Python - RegEx

Overview

Estimated time: 25–35 minutes

Use the re module to search, extract, and replace text using patterns.

Learning Objectives

• Use re.search, re.findall, and re.sub.
• Write readable patterns with raw strings and verbose mode.
• Compile patterns for reuse.

Prerequisites

• Python - Strings

Basic search and groups

import re
m = re.search(r"(\d{4})-(\d{2})-(\d{2})", "Date: 2025-09-05")
if m:
    year, month, day = m.groups()
    print(year, month, day)

Find all and replace

text = "Emails: [email protected], [email protected]"
print(re.findall(r"[\w.-]+@[\w.-]+", text))
print(re.sub(r"test.org", "example.org", text))

Compiled and verbose patterns

pattern = re.compile(r"""
    ^                # start
    [A-Za-z0-9_.-]+  # local part
    @
    [A-Za-z0-9.-]+   # domain
    $                # end
""", re.VERBOSE)
print(bool(pattern.match("[email protected]")))

Common Pitfalls

• Forgetting raw strings (r"...") when backslashes are used.
• Overcomplicated regex when a simple split or parse would do.

Checks for Understanding

1. What does re.VERBOSE enable?
2. How do you capture groups in a pattern?

Exercises

1. Extract all dates in YYYY-MM-DD from a block of text.
2. Write a regex to validate simple IPv4 addresses.