How to get @userName from @[userName] with the functionality of adding [] in the message with use of Regex?

I am working with react native and need to write down a message as @username and I am getting value from the API is @[userName]. I need to make it as I wanted with regex. I have tried and found a solution but it is also affecting to any of the square brackets [] I am adding to the message. It would be great if I can get help with this.