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Ilya Kantor 2019-09-05 14:57:06 +03:00
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9-regular-expressions/11-regexp-groups/5-parse-expression/solution.md
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@ -1,16 +1,19 @@
A regexp for a number is: `pattern:-?\d+(\.\d+)?`. We created it in previous tasks.
An operator is `pattern:[-+*/]`.
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 `-`.
Please note:
- Here the dash `pattern:-` goes first in the brackets, because in the middle it would mean a character range, while we just want a character `-`.
- A slash `/` should be escaped inside a JavaScript regexp `pattern:/.../`, we'll do that later.
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+)?`.
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+)?)`.
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:
@ -29,11 +32,11 @@ The result includes:
- `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.
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 `pattern:result.shift()`.
The full match (the arrays first item) can be removed by shifting the array `result.shift()`.
The decimal groups can be removed by making them into non-capturing groups, by adding `pattern:?:` to the beginning: `pattern:(?:\.\d+)?`.
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: