Run function on parts of text string

[wee493]

Currently I have a large string of html stored in a variable. Within the html is some images.I'm using preg_replace to find all the images

$content = preg_replace('~'.$site.'(\S+\.(?:jpe?g|png|gif))~i', "$1",$content);

 

I'm wanting to run a function on every image to cache it locally. So I need to run a function on each string preg_replace picks out. How do i do this?