Chapter 3. Regular Expressions

Chapter 3. Regular Expressions

Chapter 3. Regular Expressions

Table of Contents

Introduction

What are regular expressions?

The fullstop
Let's explore "sed" syntax
Square brackets ( [ ] ), the caret ( ^ ) and the dollar ( $ )
Using sed and pipes
The splat (asterisk) ( * )
The plus operator ( + )
Returning from detour to our discussion on curly braces …
RE's are greedy for matching patterns
Placeholders and word boundaries
Word boundaries ( < and > ) - a formal explanation

The tr command

The cut command

First Example in stages:
Second Example in stages:
Third example in stages
Exercises:

The paste command

The uniq command

The Sort command

The grep command

grep, egrep and fgrep
Exercises:
Challenge sequence:

Introduction

One of the most important (and powerful) parts of UNIX and Linux, and one that is often overlooked, is the concept of regular expressions.

Regular expressions could change your life (in the Linux world!!!) and if you don't know how to use them you will find yourself handicapped.

This chapter will not only teach you about regular expressions it will also teach how and when to use them.

First log in using your username.
Then you need files containing data. Go to the site and retrieve the file bazaar.txt. This is an extract from Eric S Raymond's book, "The Cathedral and the Bazaar" (the team highly recommends this book as per the preface to this course). All files you download from the site should be placed in your user area. Use the 'less' command to browse the file to familiarise yourself with it's contents. Repeat the process for the files:
emails1.txt emails2.txt emails3.txt columns.txt telnum.txt
Thirdly, we need a program within Linux that uses regular expressions. For this we're going to use a program called sed. sed is one of those many "Swiss army knives" that you're going to find in Linux.

	Note
	Some of the other tools that you will use regularly are: find, awk, perl and grep.


The job of the shell		What are regular expressions?