如上，我是想要结果为 12abc34
<?php  
$string = "some text (12abc34) more text";  
if(preg_match("/[(][.][)]/",$string,$matches)){  
  echo $matches;  
}  
?> 
如上，我是想要结果为 12abc34

------解决方案--------------------
$string = "some text (12abc34) more text";
preg_match('/\(([a-z0-9]+)\)/', $string, $matches);
echo $matches[1];
------解决方案--------------------
$string = "some text (12abc34) more text";
$preg = "/\d+.*\d+/";
preg_match($preg, $string, $matches);
var_dump($matches);
试试这个