Python Notes Help

Regular Expressions

Python’s re module provides tools for working with regular expressions, allowing pattern matching, searching, and text manipulation.

Searching and matching text with regex

Example using re.search() to find the first match:

import re text = 'Email: example@email.com, Phone: 123-456-789' email_match = re.search(r'\w+@\w+\.\w+', text) phone_match = re.search(r'\d{3}-\d{3}-\d{3}', text) numbers_match = re.search(r'\d', text) print(email_match.group()) print(phone_match.group()) print(numbers_match.group())
example@email.com 123-456-789 1

Example of re.findall() to find all matches:

text = 'Email: example@email.com, Phone: 123-456-789' emails = re.findall(r'\w+@\w+\.\w+', text) phones = re.findall(r'\d{3}-\d{3}-\d{3}', text) numbers = re.findall(r'\d', text) print(emails) print(phones) print(numbers)

Output:

['example@email.com'] ['123-456-789'] ['1', '2', '3', '4', '5', '6', '7', '8', '9']

Replacing text with regex

Example of re.sub() to replace all matches with a text:

text = 'Email: example@email.com, Phone: 123-456-789' replaced_text = re.sub(r'\d', '*', text) print(replaced_text)

Output:

Email: example@email.com, Phone: ***-***-***

Advanced regex patterns and examples

Regular expressions support complex patterns with metacharacters and quantifiers:

  • ^: start of string

  • $: end of string

  • .: any character

  • \d: digits

  • \s: whitespace

  • \w: alphanumeric

  • *: 0+ times)

  • +: 1+ times

  • ?: 0 or 1 time

  • {n}: exact n times

Example extracting valid URLs from text:

pattern = r'https?://[a-zA-Z0-9.-]+(?:\.[a-zA-Z]{2,})+' text = 'Visit https://example.com or http://site.org for info.' urls = re.findall(pattern, text) print(urls)

Output:

['https://example.com', 'http://site.org']
Last modified: 17 March 2025
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