How can I do arithmetic operation in JMeter on a regex string extracted from html?

The extracted String looks as follows

a+b or a-b

a and b will be integers.

As of now I did regex extractor and I have extracted the expression and assigned it to a variable.

But I am struck in doing the arithmetic operation and passing on the result value to next request.

I figured out that I can do using Bean shell processor or JSR233 processor using Java or JEXL or JS. But I don't know how to do and If some one provide me a working example, it would be useful.