regexp
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# Parse an expression
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An arithmetical expression consists of 2 numbers and an operator between them, for instance:
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- `1 + 2`
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- `1.2 * 3.4`
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- `-3 / -6`
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- `-2 - 2`
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The operator is one of: `"+"`, `"-"`, `"*"` or `"/"`.
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There may be extra spaces at the beginning, at the end or between the parts.
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Create a function `parse(expr)` that takes an expression and returns an array of 3 items:
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1. The first number.
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2. The operator.
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3. The second number.
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For example:
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```js
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let [a, op, b] = parse("1.2 * 3.4");
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alert(a); // 1.2
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alert(op); // *
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alert(b); // 3.4
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```
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