[PHP] preg_replace help

[Ben]

I am trying to build a pattern to do the following:
- Find all occurences of HREF=" and SRC=" except those that include the HTTP:// in 
their path
- Insert my own domain string (e.g.: http://www.my_domain.com)

So has an example the input string would be :
$val = '
<A href="http://www.my_domain.com/test.php";>my_domain</A>
<A href="hello.com">hello</A>
<IMG src="images/test.jpg">
<IMG src="http://www.my_domain.com/images/tstimg.jpg>
';

And I would like to end up with:
$val = '
<A href="http://www.my_domain.com/test.php";>my_domain</A>
<A href="http://www.my_domain.com/hello.com";>hello</A>
<IMG src="http://www.my_domain.com/images/test.jpg";>
<IMG src="http://www.my_domain.com/images/tstimg.jpg>
';

Thank you in advance.