Python Regular Expressions: A Gentle Introduction

Basic Matching

import re
m = re.search(r"\d+", "abc 42 def")
print(m.group())   # 42

Finding All

re.findall(r"\d+", "a1 b22 c333")
# ['1', '22', '333']

Replacing

re.sub(r"\s+", " ", "hello   world")
# 'hello world'

Useful Character Classes

• \d digit, \D non-digit
• \w word char, \W non-word
• \s whitespace, \S non-whitespace
• . any char (except newline)

Quantifiers

• * zero or more, + one or more
• ? zero or one, {n,m} n to m times

Groups

m = re.match(r"(\d{4})-(\d{2})-(\d{2})", "2026-04-22")
year, month, day = m.groups()

Regex rarely appears on HKDSE Paper 1, but it is invaluable for real-world text processing.