regular-expressions.md

Python by Example: Regular Expressions

Regular expressions (regex) match text patterns. Use them for validation, search-and-replace, and parsing. The re module provides search, findall, sub, and more. Use raw strings r"..." so backslashes aren't interpreted as escape sequences. Regex has a learning curve—start simple.

What you'll learn:

• re.search() to find one match
• re.findall() for all matches
• re.sub() to replace
• re.compile() for reusable patterns

import re

text = "Contact: alice@example.com or bob@test.org"

# Search for a pattern
match = re.search(r"\w+@\w+\.\w+", text)
if match:
    print(match.group())

# Find all matches
emails = re.findall(r"\w+@[\w.]+", text)
print(emails)

# Replace
masked = re.sub(r"\d+", "XXX", "Phone: 555-1234")
print(masked)

# Compile for reuse
pattern = re.compile(r"\b\w{4}\b")
print(pattern.findall("The quick brown fox"))

\w matches word characters; \d matches digits; \b is a word boundary. The r prefix makes it a raw string so \w isn't misinterpreted.

To run this program:

$ python source/regular-expressions.py
alice@example.com
['alice@example.com', 'bob@test.org']
Phone: XXX-XXXX
['quick', 'brown']

Tip: For simple string operations (e.g., "x" in s), prefer built-in methods. Use regex when you need pattern matching.

Try it: Use re.sub to replace all vowels in a string with *.

Source: regular-expressions.py

Next: JSON