Dennis, use this simple regex:
\([^(]+SPECIFICWORD[^)]+\)
Here is a demo:
<?php
$string = "xx(xxxx)xxxx(xxxxxxxx)xxx(xxx)xxx(xxxxSPECIFICWORDxxxxxxxxxxx)xxx";
$regex="~\([^(]+SPECIFICWORD[^)]+\)~";
echo preg_replace($regex,"\1NEWWORD",$string);
?>
The Output:
xx(xxxx)xxxx(xxxxxxxx)xxx(xxx)xxxNEWWORDxxx