正则表达式量词和字符类

2018-06-04 01:24:11

在Java Tutorial中有正则表达式量词的例子和描述。

贪婪 - 吃完整的字符串，然后退出一个字符，然后再试一次

Regex: .*foo  // greedy 
String to search: xfooxxxxxxfoo
Found "xfooxxxxxxfoo"

不情愿 - 从一开始就开始，然后一次吃一个角色

Regex: .*?foo  // reluctant quantifier
String to search: xfooxxxxxxfoo
Found "xfoo", "xxxxxxfoo"

拥有 - 吃整个字符串尝试一次匹配

Regex: .*+foo // possessive quantifier
String to search: xfooxxxxxxfoo
No match found

他们没事，我理解他们，但有人可以向我解释当正则表达式更改为角色类时会发生什么？还有其他规则吗？

Regex: [fx]*
String to search: xfooxxxxxxfoo
Found "xf","","","xxxxxxf","","","",""

Regex: [fx]*?
String to search: xfooxxxxxxfoo
Found 15 zero-length matches

Regex: [fx]*+
String to search: xfooxxxxxxfoo
Found "xf","","","xxxxxxf","","","",""

它将量词（贪婪，不情愿/懒惰，占有欲）应用于整个角色类。这意味着它将匹配（贪婪，懒惰等）字符类中的每个文字字符。

Regex: [fx]*
String to search: xfooxxxxxxfoo
Found "xf","","","xxxxxxf","","","",""

所以它寻找零或更多的f或x 。引擎找到匹配的xf 。它也匹配两个o周围的空串。然后匹配连续的x ，因为它是f或x的零个或多个 。

我会查看regex101.com了解正则表达式的更多细节，特别是左侧边栏上的调试器部分

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