UDF Luhn's formula-credit card number validation

CREATE FUNCTION luhn (@s varchar(20) )
--Luhn's formula-credit card number validation
RETURNS bit AS  
BEGIN 
declare @i int, @j int, @k int, @tmp int, @result bit
SET @result=0
IF @s is null 
GOTO lblFail
IF len(@s)=0 
GOTO lblFail
SET @i=1
while @i<=len(@s)
BEGIN
IF ISNUMERIC(substring(@s,@i,1))=0
GOTO lblFail
SET @i=@i+1
END
SET @j=0
SET @k=0
SET @i=len(@s)
while @i>0
BEGIN
IF @i>1
BEGIN
SET @tmp=(ASCII(substring(@s,@i-1,1))-48)*2
IF @tmp>9
SET @j=@j+@tmp-9
ELSE
SET @j=@j+@tmp
END
SET @k=@k+(ASCII(substring(@s,@i,1))-48)
SET @i=@i-2
END
SET @j=(@j+@k) % 10
IF @j=0
SET @result=1
lblFail:
return (@result)
END