Google Analytics' Regular Expressions can be used four areas:
1) In Google Analytics on-screen display filters:
Filter Source:
facebook|twitter|youtube|myspace|stumbleupon|technorati|delicious|reddit|digg|flickr
2) In Google Analytics Advanced Segments:
Add this Social Media Segment to your Google Analytics Profile.
3) In Google Analytics data processing Filters:
4) In Google Analytics Goal Configuration:
Here's some more information about Regular Expressions:
http://www.google.com/support/analytics/bin/answer.py?answer=55582&ctx=sibling
http://analytics.blogspot.com/2009/04/regular-expression-tips-and-tricks.html
http://www.regular-expressions.info/
Here's a great resource for testing your Regular Expressions before you deploy in Google Analytics:
http://ryanswanson.com/regexp/#start