49 lines
1.5 KiB
Markdown
49 lines
1.5 KiB
Markdown
A regexp for a number is: `pattern:-?\d+(\.\d+)?`. We created it in previous tasks.
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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.
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Note that a slash should be escaped inside a JavaScript regexp `pattern:/.../`.
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We need a number, an operator, and then another number. And optional spaces between them.
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The full regular expression: `pattern:-?\d+(\.\d+)?\s*[-+*/]\s*-?\d+(\.\d+)?`.
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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+)?)`.
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In action:
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```js run
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let reg = /(-?\d+(\.\d+)?)\s*([-+*\/])\s*(-?\d+(\.\d+)?)/;
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alert( "1.2 + 12".match(reg) );
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```
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The result includes:
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- `result[0] == "1.2 + 12"` (full match)
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- `result[1] == "1"` (first parentheses)
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- `result[2] == "2"` (second parentheses -- the decimal part `(\.\d+)?`)
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- `result[3] == "+"` (...)
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- `result[4] == "12"` (...)
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- `result[5] == undefined` (the last decimal part is absent, so it's undefined)
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We need only numbers and the operator. We don't need decimal parts.
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So let's remove extra groups from capturing by added `pattern:?:`, for instance: `pattern:(?:\.\d+)?`.
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The final solution:
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```js run
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function parse(expr) {
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let reg = /(-?\d+(?:\.\d+)?)\s*([-+*\/])\s*(-?\d+(?:\.\d+)?)/;
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let result = expr.match(reg);
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if (!result) return;
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result.shift();
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return result;
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}
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alert( parse("-1.23 * 3.45") ); // -1.23, *, 3.45
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```
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