113 lines
2.7 KiB
Markdown
113 lines
2.7 KiB
Markdown
---
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id: 587d7dba367417b2b2512ba9
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title: Positive and Negative Lookahead
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challengeType: 1
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forumTopicId: 301360
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dashedName: positive-and-negative-lookahead
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---
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# --description--
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<dfn>Lookaheads</dfn> are patterns that tell JavaScript to look-ahead in your string to check for patterns further along. This can be useful when you want to search for multiple patterns over the same string.
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There are two kinds of lookaheads: <dfn>positive lookahead</dfn> and <dfn>negative lookahead</dfn>.
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A positive lookahead will look to make sure the element in the search pattern is there, but won't actually match it. A positive lookahead is used as `(?=...)` where the `...` is the required part that is not matched.
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On the other hand, a negative lookahead will look to make sure the element in the search pattern is not there. A negative lookahead is used as `(?!...)` where the `...` is the pattern that you do not want to be there. The rest of the pattern is returned if the negative lookahead part is not present.
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Lookaheads are a bit confusing but some examples will help.
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```js
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let quit = "qu";
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let noquit = "qt";
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let quRegex= /q(?=u)/;
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let qRegex = /q(?!u)/;
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quit.match(quRegex); // Returns ["q"]
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noquit.match(qRegex); // Returns ["q"]
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```
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A more practical use of lookaheads is to check two or more patterns in one string. Here is a (naively) simple password checker that looks for between 3 and 6 characters and at least one number:
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```js
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let password = "abc123";
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let checkPass = /(?=\w{3,6})(?=\D*\d)/;
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checkPass.test(password); // Returns true
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```
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# --instructions--
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Use lookaheads in the `pwRegex` to match passwords that are greater than 5 characters long, and have two consecutive digits.
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# --hints--
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Your regex should use two positive `lookaheads`.
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```js
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assert(pwRegex.source.match(/\(\?=.*?\)\(\?=.*?\)/) !== null);
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```
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Your regex should not match `"astronaut"`
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```js
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assert(!pwRegex.test('astronaut'));
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```
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Your regex should not match `"banan1"`
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```js
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assert(!pwRegex.test('banan1'));
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```
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Your regex should match `"bana12"`
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```js
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assert(pwRegex.test('bana12'));
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```
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Your regex should match `"abc123"`
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```js
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assert(pwRegex.test('abc123'));
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```
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Your regex should not match `"12345"`
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```js
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assert(!pwRegex.test('12345'));
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```
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Your regex should match `"8pass99"`
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```js
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assert(pwRegex.test('8pass99'));
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```
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Your regex should not match `"1a2bcde"`
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```js
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assert(!pwRegex.test('1a2bcde'));
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```
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Your regex should match `"astr1on11aut"`
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```js
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assert(pwRegex.test('astr1on11aut'));
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```
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# --seed--
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## --seed-contents--
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```js
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let sampleWord = "astronaut";
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let pwRegex = /change/; // Change this line
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let result = pwRegex.test(sampleWord);
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```
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# --solutions--
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```js
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let pwRegex = /(?=\w{6})(?=\w*\d{2})/;
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```
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