正则批量轮换

正则批量替换
$str ='<img src="test/aaa/a.jpg" widht=33 height=3><img src="f/aaa/a.jpg" widht=4 height=3>'

如何批量将src前面加个域名呢

$str ='<img src="http://xxxx.com/test/aaa/a.jpg" widht=33 height=3><img src="http://xxxx.com/f/aaa/a.jpg" widht=4 height=3>'
[解决办法]

$str = '<img src="test/aaa/a.jpg" widht=33 height=3><img src="f/aaa/a.jpg" widht=4 height=3>';
echo preg_replace('/(src=")/', "$1'http://www.xxx.com/", $str);