If the specification is for a single operation with an integer on either side ...
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]