- preg_replace: documentation ( source)
<?php
echo $str = "ann@ibbeck.de,Petermann@ibbeck.de";
echo '<br>';
echo $str = preg_replace('/ann@ibbeck.de\b/i', 'kan', $str);
echo '<br>';
echo $str = preg_replace('/Petermann@ibbeck.de\b/i', 'AP', $str);
?>