Java 正則表達式——捕獲組

捕獲組分爲：

• 普通捕獲組(Expression)
• 命名捕獲組(?Expression)

1. 普通捕獲組

從正則表達式左側開始，每出現一個左括號"("記做一個分組，分組編號從 1 開始。0 代表整個表達式。

對於時間字符串：2017-04-25，表達式如下

(\\d{4})-((\\d{2})-(\\d{2}))

有 4 個左括號，所以有 4 個分組：

編號	捕獲組	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();//必須要有這句
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));

2. 命名捕獲組

每個以左括號開始的捕獲組，都緊跟着 ?，而後纔是正則表達式。

對於時間字符串：2017-04-25，表達式如下:

(?\\d{4})-(?(?\\d{2})-(?\\d{2}))

有 4 個命名的捕獲組，分別是:

編號	名稱	捕獲組	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	day	(?\d{2})	25

public static final String P_NAMED = "(?\\d{4})-(?(?\\d{2})-(?\\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===========使用名稱獲取=============");
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===========使用編號獲取=============");
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));

3. 非捕獲組

在左括號後緊跟 ?:，而後再加上正則表達式，構成非捕獲組 (?:Expression)。

對於時間字符串：2017-04-25，表達式如下：

(?:\\d{4})-((\\d{2})-(\\d{2}))

這個正則表達式雖然有四個左括號，理論上有 4 個捕獲組。但是第一組 (?:\d{4})，其實是被忽略的。當使用 matcher.group(4) 時，系統會報錯。

編號	捕獲組	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