Regular Expression Patterns in Python

Regular expressions are patterns used to match character combinations in strings. In Python, most characters match themselves, except for special control characters (+ ? . * ^ $ ( ) [ ] { } | \) which have special meanings. You can escape these control characters by preceding them with a backslash.

The following table lists the regular expression syntax available in Python ?

Basic Examples

Here are some practical examples of regular expressions in Python ?

import re

# Match digits
text = "I have 25 apples and 10 oranges"
numbers = re.findall(r'\d+', text)
print("Numbers found:", numbers)

# Match email pattern
email_text = "Contact us at info@example.com or support@test.org"
emails = re.findall(r'\w+@\w+\.\w+', email_text)
print("Emails found:", emails)

# Match word boundaries
sentence = "The cat sat on the mat"
cat_matches = re.findall(r'\bcat\b', sentence)
print("Word 'cat' found:", len(cat_matches), "times")

Numbers found: ['25', '10']
Emails found: ['info@example.com', 'support@test.org']
Word 'cat' found: 1 times

Character Classes and Quantifiers

Character classes and quantifiers are essential for flexible pattern matching ?

import re

text = "Phone numbers: 123-456-7890, (555) 123-4567, 555.123.4567"

# Match different phone number formats
phone_pattern = r'\d{3}[-.(]?\d{3}[-.)]\s?\d{4}'
phones = re.findall(phone_pattern, text)
print("Phone numbers:", phones)

# Match vowels using character class
words = "Hello World Python Programming"
vowels = re.findall(r'[aeiouAEIOU]', words)
print("Vowels found:", vowels)

# Match non-vowels
consonants = re.findall(r'[^aeiouAEIOU\s]', words)
print("Consonants found:", consonants)

Phone numbers: ['123-456-7890', '555) 123-4567', '555.123.4567']
Vowels found: ['e', 'o', 'o', 'o', 'y', 'o', 'o', 'a', 'i']
Consonants found: ['H', 'l', 'l', 'W', 'r', 'l', 'd', 'P', 't', 'h', 'n', 'P', 'r', 'g', 'r', 'm', 'm', 'n', 'g']

Grouping and Alternatives

Use parentheses for grouping and the pipe symbol for alternatives ?

import re

text = "I like cats and dogs, but not rats or bats"

# Match cats or dogs using alternation
pets = re.findall(r'cats|dogs', text)
print("Pets found:", pets)

# Match words ending in 'ats' using grouping
ats_words = re.findall(r'(c|r|b)ats', text)
print("Words ending in 'ats':", ats_words)

# Extract parts using groups
dates = "Today is 2024-03-15 and tomorrow is 2024-03-16"
date_pattern = r'(\d{4})-(\d{2})-(\d{2})'
matches = re.findall(date_pattern, dates)
print("Date parts:", matches)

Pets found: ['cats', 'dogs']
Words ending in 'ats': ['c', 'r', 'b']
Date parts: [('2024', '03', '15'), ('2024', '03', '16')]

Conclusion

Regular expressions provide powerful pattern matching capabilities in Python. Master the basic metacharacters, character classes, and quantifiers to create flexible search patterns for text processing and data validation.