Java写爬虫的时候，matcher.groupCount()返回为1，但是matcher.group(1)却抛异常

Question

Answer 1

我模仿了题意，写了测试代码，结果如下

String html = "<p><span>mytextvalue<br>";
Matcher m = Pattern.compile("<p><span>(.*?)<br>").matcher(html);
System.out.println(m.find()); //true
System.out.println(m.groupCount()); //1
System.out.println(m.group(0)); //<p><span>mytextvalue<br>
System.out.println(m.group(1)); //mytextvalue

另外

// where does m.groupCount come from
m = Pattern.compile("(group1)(group2)(group3)").matcher(html);
System.out.println(m.groupCount()); //3

增加解释说明，
看源码的注释

    /**
     * Returns the number of capturing groups in this matcher's pattern.
     *
     * <p> Group zero denotes the entire pattern by convention. It is not
     * included in this count.
     *
     * <p> Any non-negative integer smaller than or equal to the value
     * returned by this method is guaranteed to be a valid group index for
     * this matcher.  </p>
     *
     * @return The number of capturing groups in this matcher's pattern
     */
    public int groupCount() {
        return parentPattern.capturingGroupCount - 1;
    }

这里说得清楚，groupCount返回的是正则表达式的捕获分组的数量（捕获分组和非捕获分组是另外的知识点），groupCount的结果并不能说明匹配的结果。

要执行正则表达式匹配，需要执行find动作，看源码

    public boolean find() {
        int nextSearchIndex = last;
        if (nextSearchIndex == first)
            nextSearchIndex++;

        // If next search starts before region, start it at region
        if (nextSearchIndex < from)
            nextSearchIndex = from;

        // If next search starts beyond region then it fails
        if (nextSearchIndex > to) {
            for (int i = 0; i < groups.length; i++)
                groups[i] = -1;
            return false;
        }
        return search(nextSearchIndex);
    }

这样的才会给Matcher内部的成员变量groups赋值，groups[i] = -1;
这样的之后在我们执行m.group(1)的时候我们才能获得捕获分组匹配到的内容。