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Java 正则表达式的捕获组(capture group)

说明
分类：计算机

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 4System.out.printf("\nmatcher.group(4) value:%s", matcher.group(4));