public class FormValidator
    {

        public static IDictionary RegexDictionary = new Dictionary() {
        { "Phone", new Regex("^[2-9]\\d{2}-\\d{3}-\\d{4}$")},
        { "Email", new Regex(@"^(([^<>()[\]\\.,;:\s@\""]+"
          + @"(\.[^<>()[\]\\.,;:\s@\""]+)*)|(\"".+\""))@"
          + @"((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}"
          + @"\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+"
          + @"[a-zA-Z]{2,}))$"
        )},
        { "URL", new Regex(@"(http(s)?://)?([\w-]+\.)+[\w-]+(/[\w- ;,./?%&=]*)?")},
        { "ZipCode", new Regex("^[0-9]{5}$")}
        };

        public static bool IsPhoneValid(string phone)
        {
            return RegexDictionary["Phone"].IsMatch(phone);
        }

        public static bool IsEmailValid(string email)
        {
            return RegexDictionary["Email"].IsMatch(email);
        }

        public static bool IsURLValid(string url)
        {
            return RegexDictionary["URL"].IsMatch(url);
        }

        public static bool IsZipCodeValid(string url)
        {
            return RegexDictionary["ZipCode"].IsMatch(url);
        }

    }

Use

if (String.IsNullOrEmpty(email) || !FormValidator.IsEmailValid(email))
// Do something email is required