en.javascript.info/10-regular-expressions-javascript/09-regexp-groups/5-parse-expression/solution.md

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

An operator is pattern:[-+*/]. We put a dash pattern:- 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 pattern:/.../.

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

The full regular expression: pattern:-?\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: pattern:(-?\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 pattern:?:, for instance: pattern:(?:\.\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