Parse an expression

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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:

The first number.
The operator.
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 the previous task.

An operator is [-+*/]. The hyphen - goes first in the square brackets, because in the middle it would mean a character range, while we just want a character -.

The slash / should be escaped inside a JavaScript regexp /.../, we’ll do that later.

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+)?.

It has 3 parts, with \s* between them:

-?\d+(\.\d+)? – the first number,
[-+*/] – the operator,
-?\d+(\.\d+)? – the second number.

To make each of these parts a separate element of the result array, let’s enclose them in parentheses: (-?\d+(\.\d+)?)\s*([-+*/])\s*(-?\d+(\.\d+)?).

In action:

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

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

The result includes:

result[0] == "1.2 + 12" (full match)
result[1] == "1.2" (first group (-?\d+(\.\d+)?) – the first number, including the decimal part)
result[2] == ".2" (second group(\.\d+)? – the first decimal part)
result[3] == "+" (third group ([-+*\/]) – the operator)
result[4] == "12" (forth group (-?\d+(\.\d+)?) – the second number)
result[5] == undefined (fifth group (\.\d+)? – the last decimal part is absent, so it’s undefined)

We only want the numbers and the operator, without the full match or the decimal parts, so let’s “clean” the result a bit.

The full match (the arrays first item) can be removed by shifting the array result.shift().

Groups that contain decimal parts (number 2 and 4) (.\d+) can be excluded by adding ?: to the beginning: (?:\.\d+)?.

The final solution:

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

  let result = expr.match(regexp);

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

  return result;
}

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

As an alternative to using the non-capturing ?:, we could name the groups, like this:

function parse(expr) {
  let regexp = /(?<a>-?\d+(?:\.\d+)?)\s*(?<operator>[-+*\/])\s*(?<b>-?\d+(?:\.\d+)?)/;

  let result = expr.match(regexp);

  return [result.groups.a, result.groups.operator, result.groups.b];
}

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