Regular expressions are powerful tools for pattern matching and text manipulation. In the Linux world, two widely used commands for leveraging regular expressions are grep and egrep. This tutorial aims to provide a comprehensive guide on how to effectively use regular expressions with both grep and egrep commands in Linux.

What are Regular Expressions?

Regular expressions, also known as regex or pattern matching, is a sequence of characters that forms a search pattern. It allows you to match, search, and manipulate strings based on specific patterns. These patterns usually include various symbols from the keyboard.

Installing Grep and eGrep

Before we begin, let’s make sure we have the grep and egrep commands installed on our Ubuntu distribution. Most of the times, grep will come installed with the OS.

To install grep, open your terminal and run the following command:

sudo apt-get install grep

To install egrep, open your terminal and run the following command:

sudo apt-get install grep

Basic Usage of Grep

The grep command is used to search for patterns within files. Here’s an example of how to use grep:

grep "pattern" file.txt

In its simplest form, you can use regular expressions to match a specific pattern within a file. We have a file names.txt which contains below lines.

Andrew
Joseph
Jenny
Zoe
Jennifer
Alex
Alexandra

For example, let’s say we want to search for the word “Alex” in a file named names.txt. We can use the following command:

grep "Alex" names.txt
Alex Alexandra

Matching on Patterns

Character classes allow you to match a set of characters or ranges of characters. For example, if you want to match any names that contains characters ‘a’ or ‘i’ in a given string, you can use the following command:

egrep "[ai]" names.txt
Jennifer
Alexandra

This command returns output which is case-sensitive. That means, it does not match with words like Alex because it has A uppercase. To ignore case, we can use option -i.

grep -i [ai] names.txt
Andrew
Jennifer
Alex
Alexandra

Quantifiers specify the number of occurrences of a pattern that you want to match. For example, if you want to find all words that contain two consecutive vowels, you can use the following command.

egrep "[na]{2}" names.txt

This command returns all names that have either ‘a’ or ’n’ occuring twice in their names.

Alternation allows you to specify multiple patterns and matches if any of them are found. Grouping helps with capturing parts of the matched pattern for further manipulation. For example, we have adjectives.txt file with below content.

cat adjectives.txt 
beautiful texas
vibrant mumbai
multicultural toronto
scenic San Dieg

egrep "(vibrant|beautiful) mumbai" adjectives.txt
vibrant mumbai

Anchors and boundaries are used to specify the position or surroundings where a match should occur.

  • ^ indicates the start of a line.
  • $ indicates the end of a line.
  • \b indicates a word boundary.

For example, get names starting with ‘Je’ in names.txt file.

egrep "^Je" names.txt
Jenny
Jennifer

Get names ending with character e in names.txt file.

egrep "e$" names.txt
Zoe

Metacharacters have special meanings in regular expressions and are used to represent certain patterns or characters.

  • . matches any character except a newline.
  • * matches zero or more occurrences of the preceding pattern.
grep "J.n" names.txt
Jenny
Jennifer

grep "Ale*" names.txt
Alex
Alexandra

Character Classes

Some characters are widely availble as common references. These can be checked using their character class name.

  • [[:alnum:]]: Matches alphanumeric characters [a-zA-Z0-9]
  • [[:alpha:]]: Alphabetic characters [A-Za-z]
  • [[:digit:]]: Digits: 0 1 2 3 4 5 6 7 8 9
  • [[:blank:]]: Blank characters: space and tab
  • [[:lower:]]: Lower-case letters [a-z]
  • [[:upper:]]: Upper-case letters [A-Z]
  • [[:punct:]]: Punctuation characters
  • [[:xdigit:]]: Hexadecimal digits [0-9A-Fa-f]

These can be used as below.

egrep "[[:alnum:]]" names.txt
egrep "[[:punct:]]" names.txt

These are just some of the basics of using regular expressions with grep and egrep commands in Linux. As you continue your journey, you’ll discover many more powerful features and techniques.