learn-js-regexp
```js word 匹配整个单词 word.test("hello word") 测试字符中是否包含正则匹配，是返回 true,否返回 false "Hello World!".match(Hello) 返回 Array [ "Hello" ] a|b|c 匹配 a b c 中的任何一个 Ww/i 忽略大小写 /word/g 全部匹配 /hu. 匹配所有以 hu 开头的单词 /[aeiou]/ig 匹配所有符合条件的字母
"Hello World!".match(/[a-z]/gi)
"Hello World123".match(/[a-z0-9]/gi)
[aeiou0-9]/gi matches all characters that are not a vowel. Note that characters like ., !, [, @, / and white space are matched /Amy 匹配开头是 Amy 的字符串 Amy$ 匹配结尾是 Amy 的字符串
"Mississippi".match(/s+/g) 不理解，返回 [ 'ss', 'ss' ]
"Aaaaaaa".match(Aa*) 返回 Array [ "Aaaaaaa" ]
懒匹配 "<h1>Winter is coming</h1>".match(<[a-z0-9]*>)
/\w/g <==> /[A-Za-z0-9_]/g 匹配所有大小写字母、数字、`_` /\W/g 匹配除（大小写字母、数字、`_`）以外的字符
//̣g 匹配所有数字 /\D/g 匹配非数字
/\s/g 匹配全部空格 /§/g 匹配全部非空格
/Oh{3,6}\sno/g.test("Ohhh no")
/Haz{4,}ah/g z 至少重复 4 次
Tim{4}ber m 只重复 4 次
favou?rite 匹配有无 u ```
- caret character (`^`)
- greedy regex (`/C+/`)
## 用户名匹配
要求：
- 用户名只使用字符、数字
- 唯一的数字必须放在结尾，数字不能作为开头，结尾可以一个/多个数字
- 字母大小写任意
- 用户名最短 2 位
- 如果用户名只有两位，必须全部使用字母
试了几十次，找不到完美通过所有测试的方法：`/^[a-zA-Z][a-zA-Z]̣$/gm`
答案：
- `/^[a-z][a-z]+̣$|^[a-z]+̣̣$/igm`
- `/^[a-z]([0-9]{2,}|[a-z]+)̣$/`（这一个更加简洁）
## [Positive and Negative Lookahead](https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/positive-and-negative-lookahead)
[freeCodeCamp Challenge Guide: Positive and Negative Lookahead - Guide - The freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-challenge-guide-positive-and-negative-lookahead/301360)
如何使用 Positive（`(?=…)`） 和 Negative（`(?!…)`） Lookahead？`(?=\w{6})(?=\w*2̣)`
## [Check For Mixed Grouping of Characters](https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/check-for-mixed-grouping-of-characters)
[freeCodeCamp Challenge Guide: Check For Mixed Grouping of Characters - Guide - The freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-challenge-guide-check-for-mixed-grouping-of-characters/301339)
`(Franklin|Eleanor).*\sRoosevelt`
## [Reuse Patterns Using Capture Groups](https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/reuse-patterns-using-capture-groups)
[freeCodeCamp Challenge Guide: Reuse Patterns Using Capture Groups - Guide - The freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-challenge-guide-reuse-patterns-using-capture-groups/301364)
`(+̣)\s\1\s\1$`
## [Use Capture Groups to Search and Replace](https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/use-capture-groups-to-search-and-replace)
```js let str = 'one two three' let fixRegex = (\w+)\s(\w+)\s(\w+) / Change this line let replaceText = '$3 $2 $1' / Change this line let result = str.replace(fixRegex, replaceText) ```
## [Remove Whitespace from Start and End](https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/remove-whitespace-from-start-and-end)
[freeCodeCamp Challenge Guide: Remove Whitespace from Start and End - Guide - The freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-challenge-guide-remove-whitespace-from-start-and-end/301362)
```js let hello = ' Hello, World! ' let wsRegex = ^\s+|\s+$/g / Change this line let result = hello.replace(wsRegex, '') // Change this line ```
## 工具
- https://ihateregex.io/
- [Regex Vis](https://regex-vis.com/)