为什么\ R在Java 8和Java 9之间的正则表达式中表现不同？

问题描述：

以下代码在Java 8& 9，但行为不同。

The following code compiles in both Java 8 & 9, but behaves differently.

class Simple {
    static String sample = "\nEn un lugar\r\nde la Mancha\nde cuyo nombre\r\nno quiero acordarme";

    public static void main(String args[]){
        String[] chunks = sample.split("\\R\\R");
        for (String chunk: chunks) {
            System.out.println("Chunk : "+chunk);
        }
    }
}

当我用Java运行它时8它返回：

When I run it with Java 8 it returns:

Chunk : 
En un lugar
de la Mancha
de cuyo nombre
no quiero acordarme

但是当我运行它时Java 9输出不同：

But when I run it with Java 9 the output is different:

Chunk : 
En un lugar
Chunk : de la Mancha
de cuyo nombre
Chunk : no quiero acordarme

为什么？

答

Java文档不符合Unicode标准。 Javadoc误认为 \ R 应匹配的内容。它显示：

The Java documentation is out of conformance with the Unicode Standard. The Javadoc mistates what \R is supposed to match. It reads:

\ R 任何Unicode换行序列，相当于 \ u000D \ u000A | [\\ 000A\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

\R Any Unicode linebreak sequence, is equivalent to \u000D\u000A|[\u000A\u000B\u000C\u000D\u0085\u2028\u2029]

Java文档存在问题。在其有关R1.6换行符的部分，关于正则表达式的Unicode技术标准＃18 中明确说明：

That Java documentation is buggy. In its section on R1.6 Line Breaks, Unicode Technical Standard #18 on Regular Expressions clearly states:

强烈建议使用正则表达式元字符，例如\ R，以匹配所有行结尾上面列出的字符和序列（例如，在＃1中）。这将对应于与以下表达式等效的内容。 由于需要避免备份，因此表达式稍微复杂。

 (?:\u{D A}|(?!\u{D A})[\u{A}-\u{D}\u{85}\u{2028}\u{2029}]

换句话说，它只能匹配两个代码点CR + LF（回车+换行）序列或者来自该组的单个代码点，前提是不只是单独的回车符接下来是换行。这是因为 不允许备份 。对于 \ R ，CRLF必须是原子的正确运行。

In other words, it can only match a two code-point CR+LF (carriage return + linefeed) sequence or else a single code-point from that set provided that it is not just a carriage return alone that is then followed by a linefeed. That’s because it is not allowed to back up. CRLF must be atomic for \R to function properly.

因此，Java 9不再符合R1.6强烈建议的内容。此外，它现在正在做一些它应该做的事情，并且做了不这样做，在Java 8中。

So Java 9 no longer conforms to what R1.6 strongly recommends. Moreover, it is now doing something that it was supposed to NOT do, and did not do, in Java 8.

看起来是时候让谢尔曼（读作：沉雪明）再次大喊大叫。我之前和他一起工作过事实真相正式合规事宜。

Looks like it's time for me to give Sherman (read: Xueming Shen) a holler again. I've worked with him before on these nitty-gritty matters of formal conformance.

为什么\ R在Java 8和Java 9之间的正则表达式中表现不同？

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