isuniqueidentifier

CREATE FUNCTION dbo.isuniqueidentifier (@ui varchar(50))  
RETURNS bit AS  
BEGIN

RETURN case when
    substring(@ui,9,1)='-' and
    substring(@ui,14,1)='-' and
    substring(@ui,19,1)='-' and
    substring(@ui,24,1)='-' and
    len(@ui) = 36 then 1 else 0 end

END
GO