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.