You will have to override method insertString (int, String, AttributeSet)
of javax.swing.text.StyledDocument, parse the input string, and replace any occurrences
of the pattern you want with the icon.

This is roughly an outline of the steps you will need to take. For the implementation I can only
give you two examples (there is a lot of coding involved)

1. Check JTextField for
   an example on how to override insertString (top of the document, provides an example on how
   to convert input to uppercase, faily simple)
   
2. Check /demo/jfc/StylePad for an example on how to display
   a complex document (with styles and images)
   

Hope this helps

Regards

George T