Need help in wrapping text around Selected Text

Hi programmers, I am Titus Cheng. I am facing a problem that I don't know how to solve. I have two rich text box control. One for editing visually, another is for add tags, like a wysiwyg text editor. I want to wrap tags around selectedtext. For example, if I selected the text "glass" in "I am drinking the glass of milk" and change the fontstyle to bold, it will appear something like "I am drinking the [bold]glass[/bold] of milk". and If I want to change the text to red, it will appear like "I am drinking the [bold][red]glass[/red][/bold] of milk" Can anyone help me with this problem? I am not trying to change the tags to html code, but rather contextual tags.