Structure and syntax of an email address

1. The username can include:
   
1. Uppercase and lowercase letters (A-Z, a-z).
2. Latin extended characters (Ā- Ž,ā-ž).
3. Digits (0-9).
   
4. Special characters: ! # $ % & ' * + - / = ? ^ _ . { | } ~`.
5. Period (.) is allowed, but cannot be at the start, end, or appear consecutively.
   
6. Must not exceed 64 characters.
   
   
2. The domain part must follow valid domain name syntax. It can include:
   
1. Uppercase and lowercase letters (A-Z, a-z).
   
2. Digits (0-9).
   

Examples

1. Valid email addresses:
   
1. john.doe@example.com
   
2. user+mail@example.com
   
3. $B1234@example.com
   
4. _somename@example.com
   
   
   
2. Invalid email addresses:
   
1. .abce@example.com
   
2. abc..def@example.com
   
3. username1@@example.com
   
4. user231@example
   
5. the_email_which_consists_of_more_than_64_characters_in_the_username@example.com
   
6. username1.@example.com