量词范围在后面不起作用 [英] Quantifier range not working in lookbehind

问题描述

好的，所以我正在一个项目中，我需要一个可以匹配*的正则表达式，后跟1-4个空格或制表符，然后再跟一行文本.现在，我在后面查找后使用.*进行测试.但是我可以让它明确匹配1、2或4个空格/制表符，但不能匹配1-4.我正在针对以下代码块

Okay so I'm working on a project where I need a regex that can match a * followed by 1-4 spaces or tabs and then followed by a row of text. Right now I'm using .* after the lookbehind for testing purposes. However I can get it to match explicitly 1, 2, or 4 spaces/tabs but not 1-4. I'm testing against the following block

*    test line here
*   Second test
*  Third test
* Another test

这是我正在测试的两个模式(?<=(\*[ \t]{3})).*，它们按预期方式工作并匹配第二行，如果我将1替换为3，将1或2或4替换为2，则相同，形成以下内容模式(?<=(\*[ \t]{1,4})).*，它不再与任何行匹配，老实说，我不明白为什么.我尝试了Google搜索，但没有成功.我正在使用g(lobal)标志.

And these are the two patterns I'm testing (?<=(\*[ \t]{3})).* which works just as expected and matches the 2nd line, same if I replace 3 with 1, 2 or 4 however if I replace it with 1,4 forming the following pattern (?<=(\*[ \t]{1,4})).* it no longer matches any of the rows and I honestly can't understand why. I've tried googling without success. I'm using the g(lobal) flag.

推荐答案

PHP与许多其他样式一样，不支持可变长度的后向查找.唯一的支持是后置条件顶层的 alternation (|).甚至?都可以破坏模式.一种替代方法是使用:

PHP, like many flavors, doesn't support variable length lookbehind. The only support is alternation (|) at the top level of the lookbehind. Even a ? can break the pattern. An alternative is to use:

(?<=\*[ \t]|\*[ \t]{2}|\*[ \t]{3}|\*[ \t]{4}).*

或者更好的方法是中止组的后退:

Or better, abort the lookbehind for a group:

\*[ \t]{1,4}(.*)

这应该对您来说很好，因为看起来您的比赛似乎并没有重叠.

This should work well for you, since it doesn't seem like you have overlapping of your matches anyway.

从手册中:

限制向后查找的断言的内容，以使它匹配的所有字符串都必须具有固定的长度.但是，如果有几种选择，则它们不必都具有相同的固定长度.因此，允许(?<== bullock | donkey)，但是(?<！dogs?| cats?)在编译时会导致错误.匹配不同长度字符串的分支只允许在后向断言的顶层使用.

The contents of a lookbehind assertion are restricted such that all the strings it matches must have a fixed length. However, if there are several alternatives, they do not all have to have the same fixed length. Thus (?<=bullock|donkey) is permitted, but (?<!dogs?|cats?) causes an error at compile time. Branches that match different length strings are permitted only at the top level of a lookbehind assertion.

来源: http://www.php.net/manual/zh/regexp.reference.assertions.php

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