正则表达式matcher.group用法

public static void main(String[] args) {
        
    String regex = "http://www.youtube.com/channel/(\\w+)\\?feature";
        String str="http://www.youtube.com/channel/UCgYQrFwjImpTQbovggoCLlg?feature=watch";
        Pattern p = Pattern.compile(regex, Pattern.DOTALL | Pattern.CASE_INSENSITIVE);
        Matcher matcher = p.matcher(str);
        while (matcher.find()) {
            
            for(int i = 0; i <= matcher.groupCount(); i++) {
                String mat = matcher.group(i);
                System.out.println(mat);
            }
        }
        
}

打印出结果
http://www.youtube.com/channel/UCgYQrFwjImpTQbovggoCLlg?feature
UCgYQrFwjImpTQbovggoCLlg

原字符串是http://www.youtube.com/channel/UCgYQrFwjImpTQbovggoCLlg?feature=watch
正则表达式http://www.youtube.com/channel/(\\w+)\\?feature
group(0)是http://www.youtube.com/channel/UCgYQrFwjImpTQbovggoCLlg?feature
group(1)是UCgYQrFwjImpTQbovggoCLlg

group(0)是整个正则表达式对应的字符串，group(1)是第一个括号里正则表达式中对应的字符串,group(n)是第n-1个括号里正则表达式中对应的字符串