Java 中的正则表达式，\\\s 与 \\\s

Question

以下两个表达式有什么区别？

 x = x.replaceAll("\\s", "");
x = x.replaceAll("\\s+", "");

原文由 mpluse 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

第一个匹配单个空格，而第二个匹配一个或多个空格。它们是所谓的正则表达式量词，它们执行这样的匹配（取自 文档）：

 Greedy quantifiers
X?  X, once or not at all
X*  X, zero or more times
X+  X, one or more times
X{n}    X, exactly n times
X{n,}   X, at least n times
X{n,m}  X, at least n but not more than m times

Reluctant quantifiers
X?? X, once or not at all
X*? X, zero or more times
X+? X, one or more times
X{n}?   X, exactly n times
X{n,}?  X, at least n times
X{n,m}? X, at least n but not more than m times

Possessive quantifiers
X?+ X, once or not at all
X*+ X, zero or more times
X++ X, one or more times
X{n}+   X, exactly n times
X{n,}+  X, at least n times
X{n,m}+ X, at least n but not more than m times

原文由 Óscar López 发布，翻译遵循 CC BY-SA 3.0 许可协议