Variable length string validation against whitelisted characters

Question

I need to validate a variable length string against some white-listed characters. The algorithm that I have works but I have a sneaking suspicion that there is a more elegant solution. The code below hasn't been identified as a bottleneck so ease of maintenance is preferred over a clever but confusing implementation.

private static readonly Regex ReferralCodeValidator = new Regex(
 @"[ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789]",
 RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.IgnoreCase);
private const int MinLength = 3;
private const int MaxLength = 8;
public static bool IsValidReferralCode(this string code)
{
 if (string.IsNullOrWhiteSpace(code))
 {
 return false;
 }
 if (code.Length < MinLength || code.Length > MaxLength)
 {
 return false;
 }
 // Variable for clarity and easier regex troubleshooting
 var strippedCode = ReferralCodeValidator.Replace(code, string.Empty);
 return strippedCode == string.Empty;
}

I also have the following NUnit tests to validate/exercise the solution:

[Test]
[TestCase("AARPeVER", true)]
[TestCase("1234EVE", true)]
[TestCase(null, false)]
[TestCase("AARP", true)]
[TestCase("AA", false)]
[TestCase("AAr", true)]
[TestCase("123456789", false)]
[TestCase("AARP0", false, Description = "Zero isn't allowed.")]
[TestCase("AARPO", true, Description = "The letter O is allowed.")]
[TestCase("", false)]
[TestCase(" ", false)]
[TestCase("AAR ", false)]
[TestCase("AAR\n", false)]
[TestCase("Ever||12", false)]
public void IsValidReferralCode(string input, bool success)
{
 Assert.AreEqual(success, input.IsValidReferralCode());
}

Answer 1

Instead of removing valid characters, you may look for invalid ones:

 private static readonly Regex ReferralCodeValidator = new Regex(
 @"[^ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789]",
 RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.IgnoreCase);
 public static bool IsValidReferralCode(this string code)
 {
 // preliminary tests omitted for brevity
 return !ReferralCodeValidator.Match(code).Success;
 }

I can't say if this solution is more elegant (beauty is in the eye of beholder), but it surely looks more straightforward.

• vnp: @Erik Notice the ^ at the beginning of the regex. It negates the meaning - that is, it builds a blacklist from your whitelist, and the regex returns Success if the string has at least one blacklisted symbol.
• Erik: I ended up using a modified version of your code and renamed the variable ReferralCodeValidator to ReferralCodeBlackList. Here is the relevant code: return !ReferralCodeBlackList.IsMatch(code);