How do i get my account/username back if i regret to delet it?

For usernames that need to be reclaimed, send an email with your request to username@twitter.com

Per Crystal at Twitter:

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In your request, please include the following information:

*The user name you want
*Your existing user name, if you have one
*If you want to change your user name, or start a new account with the user name you're requesting.

Please note: Twitter Support will change your user name for you at the time of responding to the request so that the user name doesn't get taken by someone else. We will notify you by email when this happens.
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