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Google Analytics' Regular Expressions can be used four areas:
1) In Google Analytics on-screen display filters:

Filter Source:

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:

Here's a great resource for testing your Regular Expressions before you deploy in Google Analytics: