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29 lines
598 B
Markdown
29 lines
598 B
Markdown
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---
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id: 3ab00edf-6c63-4db2-b32e-d18f8d683329
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title: JavaScript Lookahead Assertions
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---
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# Examples
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## Positive lookahead assertion
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Text after the current location must match the assertion
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``` javascript
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const RE_AS_BS = /aa(?=bb)/;
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const match1 = RE_AS_BS.exec("aabb");
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console.log(match1[0]); // 'aa'
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```
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## Negative lookahead assertion
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Text after the current location must not match the assertion
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``` javascript
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const RE_AS_NO_BS = /aa(?!bb)/;
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console.log(RE_AS_NO_BS.test("aabb")); // false
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console.log(RE_AS_NO_BS.test("aab")); // true
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console.log(RE_AS_NO_BS.test("aac")); // true
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```
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