What is meant by capturing groups in Java reg ex?

A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern.

The capturing groups in regular expressions are used to treat multiple characters as a single unit it is represented by “()”. i.e.  if you place multiple sub patterns with in a parathesis they are treated as one group.

For example, the pattern [0-9] matches the digits 0 to 9 and the pattern {5} matches any characters. If you group these two as ([0-9]{5}), it matches a 5-digit number.

Example

Live Demo

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class COMMENTES_Example {
   public static void main(String[] args) {  
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter your name: ");
      String name = sc.nextLine();
      System.out.println("Enter your Date of birth: ");
      String dob = sc.nextLine();  
      //Regular expression to accept date in MM-DD-YYY format
      String regex = "^(1[0-2]|0[1-9])/   # For Month\n"
       + "(3[01]|[12][0-9]|0[1-9])/ # For Date\n"
       + "[0-9]{4}$                  # For Year";
      //Creating a Pattern object
      Pattern pattern = Pattern.compile(regex, Pattern.COMMENTS);
      //Creating a Matcher object
      Matcher matcher = pattern.matcher(dob);
      boolean result = matcher.matches();
      if(result) {
         System.out.println("Given date of birth is valid");
      }else {
         System.out.println("Given date of birth is not valid");
      }  
   }
}

Output

Enter your name:
Krishna
Enter your Date of birth:
09/26/1989
Given date of birth is valid

Example

Live Demo

import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class GroupTest {
   public static void main(String[] args) {
      String regex = "(.*)(\d+)(.*)";
      String input = "This is a sample Text, 1234, with numbers in between.";
      //Creating a pattern object
      Pattern pattern = Pattern.compile(regex);
      //Matching the compiled pattern in the String
      Matcher matcher = pattern.matcher(input);
      if(matcher.find()) {
         System.out.println("match: "+matcher.group(0));  
         System.out.println("First group match: "+matcher.group(1));  
         System.out.println("Second group match: "+matcher.group(2));  
         System.out.println("Third group match: "+matcher.group(3));
      }
   }
}

Output

match: This is a sample Text, 1234, with numbers in between.
First group match: This is a sample Text, 123
Second group match: 4
Third group match: , with numbers in between.