Regular expressions

In plain English

A regular expression (regex for short) is a small pattern language for finding text that follows a rule. You write patterns with the re module: re.search finds the first match, re.findall returns every match and re.sub replaces matches. Square brackets create a character class that matches any one of the characters inside, and anchors like ^ (start) and $ (end) pin the match to a position. Regex is powerful but hard to read, so for simple checks plain string methods such as startswith are clearer.

Step by step

1. 1Import re, then describe the pattern inside a raw string like r"[0-9]+".
2. 2Use re.search to find one match and .group() to get it.
3. 3Use re.findall to collect every match into a list.
4. 4Use re.sub to swap matches for replacement text.

import re

text = "Order 123 shipped, order 456 pending"

match = re.search(r"[0-9]+", text)
print(match.group())

numbers = re.findall(r"[0-9]+", text)
print(numbers)

masked = re.sub(r"[0-9]+", "###", text)
print(masked)

print(bool(re.search(r"^Order", text)))
print(bool(re.search(r"pending$", text)))

Common mistakes

• Forgetting the r prefix on a pattern, so backslashes like \d are misread.
• Expecting re.search to return a string; it returns a match object or None.
• Using a greedy .* that matches far more than you intended.
• Reaching for regex when a simple string method would be clearer.