Python Regular Expressions

What Are Regular Expressions?

Regular expressions, often called regex, describe text patterns. Python provides the re module for searching, matching, extracting, and replacing text.

import re

match = re.search(r"\d+", "Order 452")
print(match.group())

This finds the first group of digits.

Raw Strings

Regex patterns often contain backslashes. Use raw strings with r"" so Python does not treat backslashes as escape sequences first.

pattern = r"\w+@\w+\.\w+"

This makes regex patterns easier to read.

Common Pattern Symbols

• . matches almost any single character.
• \d matches a digit.
• \w matches a word character.
• \s matches whitespace.
• * means zero or more.
• + means one or more.
• ? means zero or one.
• ^ anchors the start.
• $ anchors the end.

Groups

Parentheses capture parts of a match.

match = re.search(r"(\d{4})-(\d{2})-(\d{2})", "2026-05-19")

print(match.group(1))
print(match.group(2))

Groups help extract structured pieces from text.

search, match, And fullmatch

re.search() looks anywhere in the string. re.match() checks from the beginning. re.fullmatch() requires the entire string to match.

re.fullmatch(r"\d{6}", "560001")

Use fullmatch() for validation.

Replacing Text

Use re.sub() to replace matching patterns.

cleaned = re.sub(r"\s+", " ", "Too     many   spaces")

This replaces repeated whitespace with a single space.

Common Mistakes

A common mistake is using regex for problems that simple string methods can solve. Use split(), replace(), startswith(), or in when they are clearer.

Another mistake is forgetting that re.match() starts at the beginning. If you want to find a pattern anywhere, use re.search().

Quick Summary

• Regex patterns describe text.
• Python uses the re module for regex work.
• Raw strings are recommended for regex patterns.
• search() finds a pattern anywhere.
• match() checks from the beginning.
• fullmatch() checks the whole string.
• Groups capture parts of a match.
• re.sub() replaces matching patterns.

Practice Quiz

Now practice this topic with MCQs and explanations:

Start the Python Regular Expressions quiz