What Are Regular Expressions?
The first step to understanding regular expressions is to define just exactly what they are. This section explains 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 20.1.
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 you don't know about. You've already seen an example...