CREATE FUNCTION dbo.fnDigitsOnly
        (@String NVARCHAR(4000))
RETURNS NVARCHAR(4000)
     AS
  BEGIN
         SELECT @String = STUFF(@String,N,1,'')
           FROM dbo.Tally
          WHERE N <= LEN(@String)
            AND (SUBSTRING(@String,N,1) < '0'
                 OR 
                 SUBSTRING(@String,N,1) > '9')
          ORDER BY N DESC
 RETURN @String
    END