java - 理解 Java 正则表达式

Question

Enter your regex: a?
Enter input string to search: ababaaaab
I found the text "a" starting at index 0 and ending at index 1.
I found the text "" starting at index 1 and ending at index 1.
I found the text "a" starting at index 2 and ending at index 3.
I found the text "" starting at index 3 and ending at index 3.
I found the text "a" starting at index 4 and ending at index 5.
I found the text "a" starting at index 5 and ending at index 6.
I found the text "a" starting at index 6 and ending at index 7.
I found the text "a" starting at index 7 and ending at index 8.
I found the text "" starting at index 8 and ending at index 8.
I found the text "" starting at index 9 and ending at index 9.

Enter your regex: a*
Enter input string to search: ababaaaab
I found the text "a" starting at index 0 and ending at index 1.
I found the text "" starting at index 1 and ending at index 1.
I found the text "a" starting at index 2 and ending at index 3.
I found the text "" starting at index 3 and ending at index 3.
I found the text "aaaa" starting at index 4 and ending at index 8.
I found the text "" starting at index 8 and ending at index 8.
I found the text "" starting at index 9 and ending at index 9.

Enter your regex: a+
Enter input string to search: ababaaaab
I found the text "a" starting at index 0 and ending at index 1.
I found the text "a" starting at index 2 and ending at index 3.
I found the text "aaaa" starting at index 4 and ending at index 8.  

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上面的示例来自有关量词的 Java 文档。
根据我对上面给出的示例的理解，以下是贪婪量词的工作原理:

?和 *

将查找所提到的字符是否存在。如果找不到它们前面的字符，则会将该位置标记为零长度匹配。
* 用于逐个字符匹配，而 ? 用于一组。

+

+ 将进行正则表达式的逐组匹配。它不会进行零长度匹配。

我的问题是:
我对量词的理解是正确的还是我弄乱了一些东西？
另外，是否有关于 java regex 的大脑友好型指南(Google 上没有针对初学者的指南)？

Answer 1

• goes for a character by character match while ? goes for a group.

* 匹配零个或多个。 ? 匹配零或 1。

这些都与群体或角色无关。正则表达式中的“组”或“捕获组”是一个带括号的元素，可以通过 java.util.regex.Matcher.group 方法单独提取。

(foo)* 匹配 ""、"foo" 和 "foofoo" 所以显然主体不必是单个字符。

(foo)? 匹配 "" 和 "foo" 但不匹配 "foofoo"。

• will go for group by group matching of the regex. It does not make zero-length matches.

没有。 + 与正文匹配一次或多次，因此 x* 相当于 (?:x+)?。 + 运算符可以很好地执行零长度匹配。 ()+ 匹配空字符串。