java如何匹配多个捕获组，但结果不符合预期

Question

我正在努力学习Java正则表达式。我想将几个捕获组（即j(a(va))）与另一个字符串（即this is java. this is ava, this is va）进行匹配。我希望输出为：

I found the text "java" starting at index 8 and ending at index 12.
I found the text "ava" starting at index 21 and ending at index 24.    
I found the text "va" starting at index 34 and ending at index 36.
Number of group: 2

但是，IDE仅输出：

I found the text "java" starting at index 8 and ending at index 12.
Number of group: 2

为什么会这样？我有什么遗漏吗

原始代码：

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("\nEnter your regex:");

        Pattern pattern
                = Pattern.compile(br.readLine());

        System.out.println("\nEnter input string to search:");
        Matcher matcher
                = pattern.matcher(br.readLine());

        boolean found = false;
        while (matcher.find()) {
            System.out.format("I found the text"
                    + " \"%s\" starting at "
                    + "index %d and ending at index %d.%n",
                    matcher.group(),
                    matcher.start(),
                    matcher.end());
            found = true;
            System.out.println("Number of group: " + matcher.groupCount());
        }
        if (!found) {
            System.out.println("No match found.");
        }

运行上述代码后，我输入了以下输入：

Enter your regex:
j(a(va))

Enter input string to search:
this is java. this is ava, this is va

和IDE输出：

I found the text "java" starting at index 8 and ending at index 12.
Number of group: 2

Answer 1

您的regexp只匹配整个字符串java，它不匹配ava或va。当它匹配java时，它会将捕获组1设置为ava，将捕获组2设置为va，但它本身并不匹配这些字符串。生成所需结果的regexp是：

j?(a?(va))

?使前面的项成为可选项，因此它将匹配后面没有这些前缀的项

DEMO

Answer 2

您需要正则表达式(j?(a?(va)))

Pattern p = Pattern.compile("(j?(a?(va)))");
Matcher m = p.matcher("this is java. this is ava, this is va");

while( m.find() )
{
    String group = m.group();
    int start = m.start();
    int end = m.end();
    System.out.format("I found the text"
                  + " \"%s\" starting at "
                 + "index %d and ending at index %d.%n",
                  group,
                  start,
                   end);



}

您可以看到演示here