Introduction to Regular Expressions in Shell Scripts

Regular expressions (regex) are powerful tools for pattern matching and text manipulation in shell scripts. They allow you to search, match, and replace strings efficiently. In this article, we will explore three practical examples of using regular expressions in shell scripts that will help you automate tasks and improve data processing.

Example 1: Extracting Email Addresses from a File

Context

In many scenarios, you may need to extract specific information, such as email addresses, from a large text file. This example demonstrates how to use regular expressions to achieve that efficiently.

#!/bin/bash

# Define the input file
input_file="contacts.txt"

# Use grep with a regex pattern to extract email addresses
grep -oP '[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,6}' "$input_file"

This script uses grep with the -oP options to extract and display only the email addresses found in contacts.txt. The regex pattern matches valid email formats.

Notes

• Make sure to install grep with Perl-compatible regex support if your default version does not support the -P option.
• You can redirect the output to another file using > to save the extracted emails.

Example 2: Validating Input Format

Context

You may need to validate user input, such as checking if a string matches a specific pattern, like a phone number format. Here’s how to do this in a shell script.

#!/bin/bash

# Read user input
read -p "Enter your phone number (123-456-7890): " phone

# Validate the phone number format using regex
if [[ $phone =~ ^[0-9]{3}-[0-9]{3}-[0-9]{4}$ ]]; then
    echo "Valid phone number"
else
    echo "Invalid phone number format"
fi

This script prompts the user to enter a phone number and uses regex in a conditional statement to validate the format. If the input matches the specified pattern, it confirms validity; otherwise, it notifies the user.

Notes

• The regex pattern checks for numbers in the format XXX-XXX-XXXX, where X is a digit.
• You can adjust the regex pattern to accommodate different phone number formats as needed.

Example 3: Replacing Text in a File

Context

In cases where you need to modify text within files, regular expressions can be incredibly useful. This example demonstrates how to replace all instances of a specific word or phrase.

#!/bin/bash

# Define the input file and word to replace
input_file="document.txt"
word_to_replace="old"
new_word="new"

# Use sed to replace the specified word with a new one
sed -i "s/\b$word_to_replace\b/$new_word/g" "$input_file"

This script uses sed to search for the word old in document.txt and replaces it with new. The -i option edits the file in place, while \b ensures that the script matches whole words only.

Notes

• The g flag at the end of the sed command signifies that all occurrences in the line will be replaced.
• Always make a backup of the original file before running scripts that modify files, especially with -i option.

By leveraging these examples of using regular expressions in shell scripts, you can enhance your scripting capabilities and streamline various text processing tasks.