Find Users With Valid E-Mails

** Problem #1 **
Find Users With Valid E-Mails

Table: Users
+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| name          | varchar |
| mail          | varchar |
+---------------+---------+
user_id is the primary key (column with unique values) for this table.
This table contains information of the users signed up in a website. Some e-mails are invalid.

 
Write a solution to find the users who have valid emails.
A valid e-mail has a prefix name and a domain where:
The prefix name is a string that may contain letters (upper or lower case), digits, underscore '_', period '.', and/or dash '-'. The prefix name must start with a letter.
The domain is '@leetcode.com'.
Return the result table in any order.

Example 1:

Input: 
Users table:
+---------+-----------+-------------------------+
| user_id | name      | mail                    |
+---------+-----------+-------------------------+
| 1       | Winston   | winston@leetcode.com    |
| 2       | Jonathan  | jonathanisgreat         |
| 3       | Annabelle | bella-@leetcode.com     |
| 4       | Sally     | sally.come@leetcode.com |
| 5       | Marwan    | quarz#2020@leetcode.com |
| 6       | David     | david69@gmail.com       |
| 7       | Shapiro   | .shapo@leetcode.com     |
+---------+-----------+-------------------------+
Output: 
+---------+-----------+-------------------------+
| user_id | name      | mail                    |
+---------+-----------+-------------------------+
| 1       | Winston   | winston@leetcode.com    |
| 3       | Annabelle | bella-@leetcode.com     |
| 4       | Sally     | sally.come@leetcode.com |
+---------+-----------+-------------------------+
Explanation: 
The mail of user 2 does not have a domain.
The mail of user 5 has the # sign which is not allowed.
The mail of user 6 does not have the leetcode domain.
The mail of user 7 starts with a period.

Regex Pattern: ^[A-Za-z][A-Za-z0-9_\.\-]*@leetcode(\?com)?\.com$

Let's break down the regex pattern step by step to clearly explain each part:

^: Anchor the regex pattern to match from the start of the string.
[A-Za-z]: Match any single uppercase or lowercase letter. The email prefix name must start with a letter.
[A-Za-z0-9_.-]*: Match any number of characters following the first letter in the email prefix name. It includes letters (upper or lower case), digits, underscore '_', period '.', and/or dash '-'.
@: Match the literal '@' character, which separates the prefix name and the domain.
leetcode: Match the literal 'leetcode', which is part of the email domain.
(?com)?: Make the sequence ?com optional in the email domain. Allows the pattern to match both '@leetcode.com' and '@leetcode?com'.
. : Match the literal '.' character, which separates the 'leetcode' part from the 'com' part of the domain.
com: Match the literal 'com' at the end of the email domain.
$: Anchor the regex pattern to match until the end of the string.

Solution:

SELECT *
FROM Users
WHERE REGEXP_LIKE(mail, '^[a-zA-Z][a-zA-Z0-9\_\.\-]*@leetcode.com')