d{1,2})/(?<Year>(?:\d{4}|\d{2}))")

if(r.IsMatch(date_str))
{
string year = r.Match(date_str).Groups["Year"];
if (year.Length == 2 && Convert.ToInt32(year) >= 20) {
...date is correct
}
if (year.Length == 4 && Convert.ToInt32(year) >= 1920) {
...date is correct
}
}

I didn't test it, try it...

But I think it's better to convert it to the DateTime object, for
example with DateTime.TryParseExact as G?ran has suggested

Ysgrifennodd shapper:

I'd check for a valid separator, then convert it to a DateTime and let
that check for validity using DateTime.Parse.  Checking for the
separator's easy using a Regex.  Something like:

(\d[/\.-]){3}

This will pick up on all the leap years and so forth.

As for setting a minimum year, you can do that when you convert the
string to a datetime - although you've made it a bit more difficult for
yourself by only using 2 digit dates in one of your formats (i.e. are
you going to allow 19?  Does it mean 1919 or 2019?).  So on a quick
first look I'd say you can only meaningfully do that for 4 digit dates.

HTH

Peter