> 文章片段来源：https://github.com/ziishaned/learn-regex/blob/master/translations/README-cn.md
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文章片段来源：https://github.com/ziishaned/learn-regex/blob/master/translations/README-cn.md
先行断言和后发断言都属于非捕获簇(不捕获文本 ，也不针对组合计进行计数). 先行断言用于判断所匹配的格式是否在另一个确定的格式之前, 匹配结果不包含该确定格式(仅作为约束).
例如, 我们想要获得所有跟在
$(?<=\$)[0-9\.]*$0,1,2,3,4,5,6,7,8,9,.零宽度断言如下:
| 符号 | 描述 |
|---|---|
| ?= | 正先行断言-存在 |
| ?! | 负先行断言-排除 |
| ?<= | 正后发断言-存在 |
| ?<! | 负后发断言-排除 |
?=...?=...?=...返回结果只包含满足匹配条件的第一部分表达式. 定义一个正先行断言要使用
()(?=...)正先行断言的内容写在括号中的等号后面. 例如, 表达式
(T|t)he(?=\sfat)Thethe(?=\sfat)Thethe(空格)fat"(T|t)he(?=\sfat)" => The fat cat sat on the mat.
?!...负先行断言
?!正先行断言负先行断言=!(?!...)表达式
(T|t)he(?!\sfat)Thethe(空格)fat"(T|t)he(?!\sfat)" => The fat cat sat on the mat.
?<= ...正后发断言 记作
(?<=...)(?<=(T|t)he\s)(fat|mat)fatmatThethe"(?<=(T|t)he\s)(fat|mat)" => The fat cat sat on the mat.
?<!...负后发断言 记作
(?<!...)(?<!(T|t)he\s)(cat)catThethe"(?<!(T|t)he\s)(cat)" => The cat sat on cat.