Prevent User From Entering Mistakes

<script type = "text/javascript">

function testfield() {
var un = document.getElementById("dir").value;
if (!/^(/ftp/nas)/i.test(un)) {
alert ("Invalid starting characters - must be /ftp/nas");
return false;
[Code]...

I have a code like above which doesn't allow the user to proceed with out starting the field with a "/ftp/nas" which should be the starting of a path. Is there any way to make it disabled and the user only can enter the rest of the path. for eg: user1 has directory path like /ftp/nas/user1/help

so when this user enters his path, on the field "/ftp/nas" should always be available and the user has to only enter the rest of the path. and the user is allowed to enter only / (// is not allowed) and [A-Za-z] in the field. this is just to secure the application from typing mistakes.