how to add 0 in left padding using regular expression java

To add left padding with zeros to a string in Java using a regular expression, you can use the java.util.regex.Matcher class and the replaceFirst method to insert the zeros before the string.

Here's an example of how you could do this:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    String input = "123";
    int padding = 5;

    // Define a regular expression pattern to match the beginning of the input string
    String pattern = "^";

    // Create a Matcher object
    Pattern p = Pattern.compile(pattern);
    Matcher m = p.matcher(input);

    // Insert the padding at the beginning of the input string
    String output = m.replaceFirst(String.format("%0" + padding + "d", 0));

    System.out.println(output); // 00123
  }
}

In this example, the replaceFirst method is used to insert padding number of zeros at the beginning of the input string. The String.format method is used to generate a formatted string with the desired number of zeros.

You can customize the regular expression pattern and the padding value as needed to match your specific requirements. For example, you can use a different regular expression pattern to match a different part of the input string, or you can use a different padding value to add more or fewer zeros.