lovely, this html-aware board.
When you type the correct html code for an ampersand, it prints the character, not the code. :-)
Anyway I searched the web a little (remember that word "search" it is a wonderfull word) and I found this:
$string=preg_replace("/&(?!amp)/","&",$string);
at a page where regexps are explained to the point where you actually begin to undertand them:
http://www.theknow.co.uk/zonk/regex.htm
