[ Java 常見問題 ] Reference to previous matched groups within a regex

Source From Here 
Question 
Is there a way in Java to refer to previous matched groups within the regex? 

How-To 
Sample code as below: 

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1. package howto;  
2.   
3. import java.util.regex.Matcher;  
4. import java.util.regex.Pattern;  
5.   
6. public class Test {  
7.   
8.     public static void main(String[] args) throws Exception  
9.     {  
10.         String str = "A123 A123 A123 A123";  
11.         // The outermost () is group1; So "(\\w\\d\\d\\d)" is group2.  
12.         Pattern ptn = Pattern.compile("((\\w\\d\\d\\d)(\\s+\\2)*)");  
13.         Matcher mth = ptn.matcher(str);  
14.         if(mth.find())  
15.         {  
16.             for(int i=0; i
17.             {  
18.                 System.out.printf("G%d: '%s'\n", i+1, mth.group(i+1));  
19.             }  
20.         }  
21.         else  
22.         {  
23.             System.err.printf("Miss!\n");  
24.         }  
25.     }  
26. }  

Execution result: 

G1: 'A123 A123 A123 A123'
G2: 'A123'
G3: ' A123'