Regular Expressions with re Module

What is a Regular Expression?

A regular expression (regex) is a pattern used to search, match or extract text. Python's re module powers it. Regex is great for checking emails, phone numbers, PIN codes and more.

Common Patterns

re Functions

• re.search() — find the first match anywhere.
• re.findall() — return all matches as a list.
• re.match() — match only at the start.
• re.sub() — replace matches.

Program 1: Find a Pattern

import re
text = "My roll number is 42"
match = re.search(r"\d+", text)
if match:
    print("Found number:", match.group())

r"\d+" means "one or more digits". .group() returns the matched text.

Program 2: Validate an Email

import re
pattern = r"^[\w.]+@[\w]+\.[a-z]{2,}$"
email = input("Enter email: ")
if re.match(pattern, email):
    print("Valid email")
else:
    print("Invalid email")

• [\w.]+ = name part; @ literal; [\w]+ = domain.
• \.[a-z]{2,} = a dot then at least 2 letters (com, in...).

Program 3: Extract All Phone Numbers

import re
text = "Call 9876543210 or 9123456780 today"
numbers = re.findall(r"\d{10}", text)
print("Phone numbers:", numbers)

\d{10} matches exactly 10 digits; findall returns every match as a list.

Common Mistakes

• Forgetting the r"..." raw string — backslashes get misread.
• Using match (start only) when you meant search (anywhere).
• Over-complicating patterns — start simple and test.

Practice Tasks

1. Check if a string contains any digit.
2. Validate a 6-digit PIN code.
3. Extract all words starting with a capital letter.
4. Replace all spaces in a string with hyphens using re.sub.

Summary

• Regex matches text patterns using the re module.
• Key patterns: \d digit, \w word char, + one-or-more, {n} exactly n.
• Functions: search, findall, match, sub. Always use raw strings r"...".