Of the top of my head.

IF EXISTS (SELECT * 
	   FROM   sysobjects 
	   WHERE  name = N'ufn_ValidateSSN')
	DROP FUNCTION ufn_ValidateSSN
GO

CREATE FUNCTION ufn_ValidateSSN 
	(@SSN char(11))
RETURNS int
AS
BEGIN
	DECLARE @Ret int, @i int
        SET @I = 1
	WHILE @I <= LEN(@SSN)
	BEGIN
		IF @I IN (4, 7) 
			BEGIN IF SUBSTRING(@SSN, @I,1) <> '-' BREAK END 
		ELSE 
		IF NOT (SUBSTRING(@SSN, @I,1) BETWEEN '0' AND '9') BREAK
		SET @I = @i + 1
	END
	
	RETURN CASE WHEN @I <= LEN(@SSN) THEN 0 ELSE 1 END
END

>I need to write a UDF to validate existing SSN in our system. We have found numerous SSN's with a character in the number. The numbers were imported from another system that did not validate the information.
>
>I need to look at each position in the ssn and determine if it is a character or a number.
>
>@ivalue = ASCII(SUBSTR(@cssn, @iposition, 1)
>
>Then I look to see the value of @ivalue is between 48 and 57.
>
>Any ideas?