Why does \R behave differently in regular expressions between Java 8 and Java 9?

问题

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);
        }
    }
}

When I run it with Java 8 it returns:

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

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

Why?

回答1:

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

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

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

It is strongly recommended that there be a regular expression meta-character, such as "\R", for matching all line ending characters and sequences listed above (for example, in #1). This would correspond to something equivalent to the following expression. That expression is slightly complicated by the need to avoid backup.
 (?:\u{D A}|(?!\u{D A})[\u{A}-\u{D}\u{85}\u{2028}\u{2029}]

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.

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.

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.