Chapter 17. Regular Expressions

IN THIS CHAPTER

  • Defining regular expressions

  • Looking at the basics

  • Extending our patterns

  • Creating expressions

The key to successfully working with the sed editor and the gawk program in your shell script is being comfortable using regular expressions. This is not always an easy thing to do, as trying to filter specific data from a large batch of data can (and often does) get complicated. This chapter describes how to create regular expressions in both the sed editor and the gawk program that can filter out just the data you need.

What Are Regular Expressions?

The first step to understanding regular expressions is defining just exactly what they are. This section explains just what a regular expression is and describes how Linux uses regular expressions.

A definition

A regular expression is a pattern template you define that a Linux utility uses to filter text. A Linux utility (such as the sed editor or the gawk program) matches the regular expression pattern against data as that data flows into the utility. If the data matches the pattern, it's accepted for processing. If the data doesn't match the pattern, it's rejected. This is illustrated in Figure 17-1.

The regular expression pattern makes use of wildcard characters to represent one or more characters in the data stream. There are plenty of instances in Linux where you can specify a wildcard character to represent data that you don't know about. You've already seen an example of using wildcard characters ...

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