1public static bool IsGUID(string expression)
2{
3    if (expression != null)
4    {
5        Regex guidRegEx = new Regex(@"^(\{{0,1}([0-9a-fA-F]){8}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){4}-([0-9a-fA-F]){12}\}{0,1})$");
6
7        return guidRegEx.IsMatch(expression);
8    }
9    return false;
10}

check if a string is a valid formatted GUID