java - 正则表达式在多行模式下不匹配空字符串 (Java)

Question

我刚刚观察到这种行为；

Pattern p1 = Pattern.compile("^$");
Matcher m1 = p1.matcher("");
System.out.println(m1.matches()); /* true */

Pattern p2 = Pattern.compile("^$", Pattern.MULTILINE);
Matcher m2 = p2.matcher("");
System.out.println(m2.matches()); /* false */

令我感到奇怪的是，最后一个陈述是错误的。这就是文档所说的；

By default, the regular expressions ^ and $ ignore line terminators and only match at the beginning and the end, respectively, of the entire input sequence. If MULTILINE mode is activated then ^ matches at the beginning of input and after any line terminator except at the end of input. When in MULTILINE mode $ matches just before a line terminator or the end of the input sequence. http://docs.oracle.com/javase/1.4.2...

从我得到的结果来看，它应该匹配吗？以下内容使事情变得更加困惑；

Pattern p3 = Pattern.compile("^test$");
Matcher m3 = p3.matcher("test");
System.out.println(m3.matches()); /* true */

Pattern p4 = Pattern.compile("^test$", Pattern.MULTILINE);
Matcher m4 = p4.matcher("test");
System.out.println(m4.matches()); /* true */

那这是什么？我如何理解这一点？我希望有人能对此有所了解，将不胜感激。

Answer 1

If MULTILINE mode is activated then ^ matches at the beginning of input and after any line terminator except at the end of input.

由于你在输入的末尾，^无法在多行模式下匹配。

这令人惊讶，甚至令人作呕，但根据其文档仍然如此。