Description:
------------
Trying the code, it'll give you this warning:

Warning: preg_replace() [function.preg-replace]: Compilation failed: range out of order in character class at offset 6 in D:\www\integraCMS\scripts\testOffset.php on line 4


However, if you shuffle the characters around it'll not happen.

Test script:
---------------
<?php

$value = "this @ is . a _ test-string";
$value = preg_replace('#[^\w_-.@]#','',$value);
echo $value;

?>