RegExpres Prob

I have a regexp problem I want to go through some HTML and where <img src="path/to/image.jpg"> I want to change it to this <img src="http://www.domain.com/path/to/image.jpg"> BUT if the image tag is like this <img src="http://www.domain.com/path/to/image.jpg"> already I don't want anything to happen........ I thought this would do it: ereg_replace("src=\"(^http)","src=\"http://www.domain.com/", $string); but it didn't............... Argh... Cheers John Wards