Regex Session 01: Absolute Basics
Starting from zero. This session covers literal matching, the concept of metacharacters, and your first patterns using regexr.com.
Pre-Session State
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Opened regexr.com
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Can write simple grep commands
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Understand "pattern matching" concept vaguely
Setup
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Open regexr.com in browser
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Clear the default pattern (select all, delete)
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Paste this test text in the "Text" area:
The quick brown fox jumps over the lazy dog. IP: 192.168.1.100 MAC: AA:BB:CC:DD:EE:FF User: evanusmodestus logged in at 10:30:45 Error: Connection refused to 10.50.1.20 VLAN 100 is active on port Gi1/0/24 Log: 2026-03-15T14:30:00 INFO Server started
Lesson 1: Literal Characters
Concept: Most characters match themselves literally.
Exercise 1.1: Find a word
Type in the pattern field:
fox
What happens: The word "fox" is highlighted.
Key insight: Regex is case-sensitive by default. Fox won’t match fox.
192
What happens: "192" in the IP address is highlighted.
error
What happens: Nothing matches! The text has "Error" with capital E.
Fix: Toggle the "case insensitive" flag (i) in regexr, or use Error.
Lesson 2: The Dot Metacharacter
Concept: The dot . matches ANY single character (except newline).
f.x
What happens: "fox" matches - the . matched o.
Type:
1..168
**What happens:** "192.168" matches - dots matched `9` and `2`. **Problem:** We matched digits, but `.` is too greedy. What if we want literal dots? == Lesson 3: Escaping Metacharacters **Concept:** Backslash `\` makes a metacharacter literal. === Exercise 3.1: Match literal dot Type:
192\.168
**What happens:** "192.168" matches - the `\.` matched literal `.` === Exercise 3.2: Full IP octet pattern Type:
192\.168\.1\.100
**What happens:** The full IP matches exactly. == Lesson 4: Character Classes **Concept:** Square brackets `[]` define a set of characters to match. === Exercise 4.1: Match digits Type:
**What happens:** ALL digits in the text are highlighted individually. === Exercise 4.2: Match hex characters Type:
**What happens:** All uppercase A-F letters match (from MAC address). === Exercise 4.3: Combine ranges Type:
**What happens:** All hex characters (uppercase, lowercase, digits) match. == Lesson 5: Quantifiers (Basic) **Concept:** Quantifiers specify how many times a pattern should match. === Exercise 5.1: Plus (+) - one or more Type:
[0-9]+
**What happens:** Entire numbers match as units (192, 168, 1, 100, etc.) === Exercise 5.2: Asterisk (*) - zero or more Type:
[A-Z]*
**What happens:** Uppercase sequences match, but also empty strings (many zero-length matches). === Exercise 5.3: Question mark (?) - zero or one Type:
logs?
What happens: Matches "log" or "logs" (the 's' is optional).
Summary: What You Learned
| Concept | Syntax | Example |
|---|---|---|
Literal match |
Just type it |
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Any character |
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Escape metachar |
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Character class |
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Range |
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One or more |
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Zero or more |
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Zero or one |
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Exercises to Complete
Before closing regexr, try these:
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[ ] Match all MAC address octets:
[A-F0-9]+ -
[ ] Match the username after "User: ": (hint: literal match)
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[ ] Match "VLAN" followed by a space and number:
VLAN [0-9]+ -
[ ] Match time format HH:MM:SS:
:[0-9]:[0-9]+
Post-Session Reflection
What clicked:
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<Write what made sense>
What’s still fuzzy:
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<Write what needs more practice>
Connection to work:
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<How will you use this?>
CLI Application
Try these in your terminal:
# Find lines with IP addresses (basic)
echo "Server 192.168.1.1 is up" | grep -E '[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+'
# Find lines with MAC addresses
echo "MAC: AA:BB:CC:DD:EE:FF" | grep -E '[A-Fa-f0-9]+:[A-Fa-f0-9]+'Next Session
Session 02: Character Classes & Quantifiers - Deep dive into \d, \w, \s, {n,m} quantifiers.
Session Log
| Timestamp | Notes |
|---|---|
Start |
<Record when you started> |
End |
<Record when you finished> |
Duration |
<Actual time spent> |