2021-05-05 10:13:49 -07:00
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---
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id: 587d7dba367417b2b2512ba9
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title: 正向先行斷言和負向先行斷言
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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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先行斷言 (<dfn>Lookaheads</dfn>)是告訴 JavaScript 在字符串中向前查找的匹配模式。 當想要在同一個字符串上搜尋多個匹配模式時,這可能非常有用。
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有兩種先行斷言:正向先行斷言(<dfn>positive lookahead</dfn>)和負向先行斷言(<dfn>negative lookahead</dfn>)。
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正向先行斷言會查看並確保搜索匹配模式中的元素存在,但實際上並不匹配。 正向先行斷言的用法是 `(?=...)`,其中 `...` 就是需要存在但不會被匹配的部分。
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另一方面,負向先行斷言會查看並確保搜索匹配模式中的元素不存在。 負向先行斷言的用法是 `(?!...)`,其中 `...` 是希望不存在的匹配模式。 如果負向先行斷言部分不存在,將返回匹配模式的其餘部分。
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儘管先行斷言有點兒令人困惑,但是這些示例會有所幫助。
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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);
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noquit.match(qRegex);
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```
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這兩次 `match` 調用都將返回 `["q"]`。
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先行斷言的更實際用途是檢查一個字符串中的兩個或更多匹配模式。 這裏有一個簡單的密碼檢查器,密碼規則是 3 到 6 個字符且至少包含一個數字:
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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);
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```
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# --instructions--
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在正則表達式 `pwRegex` 中使用先行斷言以匹配大於 5 個字符且有兩個連續數字的密碼。
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# --hints--
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你的正則表達式應該使用兩個正向先行斷言( `lookaheads`)。
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```js
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assert(pwRegex.source.match(/\(\?=.*?\)\(\?=.*?\)/) !== null);
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```
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您的正則表達式不應匹配字符串 `astronaut`
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```js
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2021-10-06 08:36:48 -07:00
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pwRegex.lastIndex = 0;
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assert(!pwRegex.test('astronaut'));
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```
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你的正則表達式不應匹配字符串 `banan1`
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```js
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pwRegex.lastIndex = 0;
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assert(!pwRegex.test('banan1'));
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```
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你的正則表達式應該匹配字符串 `bana12`
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```js
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pwRegex.lastIndex = 0;
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assert(pwRegex.test('bana12'));
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```
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你的正則表達式應該匹配字符串 `abc123`
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```js
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pwRegex.lastIndex = 0;
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assert(pwRegex.test('abc123'));
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```
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你的正則表達式不應匹配字符串 `12345`
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```js
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pwRegex.lastIndex = 0;
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assert(!pwRegex.test('12345'));
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```
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你的正則表達式應該匹配字符串 `8pass99`
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```js
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pwRegex.lastIndex = 0;
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assert(pwRegex.test('8pass99'));
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```
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你的正表達式不應匹配字符串 `1a2bcde`
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```js
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pwRegex.lastIndex = 0;
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assert(!pwRegex.test('1a2bcde'));
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```
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你的正則表達式應該匹配字符串 `astr1on11aut`
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```js
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pwRegex.lastIndex = 0;
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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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