Validating Username Using REGEX.

Hello Everyone!💜

Regex is one of the topics where almost every developer takes time in the beginning. So, let's practice it today by creating the regex pattern to validate a username. Let's begin!🚀

Before solving, if you are not aware of syntax for writing regular expressions do check it here!

I have taken this question from Hackerrank. Let's solve it.💪

I recommend you to try before seeing the solution.

🔎Solution

First let's take username as input

Scanner sc = new Scanner(System.in);
String userName = sc.nextLine();

Creating a regex pattern for username validation.

String regularExpression= "^[A-Za-z][A-Za-z0-9_]{7,29}$";

A valid username should start with an alphabet so, [A-Za-z].
All other characters can be alphabets, numbers or an underscore so, [A-Za-z0-9_].
Since length constraint was given as 8-30 and we had already fixed the first character, so we give {7,29}.
We use ^ and $ to specify the beginning and end of matching.

3.Now, to check the username against the regular expression.

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

Input: 
Laasya_Setty
1Hashnode
Output:
Valid
Invalid

Final Code

import java.util.regex;
import java.util.Scanner;
public class Solution {

    public static void main(String[] args){
     Scanner sc = new Scanner(System.in);
     String userName = sc.nextLine();
     String regularExpression = "^[A-Za-z][A-Za-z0-9_]{7,29}$";
     if (userName.matches(regularExpression)) {
        System.out.println("Valid");
    } else {
        System.out.println("Invalid");
    }         

    }
}