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));
命名捕获组
每个以左括号开始的捕获组，都紧跟着 ?，而后才是正则表达式。
对于时间字符串：2017-04-25，表达式如下:
(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))有 4 个命名的捕获组，分别是:
| 编号 | 名称 | 捕获组 | 匹配 |
|---|---|---|---|
| 0 | 0 | (?\d{4})-(?(?\d{2})-(?\d{2})) | 2017-04-25 |
| 1 | year | (?\d{4})- | 2017 |
| 2 | md | (?(?\d{2})-(?\d{2})) | 04-25 |
| 3 | month | (?\d{2}) | 04 |
| 4 | date | (?\d{2}) | 25 |
命名的捕获组同样也可以使用编号获取相应值。
public static final String P_NAMED = "(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))";public static final String DATE_STRING = "2017-04-25";
Pattern pattern = Pattern.compile(P_NAMED);Matcher matcher = pattern.matcher(DATE_STRING);matcher.find();System.out.printf("\n===========使用名称获取=============");System.out.printf("\nmatcher.group(0) value:%s", matcher.group(0));System.out.printf("\n matcher.group('year') value:%s", matcher.group("year"));System.out.printf("\nmatcher.group('md') value:%s", matcher.group("md"));System.out.printf("\nmatcher.group('month') value:%s", matcher.group("month"));System.out.printf("\nmatcher.group('date') value:%s", matcher.group("date"));matcher.reset();System.out.printf("\n===========使用编号获取=============");matcher.find();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));
PS:非捕获组
在左括号后紧跟 ?:，而后再加上正则表达式，构成非捕获组 (?:Expression)。
对于时间字符串：2017-04-25，表达式如下:
(?:\\d{4})-((\\d{2})-(\\d{2}))这个正则表达式虽然有四个左括号，理论上有 4 个捕获组。但是第一组 (?:\d{
