2018-10-10 22:03:03 +00:00
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---
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id: 587d7dba367417b2b2512ba9
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2021-03-15 03:20:39 +00:00
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title: 正向先行断言和负向先行断言
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2018-10-10 22:03:03 +00:00
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challengeType: 1
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2020-08-04 07:14:01 +00:00
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forumTopicId: 301360
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2021-01-13 02:31:00 +00:00
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dashedName: positive-and-negative-lookahead
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2018-10-10 22:03:03 +00:00
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---
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2020-12-16 07:37:30 +00:00
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# --description--
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2021-03-15 03:20:39 +00:00
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先行断言 (<dfn>Lookaheads</dfn>)是告诉 JavaScript 在字符串中向前查找的匹配模式。 当想要在同一个字符串上搜寻多个匹配模式时,这可能非常有用。
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2020-12-16 07:37:30 +00:00
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2021-03-15 03:20:39 +00:00
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有两种先行断言:正向先行断言(<dfn>positive lookahead</dfn>)和负向先行断言(<dfn>negative lookahead</dfn>)。
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2020-12-16 07:37:30 +00:00
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2021-03-15 03:20:39 +00:00
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正向先行断言会查看并确保搜索匹配模式中的元素存在,但实际上并不匹配。 正向先行断言的用法是 `(?=...)`,其中 `...` 就是需要存在但不会被匹配的部分。
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2020-12-16 07:37:30 +00:00
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2021-03-15 03:20:39 +00:00
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另一方面,负向先行断言会查看并确保搜索匹配模式中的元素不存在。 负向先行断言的用法是 `(?!...)`,其中 `...` 是希望不存在的匹配模式。 如果负向先行断言部分不存在,将返回匹配模式的其余部分。
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2020-12-16 07:37:30 +00:00
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2021-03-15 03:20:39 +00:00
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尽管先行断言有点儿令人困惑,但是这些示例会有所帮助。
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2020-08-04 07:14:01 +00:00
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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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2021-03-15 03:20:39 +00:00
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quit.match(quRegex);
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noquit.match(qRegex);
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2020-08-04 07:14:01 +00:00
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```
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2021-03-15 03:20:39 +00:00
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这两次 `match` 调用都将返回 `["q"]`。
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先行断言的更实际用途是检查一个字符串中的两个或更多匹配模式。 这里有一个简单的密码检查器,密码规则是 3 到 6 个字符且至少包含一个数字:
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2020-08-04 07:14:01 +00:00
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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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2020-12-16 07:37:30 +00:00
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# --instructions--
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2021-03-15 03:20:39 +00:00
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在正则表达式 `pwRegex` 中使用先行断言以匹配大于 5 个字符且有两个连续数字的密码。
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2020-08-04 07:14:01 +00:00
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2020-12-16 07:37:30 +00:00
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# --hints--
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2020-08-04 07:14:01 +00:00
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2021-03-15 03:20:39 +00:00
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你的正则表达式应该使用两个正向先行断言( `lookaheads`)。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(pwRegex.source.match(/\(\?=.*?\)\(\?=.*?\)/) !== null);
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2018-10-10 22:03:03 +00:00
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```
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2021-03-15 03:20:39 +00:00
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您的正则表达式不应匹配字符串 `astronaut`
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(!pwRegex.test('astronaut'));
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```
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2018-10-10 22:03:03 +00:00
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2021-03-15 03:20:39 +00:00
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你的正则表达式不应匹配字符串 `banan1`
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(!pwRegex.test('banan1'));
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```
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2018-10-10 22:03:03 +00:00
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2021-03-15 03:20:39 +00:00
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你的正则表达式应该匹配字符串 `bana12`
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(pwRegex.test('bana12'));
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```
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2018-10-10 22:03:03 +00:00
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2021-03-15 03:20:39 +00:00
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你的正则表达式应该匹配字符串 `abc123`
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(pwRegex.test('abc123'));
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```
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2021-03-15 03:20:39 +00:00
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你的正则表达式不应匹配字符串 `12345`
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2020-12-16 07:37:30 +00:00
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```js
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2021-02-06 04:42:36 +00:00
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assert(!pwRegex.test('12345'));
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2020-12-16 07:37:30 +00:00
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```
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2021-03-15 03:20:39 +00:00
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你的正则表达式应该匹配字符串 `8pass99`
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2020-08-04 07:14:01 +00:00
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2018-10-10 22:03:03 +00:00
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```js
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2021-02-06 04:42:36 +00:00
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assert(pwRegex.test('8pass99'));
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2018-10-10 22:03:03 +00:00
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```
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2020-08-04 07:14:01 +00:00
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2021-03-15 03:20:39 +00:00
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你的正表达式不应匹配字符串 `1a2bcde`
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2020-12-16 07:37:30 +00:00
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```js
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2021-02-06 04:42:36 +00:00
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assert(!pwRegex.test('1a2bcde'));
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2020-12-16 07:37:30 +00:00
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```
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2021-03-15 03:20:39 +00:00
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你的正则表达式应该匹配字符串 `astr1on11aut`
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2020-12-16 07:37:30 +00:00
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```js
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assert(pwRegex.test('astr1on11aut'));
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2020-12-16 07:37:30 +00:00
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```
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2021-01-13 02:31:00 +00:00
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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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2020-12-16 07:37:30 +00:00
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# --solutions--
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2021-01-13 02:31:00 +00:00
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```js
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let pwRegex = /(?=\w{6})(?=\w*\d{2})/;
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```
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