正则表达式先行，后向和原子组 [英] Regex lookahead, lookbehind and atomic groups

问题描述

我在我的正则表达式中发现了这些东西，但是我不知道该怎么用. 有人有示例，以便我尝试了解它们的工作原理吗?

I found these things in my regex body but I haven't got a clue what I can use them for. Does somebody have examples so I can try to understand how they work?

(?!) - negative lookahead
(?=) - positive lookahead
(?<=) - positive lookbehind
(?<!) - negative lookbehind

(?>) - atomic group

推荐答案

示例

给出字符串foobarbarfoo:

bar(?=bar)     finds the 1st bar ("bar" which has "bar" after it)
bar(?!bar)     finds the 2nd bar ("bar" which does not have "bar" after it)
(?<=foo)bar    finds the 1st bar ("bar" which has "foo" before it)
(?<!foo)bar    finds the 2nd bar ("bar" which does not have "foo" before it)

您也可以将它们组合在一起:

You can also combine them:

(?<=foo)bar(?=bar)    finds the 1st bar ("bar" with "foo" before it and "bar" after it)

定义

向前看肯定(?=)

在表达式B后面找到表达式A:

Definitions

Look ahead positive (?=)

Find expression A where expression B follows:

A(?=B)

前瞻性否定(?!)

查找表达式A，其中表达式B不跟在后面:

Look ahead negative (?!)

Find expression A where expression B does not follow:

A(?!B)

回避积极的(?<=)

在表达式B之前查找表达式A:

Look behind positive (?<=)

Find expression A where expression B precedes:

(?<=B)A

回避否定的(?<!)

查找表达式A，其中表达式B不在前面

Look behind negative (?<!)

Find expression A where expression B does not precede:

(?<!B)A

原子组(?>)

一个原子组退出一个组，并在该组内 first 匹配模式之后丢弃其他模式(禁用回溯功能).

Atomic groups (?>)

An atomic group exits a group and throws away alternative patterns after the first matched pattern inside the group (backtracking is disabled).

应用于foots的
• (?>foo|foot)s将匹配其第一个替代项foo，然后由于s不会立即跟随而失败，并由于禁用回溯而停止

• (?>foo|foot)s applied to foots will match its 1st alternative foo, then fail as s does not immediately follow, and stop as backtracking is disabled

一个非原子的小组将允许回溯；如果后续的后续匹配失败，它将回溯并使用其他模式，直到找到整个表达式的匹配项或所有可能性均已耗尽.

A non-atomic group will allow backtracking; if subsequent matching ahead fails, it will backtrack and use alternative patterns until a match for the entire expression is found or all possibilities are exhausted.

应用于foots的

• (foo|foot)s将:

1. 匹配其第一个替代项foo，然后失败，因为s不会立即跟随foots，并回溯到其第二个替代项；
2. 匹配其第二个替代项foot，然后在foots中紧随其后的是s并成功，然后停止.

1. match its 1st alternative foo, then fail as s does not immediately follow in foots, and backtrack to its 2nd alternative;
2. match its 2nd alternative foot, then succeed as s immediately follows in foots, and stop.

• http://www.regular-expressions.info/lookaround.html
• http://www.rexegg.com/regex-lookarounds.html

• http://www.regular-expressions.info/lookaround.html
• http://www.rexegg.com/regex-lookarounds.html

• https://regex101.com

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