renames

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

@@ -0,0 +1,49 @@
+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:
+
+```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"` (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:
+
+```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
+```