Introducing Regular Expressions

Preface

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 system—the work of Brian Kernighan, Dennis Ritchie, Ken Thompson, and others at AT&T Bell Labs—and 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 Berkeley Timesharing System, which ran on the Scientific Data Systems SDS 940. Documented in 1970, it was a rewrite by Ken Thompson of a previous editor on MIT’s Compatible Time-Sharing System and yielded one of the earliest if not first practical implementations of regular expressions in computing. (Table A-1 in Appendix A documents the regex features of QED.)

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