[kh99]

OK, there may be some more elegant way someone knows to do it (in which case, please post it), but here's what I came up with:

$word = array(
'google',
'yahoo'
);

$link = array(
'<a href="http://google.com">google</a>',
'<a href="http://yahoo.com">yahoo</a>'
);

$parts = preg_split('#(<a|</a)#i', $this->post['message'], -1, PREG_SPLIT_DELIM_CAPTURE);

$newmsg = '';
$inlink = false;
foreach ($parts as $part)
{
    if (strcasecmp($part, "<a") == 0)
        $inlink = true;
    else if (strcasecmp($part, "</a") == 0)
        $inlink = false;
    else if (!$inlink)
        $part = str_replace($word, $link, $part);
    $newmsg .= $part;
}

$this->post['message'] = $newmsg;

I haven't tested it extensively so if you try it you might want to check a lot of posts to make sure they look OK.