JAVA EE: How to define nested multiple groups in a regular expression | Java Regex

Monday 25 December 2017

How to define nested multiple groups in a regular expression | Java Regex | Regex in java

Click here to watch on Youtube :
https://www.youtube.com/watch?v=NZVtYrycZ-k&list=UUhwKlOVR041tngjerWxVccw

RegexDemo.java

import java.util.regex.Matcher;

import java.util.regex.Pattern;

/**
 * Groups Inside Groups
 * 
 * It is possible to have groups inside group in a regular
 * expression.
 *
 */

public class RegexDemo

{

    public static void main(String[] args)

    {

        String text = "John writes about this, and John Doe writes about that,"

                + " and John Wayne writes about everything.";

        /*
         * Notice how the two groups from the examples earlier are now
         * nested inside a larger group. (again, you cannot see the
         * space at the end of the expression, but it is there).
         * 
         * When groups are nested inside each other, they are numbered
         * based on when the left paranthesis of the group is met.
         * Thus, group 1 is the big group. Group 2 is the group with
         * the expression John inside. Group 3 is the group with the
         * expression .+? inside. This is important to know when you
         * need to reference the groups via the groups(int groupNo)
         * method.
         */

        String regex = "((John) (.+?)) ";

        Pattern pattern = Pattern.compile(regex);

        Matcher matcher = pattern.matcher(text);

        while (matcher.find())

        {

            System.out.println("Group1 = "+matcher.group(1));

            System.out.println("Group2 = "+matcher.group(2));

            System.out.println("Group3 = "+matcher.group(3));

            System.out.println("--------------------------------");

        }

    }

}

Output

Group1 = John writes

Group2 = John

Group3 = writes

--------------------------------

Group1 = John Doe

Group2 = John

Group3 = Doe

--------------------------------

Group1 = John Wayne

Group2 = John

Group3 = Wayne

--------------------------------