Valid Username Regular Expression

You are updating the username policy on your company’s internal networking platform. According to the policy, a username is considered valid if all the following constraints are satisfied:

The username consists of 8 to 30 characters inclusive. If the username consists of less than 8 or greater than 30 characters, then it is an invalid username.
The username can only contain alphanumeric characters and underscores (_). Alphanumeric characters describe the character set consisting of lowercase characters [a-z], uppercase characters [A-Z], and digits [0-9].
The first character of the username must be an alphabetic character, i.e., either lowercase character [a-z] or uppercase character [A-Z].

For example:

Username	Validity
Julia	INVALID; Username length < 8 characters
Samantha	VALID
Samantha_21	VALID
1Samantha	INVALID; Username begins with non-alphabetic character
Samantha?10_2A	INVALID; ‘?’ character not allowed

Update the value of regularExpression field in the UsernameValidator class so that the regular expression only matches with valid usernames.

Input Format

The first line of input contains an integer n, describing the total number of usernames. Each of the next n lines contains a string describing the username. The locked stub code reads the inputs and validates the username.

Constraints

1 < n < 100
The username consists of any printable characters.

Output Format

For each of the usernames, the locked stub code prints Valid if the username is valid; otherwise Invalid each on a new line.

Sample Input 0

8
Julia
Samantha
Samantha_21
1Samantha
Samantha?10_2A
JuliaZ007
Julia@007
_Julia007

Sample Output 0

Invalid
Valid
Valid
Invalid
Invalid
Valid
Invalid
Invalid

Explanation 0

Refer diagram in the challenge statement.

Solution:

import java.util.Scanner;
class UsernameValidator {
    /*
     * Write regular expression here.
     */
    public static final String regularExpression = "(^\\b\\w{0}[a-zA-Z]\\w{6,28}[a-zA-Z0-9_]\\b$)";
}


public class Solution {
    private static final Scanner scan = new Scanner(System.in);
    
    public static void main(String[] args) {
        int n = Integer.parseInt(scan.nextLine());
        while (n-- != 0) {
            String userName = scan.nextLine();

            if (userName.matches(UsernameValidator.regularExpression)) {
                System.out.println("Valid");
            } else {
                System.out.println("Invalid");
            }           
        }
    }
}