正则表达式与多行模式（Java）中的空字符串不匹配 [英] Regular expression doesn't match empty string in multiline mode (Java)

问题描述

我刚观察到这种行为;

I just observed this behavior;

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 */

令我觉得最后一句话是假的奇怪。这就是文档所说的;

It strikes me as odd that the last statement is false. This is what the docs say;

默认情况下，正则表达式^和$忽略行终止符，并且只分别匹配整个输入的开头和结尾序列。如果激活MULTILINE模式，则^在输入开始时和任何行终止符之后匹配，但输入结束时除外。当处于MULTILINE模式时，$匹配行终止符或输入序列的结尾。 http://docs.oracle.com/javase /1.4.2 ......

从我得到的，它应该匹配？以下使事情变得更加混乱;

From what I get from this, it should match? The following makes things even more confusing;

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？我怎么理解这个？我希望有人可以对此有所了解，真的很感激。

So what is this? How do I make sense of this? I hope someone can shed some light on this, would be really appreciated.

推荐答案

如果激活MULTILINE模式，那么^匹配
输入的开头和在任何行终止符之后，除了在输入结束时。

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， ^ 在多线模式下无法匹配。

Since you are at the end of input, ^ can't match in multiline mode.

这是令人惊讶的，甚至令人作呕，但仍然根据其文档。

This is surprising, even disgusting, but nevertheless according to its documentation.

这篇关于正则表达式与多行模式（Java）中的空字符串不匹配的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！