🔤 Linux/Unix: Regex Concepts & Practice – Mastering Pattern Matching in Shell

🧲 Introduction – Why Learn Regular Expressions (Regex) in Linux?

Regular expressions (regex) are the heart of text searching, filtering, and data validation in Linux/Unix. Whether you’re using grep, sed, awk, or scripting in Bash, regex helps you create powerful, flexible patterns for matching strings and structures in files, logs, and outputs.

🎯 In this guide, you’ll learn:

• Key regex symbols and their meanings
• Practical examples with tools like grep, sed, and awk
• Real-world usage for filtering logs, validating input, and processing data

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🔢 Basic Regex Syntax & Meaning

Symbol	Meaning	Example	Matches
.	Any single character	a.c	abc, a1c, a-c
*	Zero or more of the preceding char	lo*	l, lo, loo, looo
+	One or more of the preceding char	go+	go, goo, gooo
?	Zero or one occurrence	colou?r	color, colour
[]	Match any one character in brackets	[aeiou]	a, e, i, o, u
[^]	Match any character NOT in brackets	[^0-9]	Any non-digit character
^	Start of a line	^root	Lines starting with root
$	End of a line	.com$	Lines ending in .com
`	`	OR pattern	`cat
()	Group patterns	(ab)+	ab, abab, ababab
{}	Quantifier for repetitions	a{2,4}	aa, aaa, aaaa

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🔍 Using Regex with grep, egrep, awk, and sed

🔸 1. grep – Basic Pattern Search

✅ Example: Match lines that start with “root”

grep "^root" /etc/passwd

📤 Output:

root:x:0:0:root:/root:/bin/bash

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🔸 2. egrep – Extended Regex Support

✅ Example: Match either “cat” or “dog”

egrep "cat|dog" animals.txt

📤 Output:

I love my dog.
The cat is sleeping.

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🔸 3. awk – Pattern Matching by Fields

✅ Example: Match rows where the second column is a number

awk '$2 ~ /^[0-9]+$/' data.txt

📤 Output:

user1 42
admin 103

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🔸 4. sed – Stream Editing with Regex

✅ Example: Replace lines ending with .html to .php

sed 's/\.html$/\.php/' urls.txt

📤 Output:

index.php
about.php

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🧪 Practice Regex – Real-World Scenarios

🔎 Find all lines with a 3-digit number

grep -E "[0-9]{3}" data.txt

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🔎 Extract valid email addresses

grep -E "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-z]{2,}" file.txt

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🔎 Replace multiple spaces with one space

sed 's/  */ /g' messy.txt

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🔎 Validate phone number format (XXX-XXX-XXXX)

grep -E "^[0-9]{3}-[0-9]{3}-[0-9]{4}$" phones.txt

📤 Output:

123-456-7890
555-000-1111

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📌 Summary – Recap & Next Steps

Regex enables dynamic pattern matching and text transformation in Linux. From validating formats to filtering logs, it’s a crucial skill for any sysadmin or developer.

🔍 Key Takeaways:

• Learn key regex symbols: ^, $, *, +, ?, {}, [], (), |
• Use grep for search, sed for substitution, and awk for field-aware pattern matching
• Practice regex to handle complex data parsing tasks

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❓ FAQs

❓ What’s the difference between grep and egrep?
✅ egrep supports extended regex like +, ?, | without escaping. grep requires escaping them.

❓ How do I match an exact word using regex?
✅ Use:

grep -w "word" file.txt

❓ Can regex be used inside shell scripts?
✅ Absolutely! [[ "$var" =~ pattern ]] is valid in Bash.

❓ How do I test regex interactively?
✅ Use tools like grep, sed, or online playgrounds like regex101.com.

❓ Can I replace only matching groups in sed?
✅ Yes, with \1, \2 etc. Example:

sed -E 's/(.*)\.html/\1.php/' file.txt

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