Email validation - the point is to show that the extension method doesn't actually do anything. The real logic is in the validator class.

email_validation.cs

public class EmailValidator
{
    private const string PATTERN = "^[A-Z0-9._%+-]+@[A-Z0-9.-]+\\.[A-Z]{2,4}$";

    public static bool IsValid(string s)
    {
        if (string.IsNullOrEmpty(s))
            return false;

        // Regex from http://www.regular-expressions.info/email.html
        bool isMatch = Regex.IsMatch(s, PATTERN, RegexOptions.IgnoreCase);
        return isMatch;
    }
}

public static class StringExtensions
{
    public static bool IsEmail(this string str)
    {
        return EmailValidator.IsValid(str);
    }
}