背景
最近系统性的学习了遍正则, 开始看的是中文档，看到先行断言和后行断言时，又开始蒙逼了。我完全不能理解“先行断言”和“后行断言”这两个词的意思。
学习
打开英文文档, 得到下面这两张表
| Characters / constructs | Corresponding article |
|---|---|
\, ., \cX, \d, \D, \f, \n, \r, \s, \S, \t, \v, \w, \W, \0, \xhh, \uhhhh, \uhhhhh, [\b] |
Character classes |
^, $, x(?=y), x(?!y), (?<=y)x, (?<!y)x, \b, \B |
Assertions |
(x), (?:x), (?<Name>x), `x |
y, [xyz], [^xyz]`, `*Number*` |
*, +, ?, x{*n*}, x{*n*,}, x{*n*,*m*} |
Quantifiers |
\p{*UnicodeProperty*}, \P{*UnicodeProperty*} |
Unicode property escapes |
好吧，断言就是Assertions，先行和后行表示位置，看来还是很信达雅的，就是没有看到原文的话还真看不懂了，之后还是要多翻英文档了。其中“先行” 以应 ‘lookahead’ （看头），“后行”为 ‘lookbehind’(看尾)。
各种断言也在这里记录一下吧
| Characters | 中文 | Meaning |
|---|---|---|
x(?=y) |
先行断言 | Lookahead assertion: (positive lookahead) Matches “x” only if “x” is followed by “y”. For example, /Jack(?=Sprat)/ matches “Jack” only if it is followed by “Sprat”. `/Jack(?=Sprat |
x(?!y) |
正向否定查找(这个翻译的也不好，反正我不理解) | Negative lookahead assertion: Matches “x” only if “x” is not followed by “y”. For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. /\d+(?!\.)/.exec('3.141') matches “141” but not “3”. |
(?<=y)x |
后行断言 | Lookbehind assertion: Matches “x” only if “x” is preceded by “y”. For example, /(?<=Jack)Sprat/ matches “Sprat” only if it is preceded by “Jack”. `/(?<=Jack |
(?<!y)x |
反向否定查找 | Negative lookbehind assertion: Matches “x” only if “x” is not preceded by “y”. For example, /(?<!-)\d+/ matches a number only if it is not preceded by a minus sign. /(?<!-)\d+/.exec('3') matches “3”. /(?<!-)\d+/.exec('-3') match is not found because the number is preceded by the minus sign. |
图型展示一下
可以在 https://regexper.com/ 这个网站测试一下正则还是
先行断言/positive lookahead
正向否定/negative lookahead
https://regexper.com/不支持 lookbehind，就不展示了。
后记
还是是英文吧 positive lookahead, negative lookahead, positive lookbehind, negative lookbehind 好记多了