Back to the lesson

Parse an expression

An arithmetical expression consists of 2 numbers and an operator between them, for instance:

  • 1 + 2
  • 1.2 * 3.4
  • -3 / -6
  • -2 - 2

The operator is one of: "+", "-", "*" or "/".

There may be extra spaces at the beginning, at the end or between the parts.

Create a function parse(expr) that takes an expression and returns an array of 3 items:

  1. The first number.
  2. The operator.
  3. The second number.

For example:

let [a, op, b] = parse("1.2 * 3.4");

alert(a); // 1.2
alert(op); // *
alert(b); // 3.4

A regexp for a number is: -?\d+(\.\d+)?. We created it in previous tasks.

An operator is [-+*/]. We put a dash - the first, because in the middle it would mean a character range, we don’t need that.

Note that a slash should be escaped inside a JavaScript regexp /.../.

We need a number, an operator, and then another number. And optional spaces between them.

The full regular expression: -?\d+(\.\d+)?\s*[-+*/]\s*-?\d+(\.\d+)?.

To get a result as an array let’s put parentheses around the data that we need: numbers and the operator: (-?\d+(\.\d+)?)\s*([-+*/])\s*(-?\d+(\.\d+)?).

In action:

let reg = /(-?\d+(\.\d+)?)\s*([-+*\/])\s*(-?\d+(\.\d+)?)/;

alert( "1.2 + 12".match(reg) );

The result includes:

  • result[0] == "1.2 + 12" (full match)
  • result[1] == "1" (first parentheses)
  • result[2] == "2" (second parentheses – the decimal part (\.\d+)?)
  • result[3] == "+" (…)
  • result[4] == "12" (…)
  • result[5] == undefined (the last decimal part is absent, so it’s undefined)

We need only numbers and the operator. We don’t need decimal parts.

So let’s remove extra groups from capturing by added ?:, for instance: (?:\.\d+)?.

The final solution:

function parse(expr) {
  let reg = /(-?\d+(?:\.\d+)?)\s*([-+*\/])\s*(-?\d+(?:\.\d+)?)/;

  let result = expr.match(reg);

  if (!result) return;
  result.shift();

  return result;
}

alert( parse("-1.23 * 3.45") );  // -1.23, *, 3.45