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Java Regex 3 - Username Checker
Regular expressions (regexp) help us match or search for patterns in strings. In this problem, you will be given a username. 
Your task is to check whether that username is valid. A valid username will have the following properties:
The username can contain alphanumeric characters and/or underscores(_).
The username must start with an alphabetic character.
8 ≤ |Username| ≤ 30.
To simplify your task, we have provided a portion of the code in the editor. You just need to write down the regex pattern 
which will be used to validate the username input.
Input Format
The first line of input contains an integer N, representing the number of testcases. Each of the next N lines contains a 
string that represents a username.
Constraints
The username consists of any printable characters.
Output Format
For each username, output Valid if the username is valid; otherwise, output Invalid.
Sample Input
alpha_naheed
xahidbuffon
123Swakkhar
Sample Output
 String pattern = "^[a-zA-Z][a-zA-Z0-9_]{7,29}$";

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