Backreferences

Capturing groups can remember text matched by the expression they enclose. Backreferencing is a feature that use the remembering capacity of Capturing groups.

Capturing Group	Description
\n	Matches with whatever the nth capturing group matched.
\k<name>	Matches with whatever the named-capturing group "name"matched.

For Example, regular expression '(\d{4}) \1' match the 4 digits which are repeating twice.

Enter Regular Expression

(\d{4}) \1

Enter the string

1234 1234

I found the text 1234 1234 starting at index 0 Ending at index 9

Enter the string

1234 5678 5678 1234

I found the text 5678 5678 starting at index 5 Ending at index 14

Another Example, regular expression '([a-z,A-Z]*) \1' match if same striing repeated twice.

Enter Regular Expression

([a-z,A-Z]*) \1

Enter the string

hari hari

I found the text hari hari starting at index 0 Ending at index 9

Enter the string

krishna krishna

I found the text krishna krishna starting at index 0 Ending at index 15

Enter the string

my name name is krishna

I found the text starting at index 2 Ending at index 3

I found the text name name starting at index 3 Ending at index 12

I found the text starting at index 12 Ending at index 13

I found the text starting at index 15 Ending at index 16

Enter Regular Expression

(a((b){2})) \2

Enter the string

abb bb

I found the text abb bb starting at index 0 Ending at index 6

Example 1 : Backreferencing using naming

import java.util.regex.*;
        
public class CaptureGroupEx {
    public static void main(String args[]){
        String name = "(?<number>\\d+)\\s\\k<number>";
        Matcher m = Pattern.compile(name).matcher("1234 1234");
        
        while (m.find()) {
            System.out.print("I found the text " + m.group());
            System.out.print(" starting at index " + m.start());
            System.out.println(" Ending at index " + m.end());
        }        
    }
}

Output

I found the text 1234 1234 starting at index 0 Ending at index 9

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