regexp

9-regular-expressions/11-regexp-groups/04-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:[-+*/]`. The hyphen `pattern:-` 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 `pattern:/.../`, we'll do that later.
+
+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+)?`.
+
+It has 3 parts, with `pattern:\s*` between them:
+1. `pattern:-?\d+(\.\d+)?` - the first number,
+1. `pattern:[-+*/]` - the operator,
+1. `pattern:-?\d+(\.\d+)?` - the second number.
+
+To make each of these parts a separate element of the result array, let's enclose them in parentheses: `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, 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) `pattern:(.\d+)` can be excluded 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
+```

9-regular-expressions/11-regexp-groups/04-parse-expression/task.md

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+# 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:
+
+```js
+let [a, op, b] = parse("1.2 * 3.4");
+
+alert(a); // 1.2
+alert(op); // *
+alert(b); // 3.4
+```