title: Find all words starts with @@ and ends with @@ in long string
body: <p>I have a quite big string. In that big string, I want to get all UNIQUE words starts with @@ and ends with @@. Between @@ could be text, number or alphanumeric or anything.</p>
  
  <p>Once I get all the UNIQUE words starting @@ and ends with @@, I want to replace each word with   a value which matches a key in a different array.</p>
  
  <p>Looking for the solution in C#.</p>