public static final String regularExpression = "^[a-zA-Z][a-zA-Z0-9_]{7,29}$";

import java.io.; import java.util.;

public class Solution {

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Scanner input = new Scanner(System.in);
   int count = input.nextInt();
    input.nextLine();

    for(int i=0;i<count;i++){
    String username = input.nextLine();


    if(checkUsername(username)){
        System.out.println("Valid");
    }
    else{
        System.out.println("Invalid");
    }
    }

    input.close();

}
public static boolean checkUsername(String username){
    int length = username.length();

    if(length<8 || length>30){
        return false;
    }
    if(!Character.isLetter(username.charAt(0))){
        return false;
    }

    for(int i=1;i<length;i++){
       char ch = username.charAt(i);
       if(!Character.isLetterOrDigit(ch) && ch!='_'){
        return false;
       }
    }
    return true;
}

}

JAVA SOLUTION

import java.io.; import java.util.;

public class Solution {

public static void validator(String word){

    // First character must be a letter : ^[a-zA-Z]

    // Following characters can be either letters , numbers or _ : [a-zA-Z_0-9]

    // Must have 8-30 characters so after the first validation to the last character 
    // can have 7 to 29 =  {7,29}$

    if (word.matches("^[a-zA-Z][a-zA-Z_0-9]{7,29}$")) {
        System.out.println("Valid");
    }
    else{
       System.out.println("Invalid"); 
    }

}

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */

    Scanner sc = new Scanner(System.in);
    int numWords = sc.nextInt();

    sc.nextLine();

    for (int i = 0; i < numWords; i++) {
        String word = sc.nextLine();
        validator(word);

    }

    sc.close();


}

}

public static final String regularExpression = "^[a-zA-Z][a-zA-Z0-9_]{7,29}$";

TestCase for Julia is wrong