Here is a nice function that works for up to 8 characters!

CREATE

FUNCTION dbo.fnFirstDifference

(

@FirstString VARCHAR(8),

@SecondString VARCHAR(8)

)

RETURNS VARCHAR(17)

AS

BEGIN

DECLARE @var VARBINARY(8),

@pos TINYINT

SELECT @var = CAST(CAST(@FirstString AS BINARY(8)) AS BIGINT) ^ CAST(CAST(@SecondString AS BINARY(8)) AS BIGINT),

@pos = PATINDEX('%[^0]%', SUBSTRING(master.dbo.fn_sqlvarbasetostr(@var), 3, 16)),

@pos = (1 + @pos) / 2

RETURN LEFT(@FirstString, @pos) + '-' + LEFT(@SecondString, @pos)

END