Replacing Element / Doc Write From Regex

var bakaBanner;
bakaBanner = document.getElementById('head');
if (bakaBanner) {
var str=//images/banners/[A-Za-z0-9_]+.[a-z]+/;
document.write(str.replace(//images/banners/[A-Za-z0-9_]+.[a-z]+/, "http://img218.imageshack.us/img218/5904/revyblacklagoon1.png"));
}

Alright, here's the deal... This is a script to be used in Greasemonkey. The page this is for changes their banner every 5 minutes, the name changes and it could be either jpg or png.

I'm fairly certain my regex itself:
/images/banners/[A-Za-z0-9_]+.[a-z]+
Is correct... Here's an example of what it's looking through:
<div id="head" style="background-image: url(/images/banners/kiminitodoke.jpg);" >
<div id="header_left">
<div id="rss">

My guess is that it has something to do with this line:
document.write(str.replace(//images/banners/[A-Za-z0-9_]+.[a-z]+/, "http://img218.imageshack.us/img218/5904/revyblacklagoon1.png"));