lookahead of regexp of javascript

<script type="text/javascript" charset="utf-8">
 function doc (argument) {
  document.write(argument);
  document.write("<br />\n");
 }
 
 var pp = /^\d(?=\.\d+)/; // pattern positive lookahead
 doc('3.14'.match(pp));
 doc('3.'.match(pp));
 doc('.14'.match(pp));
 
 var pn = /^\d(?!\.\d+)/; // pattern negative lookahead
 doc('3'.match(pn));
 doc('3.14'.match(pn));
 doc('.14'.match(pn));
</script>