Regex matching a string that DOESN'T contain a given word

From: alan (alanbrown_at_pacbell.net)
Date: 08/31/04 

Date: 31 Aug 2004 12:07:25 -0700

I have a tough little nut to crack, and I'm no regexpert so I thought
I'd see if anyone out there had a solution.

I need to match strings that represent directory structures. I want
to know if I have a directory represented in my file that is
underneath directory a/b/c

However, I'm not interested if it's directory a/b/c/tests/...

so I need someway to filter that out of my matches.

But I have to do it in one regex expression because I'm forced to use
a grepping tool from within my application that takes a regular
expression.

Surely this isn't as hard as it seems like it is to me.

Can anyone help me with the pattern for this?

alan