Another simple function in c# which is usefull for validating a guid passed in a url string which may have been modified to try to invoke a bug / issues in an asp.net application.

public static bool IsGUID(string expression)
{
    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})$");
    return guidRegEx.IsMatch(expression);
}