Exact string replacement...

Hi there, Please help me out in the following... <?php $is='is'; echo eregi_replace(" ".$is." "," ","This is a test string which contains is in dismissal"); ?> Actually I want to replace "is" alone from the string and not all the words that contains is... I have tried with "^".$is."$" ---> but yields nothing.... Also tried with "\b"... thanks and regards, Desikan -- Desikan desikb4u@fastmail.fm