CREATE FUNCTION [SimpleMath](@expression VARCHAR(max))
RETURNS INT
AS
BEGIN

	DECLARE
		@position INT,
		@operator CHAR(1),
		@left INT,
		@right INT,
		@result INT

	SELECT
		@position = PATINDEX('%[*/+-]%', @expression),
		@operator = SUBSTRING(@expression, @position, 1),
		@left = CONVERT(INT, LTRIM(RTRIM(LEFT(@expression, @position - 1)))),
		@right = CONVERT(INT, LTRIM(RTRIM(RIGHT(@expression, LEN(@expression) - (@position + 1))))),
		@result = CASE @operator
				WHEN '*' THEN @left * @right
				WHEN '/' THEN @left / @right
				WHEN '+' THEN @left + @right
				WHEN '-' THEN @left - @right
			END

 RETURN @result
END
GO

;WITH [data] ([expression]) AS (
  SELECT '1 + 5' UNION ALL
  SELECT '10 * 4' UNION ALL
  SELECT '15 / 3' UNION ALL
  SELECT '100 - 25'
 )
SELECT
 [expression],
 [result] = [dbo].[SimpleMath]([expression])
FROM [data]