Easy defintion please. I looked it up already online and the definition I found almost melted my eyes.

REgular EXpressions, a powerful tool for manipulating text and data... If you don't use them yet, you will discover in this book a whole new world of mastery over your data. If you already use them, you'll appreciate this book's unprecedented detail and breadth of coverage...

With regular expressions, you can save yourself time and aggravation while dealing with documents, mail messages, log files -- you name it -- any type of text or data. For example, regular expressions can play a vital role in constructing a World Wide Web CGI script, which can involve text and data of all sorts...

Regular expressions are not a tool in and of themselves, but are included as part of a larger utility. The classic example is grep. These days, regular expressions can be found everywhere, such as in:

* Scripting languages (including Perl, Tcl, awk, and Python)
* Editors (including Emacs, vi, and Nisus Writer)
* Programming environments (including Delphi and Visual C++)


While many of these tools originated on UNIX, they are now available for a wide variety of platforms, including DOS/Windows and MacOS, so you can use them in your home environment. Additionally, many favorite programming languages offer regular-expression libraries, so you can include support for them in your own programs, and yes, even applets.

There can be certain subtle, but valuable, ways to think when you're using regular expressions, and these can be taught. Jeffrey Friedl has spent years helping people on the Net understand and use regular expressions. In this book he leads you through the steps of knowing exactly how to craft a regular expression to get the job done.