Chapter 1. Introduction to Regular Expressions

Here's the scenario: you're given the job of checking the pages on a web server for doubled words (such as "this this"), a common problem with documents subject to heavy editing. Your job is to create a solution that will:

• Accept any number of files to check, report each line of each file that has doubled words, highlight (using standard ANSI escape sequences) each doubled word, and ensure that the source filename appears with each line in the report.

• Work across lines, even finding situations where a word at the end of one line is repeated at the beginning of the next.

• Find doubled words despite capitalization differences, such as with 'The the···', as well as allow differing amounts of whitespace (spaces, tabs, newlines, and the like) to lie between the words.

• Find doubled words even when separated by HTML tags. HTML tags are for marking up text on World Wide Web pages, for example, to make a word bold: '···it is <B>very</B> very important···'.

That's certainly a tall order! But, it's a real problem that needs to be solved. At one point while working on the manuscript for this book, I ran such a tool on what I'd written so far and was surprised at the way numerous doubled words had crept in. There are many programming languages one could use to solve the problem, but one with regular expression support can make the job substantially easier.

Regular expressions are the key to powerful, flexible, and efficient text processing. Regular expressions themselves, with a general pattern notation almost like a mini programming language, allow you to describe and parse text. With additional support provided by the particular tool being used, regular expressions can add, remove, isolate, and generally fold, spindle, and mutilate all kinds of text and data.

It might be as simple as a text editor's search command or as powerful as a full text processing language. This book shows you the many ways regular expressions can increase your productivity. It teaches you how to think regular expressions so that you can master them, taking advantage of the full magnitude of their power.

A full program that solves the doubled-word problem can be implemented in just a few lines of many of today's popular languages. With a single regular-expression search-and-replace command, you can find and highlight doubled words in the document. With another, you can remove all lines without doubled words (leaving only the lines of interest left to report). Finally, with a third, you can ensure that each line to be displayed begins with the name of the file the line came from. We'll see examples in Perl and Java in the next chapter.

The host language (Perl, Java, VB.NET, or whatever) provides the peripheral processing support, but the real power comes from regular expressions. In harnessing this power for your own needs, you learn how to write regular expressions to identify text you want, while bypassing text you don't. You can then combine your expressions with the language's support constructs to actually do something with the text (add appropriate highlighting codes, remove the text, change the text, and so on).