How to do it...

Regular expressions are composed of text fragments and symbols with special meanings. Using these, we can construct a regular expression to match any text. Regular expressions are the basis for many tools. This section describes regular expressions, but does not introduce the Linux/Unix tools that use them. Later recipes will describe the tools.

Regular expressions consist of one or more elements combined into a string. An element may be a position marker, an identifier, or a count modifier. A position marker anchors the regular expression to the beginning or end of the target string. An identifier defines one or more characters. The count modifier defines how many times an identifier may occur.

Before we look at some sample regular expressions, let's look at the rules.