I found the following regex to be working:-

"^[A-Za-z][a-zA-Z0-9_]{7,29}$" ;

import java.io.; import java.util.; import java.util.regex.*; public class Solution {

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
String empty = scanner.nextLine();
Pattern specialCharactersString = Pattern.compile("[!@#$,'^%&*()+=|<>?{}\\[\\]~-]");
Matcher hasSpecial;
if (1 <= n && n <= 100) {
    String[] username = new String[n];
    for (int i = 0; i < n; i++) {
        username[i] = scanner.nextLine();
    }
    for (int i = 0; i < n; i++) {
        hasSpecial = specialCharactersString.matcher(username[i]);
        if ((username[i].length() >= 8) && (username[i].length() <= 30) && Character.isAlphabetic(username[i].charAt(0)) && !(hasSpecial.find())) {
            System.out.println("Valid");
        } else {
            System.out.println("Invalid");
        }
    }
}

} }

you have to change only in "NULL" to this " ^[A-Za-z]\w{7,29}$"

Here are the solution of Java Valid Username Regular Expression Hacker Rank Solution