# Ruby evaluate without eval?

How could I evaluate at mathematical string without using eval?

Example:

mathstring = "3+3"

Anyway that can be evaluated without using eval?

Maybe something with regex..?

## Answers

You must either or eval it, or parse it; and since you don't want to eval:

mathstring = '3+3' i, op, j = mathstring.scan(/(\d+)([+\-*\/])(\d+)/)[0] #=> ["3", "+", "3"] i.to_i.send op, j.to_i #=> 6

If you want to implement more complex stuff you could use RubyParser (as @LBg wrote here - you could look at other answers too)

I'm assuming you don't want to use eval because of security reasons, and it is indeed very hard to properly sanitize input for eval, but for simple mathematical expressions perhaps you could just check that it only includes mathematical operators and numbers?

mathstring = "3+3" puts mathstring[/\A[\d+\-*\/=. ]+\z/] ? eval(mathstring) : "Invalid expression" => 6

You have 3 options:

- In my honest opinion best - parse it to Reverse Polish Notation and then parse it as equation
- As you say use RegExps
Fastest, but dangerous and by calling eval but not Kernel#eval

RubyVM::InstructionSequence.new(mathstring).eval

Sure--you'd just want to somehow parse the expression using something other than the bare Ruby interpreter.

There appear to be some good options here: https://www.ruby-toolbox.com/search?q=math

Alternatively, it probably wouldn't be *that* hard to write your own parser. (Not that I've seriously tried--I could be totally full of crap.)

Dentaku seems (I haven't used it yet) like a good solution - it lets you check your (mathematical and logical) expressions, and to evaluate them.

calculator = Dentaku::Calculator.new calculator.evaluate('kiwi + 5', kiwi: 2)