Java Regex - Capturing Group Reference in Replacement String

The capturing groups can be referenced in the matcher's replacement string.

Syntax:

1. ${groupName}: Using capturing group name 'groupName'. We must have defined the group name in the regex.
2. $groupNumber: if we want to use group number instead of group name. May be we don't have related group name defined in the regex.

Examples:

Asume our example input string contains some sort of alphanumeric code with this format: a alphabet followed by two digits. We want to use java regex to interchange their positions i.e. the two digits followed by the alphabet.

/* Using capturing group name*/
Pattern.compile("(?<aCode>[a-z]{1,1})(?<dCode>[0-9]{2,2})")
       .matcher("a38 d45")
       .replaceAll("${dCode}${aCode}");//result: '38a 45d'


/*Regex breakdown: (?<aCode>[a-z]{1,1})(?<dCode>[0-9]{2,2})
   
(
Starting the first capturing group.
 ?<aCode>
Giving name to this capturing group: 'aCode.'
 [a-z]{1,1}
Must contain exactly one alphabet.
)
Closing the first capturing group.
(
Start the second capturing group.
 ?<dCode>
Giving name to this capturing group: 'dCode'.
 [0-9]{2,2}
Must contain exactly two digits.
)
Closing the second group.
*/




/* Using capturing group number*/
Pattern.compile("([a-z]{1,1})([0-9]{2,2})")
       .matcher("a38 d45")
       .replaceAll("$2$1");//result: '38a 45d'


/*Regex breakdown: ([a-z]{1,1})([0-9]{2,2})
   
(
Starting the first capturing group.
 [a-z]{1,1}
Must contain exactly one alphabet.
)
Closing the first capturing group.
(
Start the second capturing group.
 [0-9]{2,2}
Must contain exactly two digits.
)
Closing the second group.
*/

It's important to know that if we want to use $ in the replacement string as a literal, we have to escape it with a double backslashes i.e. \\$. Also we should always escape a single slash \ in the replacement string i.e. \\\\.

Example Project

Dependencies and Technologies Used:

• JDK 1.8
• Maven 3.0.4