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]