Michael Fitzgerald - Introducing Regular Expressions

Beijing Cambridge Farnham Kln Sebastopol Tokyo

This book shows you how to write regular expressions through examples. Its goal is to make learning regular expressions as easy as possible. In fact, this book demonstrates nearly every concept it presents by way of example so you can easily imitate and try them yourself.

Regular expressions help you find patterns in text strings. More precisely, they are specially encoded text strings that match patterns in sets of strings, most often strings that are found in documents or files.

Regular expressions began to emerge when mathematician Stephen Kleene wrote his book Introduction to Metamathematics (New York, Van Nostrand), first published in 1952, though the concepts had been around since the early 1940s. They became more widely available to computer scientists with the advent of the Unix operating systemthe work of Brian Kernighan, Dennis Ritchie, Ken Thompson, and others at AT&T Bell Labsand its utilities, such as sed and grep , in the early 1970s.

The earliest appearance that I can find of regular expressions in a computer application is in the QED editor. QED, short for Quick Editor, was written for the documents the regex features of QED.)

Ill use a variety of tools to demonstrate the examples. You will, I hope, find most of them usable and useful; others wont be usable because they are not readily available on your Windows system. You can skip the ones that arent practical for you or that arent appealing. But I recommend that anyone who is serious about a career in computing learn about regular expressions in a Unix-based environment. I have worked in that environment for 25 years and still learn new things every day.

Those who dont understand Unix are condemned to reinvent it, poorly. Henry Spencer

Some of the tools Ill show you are available online via a web browser, which will be the easiest for most readers to use. Others youll use from a command or a shell prompt, and a few youll run on the desktop. The tools, if you dont have them, will be easy to download. The majority are free or wont cost you much money.

This book also goes light on jargon. Ill share with you what the correct terms are when necessary, but in small doses. I use this approach because over the years, Ive found that jargon can often create barriers. In other words, Ill try not to overwhelm you with the dry language that describes regular expressions. That is because the basic philosophy of this book is this: Doing useful things can come before knowing everything about a given subject.

There are lots of different implementations of regular expressions. You will find regular expressions used in Unix command-line tools like vi ( vim ), grep , and sed , among others. You will find regular expressions in programming languages like Perl (of course), Java, JavaScript, C# or Ruby, and many more, and you will find them in declarative languages like XSLT 2.0. You will also find them in applications like Notepad++, Oxygen, or TextMate, among many others.

Most of these implementations have similarities and differences. I wont cover all those differences in this book, but I will touch on a good number of them. If I attempted to document all the differences between all implementations, Id have to be hospitalized. I wont get bogged down in these kinds of details in this book. Youre expecting an introductory text, as advertised, and that is what youll get.