up

Author: iliakan

10-regular-expressions-javascript/09-regexp-groups/2-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 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
+```

10-regular-expressions-javascript/09-regexp-groups/2-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
+```