What is Regular Expression
According to Wikipedia a regular expression provides a concise and flexible means for “matching” (specifying and recognizing) strings of text, such as particular characters, words, or patterns of characters. Abbreviations for “regular expression” include “regex” and “regexp”.
Where Are Regular Expressions Used
Google Analytics allows the use of regular expressions in profile filters to allow complex implementation search reports using regular expressions (see example: How to Exclude Internal Traffic from Google Analytics, create custom reports with regular expression filters and create advanced segments with regular expression.
Create Profile Filters
Create Custom Reports
Create Advanced Segments
Learning About Regular Expression
There are several resources available on the web. Lunametrics has also written a quick guide on Regular Expression for Google Analytics.
We are also working on a guide and if you have any examples of how you have used regular expression with Google Analytics then please send those to us.