<?php
$str='[movie=http://a.com/a.mp3]';           //X格式为除了.mp3和.wmv以外的任何字符串
$ptn="/\[movie\=.*(?<!mp3|wmv)]$/";
$str=preg_replace($ptn,'x',$str);
echo   $str; 
?>

------解决方案--------------------
PHP code
<?php
$str='[movie=http://a.com/a.mp3]';
//$str='[movie=http://a.com/a.mp]';
$ptn="/\[movie\=(?>.*\.)(?:(?!mp3|wmv).+)\]/";

if(preg_match($ptn,$str,$g))   {
 echo   'Y';
 print_r($g);
}   else   {
 echo   'N';
}
?>