Regular Expressions Cookbook, 2nd Edition

4.5. Validate Traditional Date Formats, Excluding Invalid Dates

Problem

You want to validate dates in the traditional formats mm/dd/yy, mm/dd/yyyy, dd/mm/yy, and dd/mm/yyyy, as shown in Recipe 4.4. But this time, you also want to weed out invalid dates, such as February 31^st.

Solution

C#

The first solution requires the month to be specified before the day. The regular expression works with a variety of flavors:

^(?<month>[0-3]?[0-9])/(?<day>[0-3]?[0-9])/(?<year>(?:[0-9]{2})?[0-9]{2})$

Regex options: None

Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10

This is the complete solution implemented in C#:

DateTime foundDate;
Match matchResult = Regex.Match(SubjectString,
    "^(?<month>[0-3]?[0-9])/(?<day>[0-3]?[0-9])/" +
    "(?<year>(?:[0-9]{2})?[0-9]{2})$");
if (matchResult.Success) {
    int year = int.Parse(matchResult.Groups["year"].Value);
    if (year < 50) year += 2000;
    else if (year < 100) year += 1900;
    try {
        foundDate = new DateTime(year,
            int.Parse(matchResult.Groups["month"].Value),
            int.Parse(matchResult.Groups["day"].Value));
    } catch {
        // Invalid date
    }
}

The second solution requires the day to be specified before the month. The only difference is that we’ve swapped the names of the capturing groups in the regular expression.

^(?<day>[0-3]?[0-9])/(?<month>[0-3]?[0-9])/(?<year>(?:[0-9]{2})?[0-9]{2})$

Regex options: None

Regex flavors: .NET, Java 7, XRegExp, PCRE 7, Perl 5.10

The C# code is unchanged, except for the regular expression:

DateTime foundDate; Match matchResult = Regex.Match(SubjectString, ...