捕获组分为：
- 普通捕获组(Expression)
-
命名捕获组(?
Expression)
普通捕获组
从正则表达式左侧开始，每出现一个左括号"("记做一个分组，分组编号从 1 开始。0 代表整个表达式。
对于时间字符串：2017-04-25，表达式如下
(\\d{4})-((\\d{2})-(\\d{2}))
有 4 个左括号，所以有 4 个分组:
| 编号 | 捕获组 | 匹配 |
|---|---|---|
| 0 | (\d{4})-((\d{2})-(\d{2})) | 2017-04-25 |
| 1 | (\d{4}) | 2017 |
| 2 | ((\d{2})-(\d{2})) | 04-25 |
| 3 | (\d{2}) | 04 |
| 4 | (\d{2}) | 25 |
public static final String DATE_STRING = "2017-04-25";
public static final String P_COMM = "(\\d{4})-((\\d{2})-(\\d{2}))";
Pattern pattern = Pattern.compile(P_COMM);
Matcher matcher = pattern.matcher(DATE_STRING);
matcher.find();//必须要有这句
System.out.printf("\nmatcher.group(0) value:%s", matcher.group(0));
System.out.printf("\nmatcher.group(1) value:%s", matcher.group(1));
System.out.printf("\nmatcher.group(2) value:%s", matcher.group(2));
System.out.printf("\nmatcher.group(3) value:%s", matcher.group(3));
System.out.printf("\nmatcher.group(4) value:%s", matcher.group(4));
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