站长资讯网捕获组分为：
- 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{4})，其实是被忽略的。当使用 matcher.group(4) 时，系统会报错。
| 编号 | 捕获组 | 匹配 |
|---|---|---|
| 0 | (d{4})-((d{2})-(d{2})) | 2017-04-25 |
| 1 | ((d{2})-(d{2})) | 04-25 |
| 2 | (d{2}) | 04 |
| 3 | (d{2}) | 25 |
public static final String P_UNCAP = "(?:\d{4})-((\d{2})-(\d{2}))"; public static final String DATE_STRING = "2017-04-25"; Pattern pattern = Pattern.compile(P_UNCAP); Matcher matcher = pattern.matcher(DATE_STRING); 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)); // Exception in thread "main" java.lang.IndexOutOfBoundsException: No group 4 System.out.printf("nmatcher.group(4) value:%s", matcher.group(4));
总结
- 普通捕获组使用方便；
- 命名捕获组使用清晰；
- 非捕获组目前在项目中还没有用武之地。
原文地址：https://blog.csdn.net/just4you/article/details/70767928