Python Academy · Lesson

Regex Fundamentals

Learn metacharacters, character classes, quantifiers, and anchors.

Lesson 1 of 414 steps

Regex Fundamentals is a free Python Academy lesson on CoddyKit. This is lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Python Academy learning path, and your progress syncs across the web and the CoddyKit app. The Python Academy course includes 4 lessons in total.

Introduction

Regular expressions are patterns for matching text. Python's re module provides the tools to use them.

What is a Regex?

A regex is a sequence of characters forming a search pattern. r'\d+' matches one or more digits.

import re
print(re.findall(r'\d+', 'abc 123 def 456'))

Literal Characters

Most characters match themselves: r'cat' matches the string 'cat' literally.

import re
print(re.search(r'cat', 'the cat sat'))

Dot (.) Wildcard

The dot . matches any character except newline. r'c.t' matches 'cat', 'cut', 'c9t'.

import re
print(re.findall(r'c.t', 'cat cut c9t caaat'))

Character Classes

[aeiou] matches any vowel. [a-z] matches any lowercase letter. [^abc] matches anything NOT in {a,b,c}.

import re
print(re.findall(r'[aeiou]', 'hello world'))

Quantifiers

* zero or more. + one or more. ? zero or one. {3} exactly 3. {2,4} between 2 and 4.

import re
print(re.findall(r'\d{2,4}', '1 12 123 1234 12345'))

Anchors

^ matches start of string. $ matches end. \b matches a word boundary.

import re
print(re.findall(r'^\w+', 'hello world'))
print(re.findall(r'\b\w{4}\b', 'cat cats come home'))

Shorthand Classes

\d = digit [0-9]. \w = word char [a-zA-Z0-9_]. \s = whitespace. Uppercase versions are negations: \D, \W, \S.

import re
print(re.findall(r'\w+', 'hello, world!'))

Alternation |

r'cat|dog' matches either 'cat' or 'dog'. Parentheses group: r'(cat|dog)s' matches 'cats' or 'dogs'.

import re
print(re.findall(r'cat|dog', 'I have a cat and a dog'))

Greedy vs Lazy

.* is greedy — matches as much as possible. .*? is lazy — matches as little as possible.

import re
html = '<b>bold</b> and <i>italic</i>'
print(re.findall(r'<.*>', html))   # greedy
print(re.findall(r'<.*?>', html))  # lazy

Raw Strings for Regex

Always use r'...' for regex patterns to avoid double-backslash escaping: r'\d' instead of '\\d'.

import re
# Without r: '\\d+' is needed
# With r:    '\d+' is sufficient
print(re.findall(r'\d+', 'abc123'))

Quick Check

What does the \b anchor match in a regular expression?

Answer it in the interactive lesson

Recap

Regex basics: . (any char), [] (class), * + ? {} (quantifiers), ^ $ \b (anchors), \d \w \s (shorthand), | (alternation), greedy vs lazy. Always use r'' strings.

Keep Going

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Frequently Asked Questions

Is the “Regex Fundamentals” lesson free?

Yes — the full text of “Regex Fundamentals” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Python Academy course, upgrade to CoddyKit PRO. The Python Academy course includes 4 lessons in total.

What will I learn in “Regex Fundamentals”?

Learn metacharacters, character classes, quantifiers, and anchors. You practise Python Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Python Academy?

No prior experience is required. Python Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Regex Fundamentals” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Python Academy lesson?

Yes. Every Python Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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