所有格量词到底如何工作? [英] How exactly does the possessive quantifier work?

问题描述

在页面最后，尝试解释贪婪，勉强和所有格修饰符的工作方式:

At the end of the page there is at attempted explanation of how do greedy, reluctant and possessive quantifiers work: http://docs.oracle.com/javase/tutorial/essential/regex/quant.html

但是我尝试了一个例子，但我似乎并没有完全理解它.

However I tried myself an example and I don't seem to understand it fully.

我将直接粘贴结果:

Enter your regex: .*+foo
Enter input string to search: xfooxxxxxxfoo
No match found.

Enter your regex: (.*)+foo
Enter input string to search: xfooxxxxxxfoo
I found the text "xfooxxxxxxfoo" starting at index 0 and ending at index 13.

为什么第一个reg.exp.找不到匹配项，第二个匹配吗? 这两个reg.exp.的确切区别是什么?

Why does the first reg.exp. find no match and the second does? What is the exact difference between those 2 reg.exp.?

推荐答案

在另一个量词后的+表示不允许正则表达式引擎回溯到以前的令牌已匹配的任何内容". (请参见此处所有修饰语的教程.)

The + after another quantifier means "don't allow the regex engine to backtrack into whatever the previous token has matched". (See a tutorial on possessive quantifiers here).

因此，当您将.*foo应用于"xfooxxxxxxfoo"时，.*首先会匹配整个字符串.然后，由于无法匹配foo，因此正则表达式引擎会一直回溯，直到可能为止，当.*匹配"xfooxxxxxx"并且foo匹配"foo"时实现匹配.

So when you apply .*foo to "xfooxxxxxxfoo", the .* first matches the entire string. Then, since foo can't be matched, the regex engine backtracks until that's possible, achieving a match when .* has matched "xfooxxxxxx" and foo has matched "foo".

现在，附加的+可以防止发生回溯，因此匹配失败.

Now the additional + prevents that backtracking from happening, so the match fails.

编写(.*)+foo时. +具有完全不同的含义；现在它的意思是前面的一个或多个令牌".顺便说一下，您已经创建了嵌套量词.如果将该正则表达式应用于"xfoxxxxxxxxxfox"之类的字符串，则会遇到灾难性回溯.

When you write (.*)+foo. the + takes on an entirely different meaning; now it means "one or more of the preceding token". You've created nested quantifiers, which is not a good idea, by the way. If you apply that regex to a string like "xfoxxxxxxxxxfox", you'll run into catastrophic backtracking.

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