WordPress text replacement function

Hello, I would like a simple WordPress plugin or function (preferably a function - no need for additional plugin files if possible) that replaces any word that begins with the @ symbol, within the post body on the front-end, with a link to a subdomain with that same name on a specified site. I'd like it to be easy to change the site name and the div or divs where this is applied in the function code. For example: @user should become <a href="[login to view URL]">@user</a> @aDifferentUser should become <a href="[login to view URL]">@aDifferentUser</a> ...and so forth. Only the name would differ, and be lowercase in the URL itself. Instances with a word also before the @ however should not be affected, so for example an e-mail adress would be left alone. I would also like there to be possible exception if it's ever needed, so I can add in a @user instance without it linking too, maybe for example @@user could simply appear as @user, without a link. I'm flexible on the format for this - whatever is easy to implement as the exception/override. All of this shouldn't have any effect on the admin panel; post edits there. Only front-end. Looking forward to your offers, thank you!