Hi Andrew,
first let me say that what you need (mantain the short URL in the browser) could be done by the Enterprise version of ReDJ (feature internal redirect), but I would suggest you to fist try to get the redirect you need with Community version (with a standard 301 redirect).
About the matching rule, this could be:
From URL: /item/[0-9]{1,3}-
But I'm afraid that what you really need is a little bit different. If you would like that the user write the shortest URL and get the page for the longest one:
User call http://www.yoursite.com/dir1/name1 and get the content of http://www.yoursite.com/dir1/item/123-name1
If so, let me say you can't do this with ReDJ. The reason is that there's no way (e.g. with macros) for ReDJ to know the missing information needed to build the real destination, /item/123-, because the digits are different for any call.
Let me know,
Luigi