Python 24-Day Course - Day 16: Regular Expressions

Day 16: Regular Expressions

Basic Usage

import re

text = "Phone: 010-1234-5678, Email: user@example.com"

# Pattern search
phone = re.search(r"\d{3}-\d{4}-\d{4}", text)
if phone:
    print(phone.group())  # 010-1234-5678

Key Pattern Syntax

Pattern	Description	Example
\d	Digit	\d+ -> “123”
\w	Letter/digit/underscore	\w+ -> “hello_1”
\s	Whitespace	\s+ -> ” “
.	Any character	a.c -> “abc”
*	0 or more	ab*c -> “ac”, “abc”
+	1 or more	ab+c -> “abc”
?	0 or 1	colou?r -> “color”
{n,m}	n to m repetitions	\d{2,4} -> “12”, “1234”
[]	Character class	[aeiou] -> vowels
^ / $	Start / End	^Hello
()	Group capture	(\d+)-(\d+)

Key Functions

text = "3 apples, 5 bananas, 12 cherries"

# findall: find all matches
numbers = re.findall(r"\d+", text)
print(numbers)  # ['3', '5', '12']

# sub: substitution
cleaned = re.sub(r"\d+", "N", text)
print(cleaned)  # N apples, N bananas, N cherries

# split: splitting
parts = re.split(r",\s*", text)
print(parts)  # ['3 apples', '5 bananas', '12 cherries']

Group Capture

pattern = r"(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})"
match = re.search(pattern, "Today is 2026-04-20")

if match:
    print(match.group("year"))   # 2026
    print(match.group("month"))  # 04
    print(match.group("day"))    # 20

Email Validation Example

def is_valid_email(email):
    pattern = r"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$"
    return bool(re.match(pattern, email))

print(is_valid_email("user@example.com"))   # True
print(is_valid_email("invalid-email"))      # False

Today’s Exercises

1. Write a regular expression that extracts all URLs from a text.
2. Validate password strength (at least 8 characters with uppercase, lowercase, digits, and special characters).
3. Create a function using regular expressions to strip HTML tags.