Calling Regex Experts

[Dan Wilson]

Ok... so I claim to have a working knowledge or regex, but I cannot seem
to get this one right.  Here's the problem:

I have text which contains multiple values that I want to manipulate.
Let's say it looks something like this:

"... blahblah the brown fox jumped over $blahblah ..."

I want to manipulate/replace the 'blahblah', but not the '$blahblah'.
How would I go about doing this?  I've got the following regex, but it
doesn't do the trick:

"/(([^\$])blahblah)([^_])/is"

Any help would be great!

-Dan