C#

public class SettingSection : ConfigurationSection
{
   [ConfigurationProperty("appId", IsKey=true, IsRequired=true), ConfigurationValidator(typeof(AppIdValidator))]
   public string AppId
   {
      get { return (string)base["appId"]; }
   }
}

public class AppIdValidator : ConfigurationValidatorBase
{
    public override bool CanValidate(Type type)
    {
        return true;
    }

    public override void Validate(object value)
    {
       string val = (string)value;
       Regex regex = new Regex(@"\A\d{4}-\d{4}-\d{4}-\d{4}\z");
       if (regex.IsMatch(val))
       {
          return;
       }
       else
       {
          throw new FormatException("Input string is not in correct format");
       }
   }
}