(?!someregexp)

where “someregexp” is a regular expression to match. The negative lookahead will reverse the logic for you. So when I needed to find all Master’s degree programs except Master of Science, I used something like

master(?!\sof\sscience)

to match say “master of arts” or “master” but not “master of science”.

The lookahead doesn’t move the position, so you can think of it as saving the spot it was in, checking forward, then returning. If you like regular expressions, you should experiment with lookaheads to get a feel for what they can do. I like to use rublar, a ruby regular expression editor and tester.