mod rewrite rule help

how do i make a mod rewrite rule that changes all URLs without filename extensions? like, if someone goes to domain.com/robots.txt or domain.com/something.php it will go there, but if they just go to domain.com/username it will run as domain.com/user.php?user=username

not sure how this would be done.

thanks

Not with html or css. http://www.yourhtmlsource.com/sitemanagement/urlrewriting.html