components

9-regular-expressions/09-regexp-groups/5-parse-expression/solution.md

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+A regexp for a number is: `pattern:-?\d+(\.\d+)?`. We created it in previous tasks.
+
+An operator is `pattern:[-+*/]`. We put the dash `pattern:-` 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:
+
+```js run
+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] == "1.2"` (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.
+
+The full match (the arrays first item) can be removed by shifting the array `pattern:result.shift()`.
+
+The decimal groups can be removed by making them into non-capturing groups, by adding `pattern:?:` to the beginning: `pattern:(?:\.\d+)?`.
+
+The final solution:
+
+```js run
+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
+```