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TinyExpr does what you want, and more.

TinyExpr is a very small recursive descent parser and evaluation engine for math expressions. It's handy when you want to add the ability to evaluation math expressions at runtime without adding a bunch of cruft to you project.

In addition to the standard math operators and precedence, TinyExpr also supports the standard C math functions and runtime binding of variables.

You'd just need to save tinyexpr.h and tinyexpr.c next to your Sketch.

#include "tinyexpr.h"

void setup() {
  Serial.begin(115200);
}

void loop() {
  char term[] = "12 + (2 * 5)";
  Serial.println(term);
  int error;
  double result = te_interp(term, &error);
  if (error){
    Serial.println("Problem with expression.");
  } else {
    Serial.printf(" = %.10g\n", result);
  }
  delay(1000);
}

It outputs:

12 + (2 * 5)
 = 22

TinyExpr knows the order of operations, and it understands many other mathematical expressions:

> 12 + 2 * 5
 = 22
> 3^5
 = 243
> exp(7)
 = 1096.633158
> (1 + sqrt(5)) / 2
 = 1.618033989
> 1 + 2 * 3 + 4 * 5
 = 27
> sin(pi/2)
 = 1
> sin(pi/3)
 = 0.8660254038