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Consider yourself as a third-party service provider who provides authentication to the users who holds account in your server. For giving the credentials to use your server you are providing a standalone usemame to the user of their wish with some restrictions Deterministic Finite Automata which will accept all possible valid usemames. Usernames can contain letters (a-z), numbers (0.2.4.6. 8) Only the specified Special Characters (. $) are allowed. Username should not have any capital letters. Username must end only with the numbers mentioned above. Special characters are allowed only in between the usernames A maximum of only one special character allowed and only one number is allowed