String contains only a given set of characters

Question

I need to know if a given string is a valid DateTime format string because the string may represent other things. I tried DateTime.ParseExact(somedate.ToString(format), format)

Answer 1

Thank you everyone. I 'upped' all of you and settled on a brute force implementation that doesn't use a Dictionary/HashSet and doesn't convert chars to strings:

private const string DateTimeFormatCharacters = "yYmMdDhHsS";
private static bool IsDateTimeFormatString(string input)
{
    foreach (char c in input)
        if (DateTimeFormatCharacters.IndexOf(c) < 0)
            return false;
    return true;
}

Answer 2

Slightly shorted Dan Tao's version since string represents an implementation of IEnumerable&lt&char>

   [TestClass]
   public class UnitTest1 {
      private HashSet<char> _legalChars = new HashSet<char>("yYmMdDsShH".ToCharArray());

      public bool IsPossibleDateTimeFormat(string format) {
         if (string.IsNullOrEmpty(format))
            return false; // or whatever makes sense to you
         return !format.Except(_legalChars).Any();
      }

      [TestMethod]
      public void TestMethod1() {
         bool result = IsPossibleDateTimeFormat("yydD");
         result = IsPossibleDateTimeFormat("abc");
      }
   }

Answer 3

Something like

Regex regex = new Regex("^(y|Y|m|M|d|D|s|S|h|H)+$");
if (regex.IsMatch('DateTime String'))
{
    // 'valid' 
}

if you're literally searching for those characters and not the numerical representation for a given date and time

Answer 4

With .NET2, you need to roll your own check for this. For example, the following method uses a foreach to check:

bool FormatValid(string format)
{
    string allowableLetters = "yYmMdDsShH";

    foreach(char c in format)
    {
         // This is using String.Contains for .NET 2 compat.,
         //   hence the requirement for ToString()
         if (!allowableLetters.Contains(c.ToString()))
              return false;
    }

    return true;
}

If you had the option of using .NET 3.5 and LINQ, you could use Enumerable.Contains to work with characters directly, and Enumerable.All. This would simplify the above to:

bool valid = format.All(c => "yYmMdDsShH".Contains(c));

Answer 5

There's a new project, NLib, which can do this much faster:

if (input.IndexOfNotAny(new char[] { 'y', 'm', 'd', 's', 'h' }, StringComparison.OrdinalIgnoreCase) < 0)
{
    // Valid
}

Answer 6

Like this:

static readonly Regex Validator = new Regex(@"^[yYmMdDsShH]+$");

public static bool IsValid(string str) {
    return Validator.IsMatch(str);
}

The regex works like this:

• ^ matches the beginning of the string
• [...] matches any of the characters that appear in the brackets
• + matches one or more characters that match the previous item
• $ matches the end of the string

Without the ^ and $ anchors, the regex will match any string that contains at least one valid character, because a regex can match any substring of the string use pass it. The ^ and $ anchors force it to match the entire string.