preg_match_all("/<[^>：]*?:[^>：]*?/>/isU", $_str, $tag, PREG_SET_ORDER); 

$str='<my:file1 />';
preg_match_all('/[^>]*>/',$str,$array);
print_r($array); 

Array
(
    [0] => Array
        (
            [0] => <my:file1 />
        )

)

$str='<sys:date />dfsdf<my:file1 />k;k;<my:file1 />werewr<my:file1/>';
echo preg_replace('/<sys:date\s*\/>|<my:file1\s*\/>/','',$str);

$str=' <sys:date />dfsdf<my:dadfad/>dfadfjasdj <my:file1 />kjf kadflaj<img:img/img1.gif    />'; 
preg_match_all("/<(?<keyword>[^>:]+):(?<expression>[^>:]+)\s*\/>/",$str,$tag , PREG_SET_ORDER );