Regular expressions are one of the most powerful, and hence, the most complicated tools for matching strings. You can think of a regular expression as a "pattern" that can be used to match against some string. Regular expressions are far more versatile than the simple "eq" and "ne" operators and include a wide variety of modifiers and tricks. Other books have detailed chapters focusing on the use of regular expressions, so we will only touch upon a few common uses of regular expressions found in CGI applications.