I have this text field for people to type their username

When someone types their username, I would like another (disabled) text field to dynamically update to show the user a preview of their profile link, which is in the format of "http://site.com/profile/username"

ie. if user types "funn" in the username field, the profile text field will display "http://site.com/profile/funn/"
if the user subsequently types a "y" to make "funny", the profile field will update to "http://site.com/profile/funny/"

Can someone show me how to do this, or direct me to an example?