[jQuery] How do I make a partial textarea autocomplete?

[jQuery] How do I make a partial textarea autocomplete?


Hello,
Is there an example of partial autocomplete? By 'partial' I mean, only
a part of textarea gets suggestions.
Users in my project are supposed to start a message in a textarea with
'Dear <UserName>'
I want to make this <UserName> part either clickable like in Office
documents custom fields, and when a user is editing this part I want
to show a list of options (usernames from a database).
I can work out the backend, but I have no clue where to begin to make
such a textarea. Please help.
Thanks a ton.
Sam