Text Rewrite JavaScript

[vanbao]

Is there such a script that whenever it sees the url below,
http://youtube.com/v/PRv_czxZKUw

it will rewrite into

http://img.youtube.com/vi/PRv_czxZKUw/1.jpg


In my template, I have the following codes to display and play the video:

<script>
var image = "<?php echo $custom_34; ?>"
if (image != "")
{
document.write("&nbsp;&nbsp;&nbsp;<a href=\"http://youtube.com/v/PRv_czxZKUw&autoplay=1\" rel=\"iframe\" rev=\"width::425::height::355::scrolling::no::frameborder::1\" title=\"+title.substr(0,60)\"><img src=\"http://img.youtube.com/vi/PRv_czxZKUw/2.jpg"></a>")
}
else {
document.write("<a href=\"/members/user_login.php\"><img src=\"template/Yellow/images/no_video.png\" class=\"borderimage\" onMouseover=\"borderit(this,'green')\" onMouseout=\"borderit(this,'white')\"></a>")
}
</script>

The link in blue is what the user will enter as the source of the video. My question no is if there is any script that will take the link in blue and modify to to resamble the tag in red. I see that the difference between the link in blue and red are the addition of "img,vi,2.jpg" in the red link.

Or if there is a script that take the video link that the user enter and strip out the video reference code PRv_czxZKUw. If this is possible, I can setup my template to inclue "http://img.youtube.com/vi/XXXXXXX/2.jpg as fixed with XXXXXX=PRv_czxZKUw part being the variable that the script will insert.