75 lines
3.5 KiB
Markdown
75 lines
3.5 KiB
Markdown
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---
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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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---
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## Description
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<section id='description'>
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<code>Lookaheads</code> 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 <code>lookaheads</code>: <code>positive lookahead</code> and <code>negative lookahead</code>.
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A <code>positive lookahead</code> 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 <code>(?=...)</code> where the <code>...</code> is the required part that is not matched.
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On the other hand, a <code>negative lookahead</code> will look to make sure the element in the search pattern is not there. A negative lookahead is used as <code>(?!...)</code> where the <code>...</code> 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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<blockquote>let quit = "qu";<br>let noquit = "qt";<br>let quRegex= /q(?=u)/;<br>let qRegex = /q(?!u)/;<br>quit.match(quRegex); // Returns ["q"]<br>noquit.match(qRegex); // Returns ["q"]</blockquote>
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A more practical use of <code>lookaheads</code> 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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<blockquote>let password = "abc123";<br>let checkPass = /(?=\w{3,6})(?=\D*\d)/;<br>checkPass.test(password); // Returns true</blockquote>
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</section>
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## Instructions
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<section id='instructions'>
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Use <code>lookaheads</code> in the <code>pwRegex</code> to match passwords that are greater than 5 characters long and have two consecutive digits.
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</section>
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## Tests
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<section id='tests'>
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```yml
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- text: Your regex should use two positive <code>lookaheads</code>.
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testString: 'assert(pwRegex.source.match(/\(\?=.*?\)\(\?=.*?\)/) !== null, "Your regex should use two positive <code>lookaheads</code>.");'
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- text: Your regex should not match <code>"astronaut"</code>
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testString: 'assert(!pwRegex.test("astronaut"), "Your regex should not match <code>"astronaut"</code>");'
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- text: Your regex should not match <code>"airplanes"</code>
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testString: 'assert(!pwRegex.test("airplanes"), "Your regex should not match <code>"airplanes"</code>");'
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- text: Your regex should not match <code>"banan1"</code>
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testString: 'assert(!pwRegex.test("banan1"), "Your regex should not match <code>"banan1"</code>");'
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- text: Your regex should match <code>"bana12"</code>
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testString: 'assert(pwRegex.test("bana12"), "Your regex should match <code>"bana12"</code>");'
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- text: Your regex should match <code>"abc123"</code>
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testString: 'assert(pwRegex.test("abc123"), "Your regex should match <code>"abc123"</code>");'
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- text: Your regex should not match <code>"123"</code>
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testString: 'assert(!pwRegex.test("123"), "Your regex should not match <code>"123"</code>");'
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- text: Your regex should not match <code>"1234"</code>
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testString: 'assert(!pwRegex.test("1234"), "Your regex should not match <code>"1234"</code>");'
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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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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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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var pwRegex = /(?=\w{5})(?=\D*\d{2})/;
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```
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</section>
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