正則表達式的捕獲組（Java）

 捕獲組分類

1. 普通捕獲組(Expression)
2. 命名捕獲組(?<name>Expression)

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

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));

命名捕獲組

每個以左括號開始的捕獲組，都緊跟着“?”，而后才是正則表達式。

對於時間字符串：2017-04-25，表達式如下

(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))

有4個命名的捕獲組，分別是

命名的捕獲組同樣也可以使用編號獲取相應值

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));

PS:非捕獲組
在左括號后緊跟“?:”，而后再加上正則表達式，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}))
1
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。

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));

 

//如果同時匹配到了不同位置的字符串，要取捕獲組哪個位置的字符串
//這個位置在循環中索引值不能變，如m.group(2)中的2就不能變
String s=" from aaa from bbb"
Pattern p = Pattern.compile("\\s+(from|join)\\s+(\\w+)");
Matcher m = p.matcher(hql);
while (m.find()) {
   System.out.println(m.group(2));
}
//輸出結果：
aaa
bbb