Regular expressions (regex) are powerful tools for searching, extracting, and replacing text patterns. Python’s built-in re module provides operations similar to those found in Perl, making it easy to harness the power of regular expressions.
Key Fact: The
remodule provides regular expression matching operations similar to those found in Perl.
Regular expressions often use the backslash character (\) to indicate special forms or to allow special characters to be used without invoking their special meaning. However, standard Python string literals consume backslashes (e.g. \n), so you must use raw strings (r"...") to pass backslashes directly to the regex engine.
import re
text = 'This is a backslash: \\'
# r'\\' means a raw string with two backslashes. The regex engine sees \\ and matches a literal \
match = re.search(r'\\', text)
print('Found backslash!' if match else 'Not found')Found backslash!
A common pitfall is misunderstanding the difference between re.match() and re.search().
re.match() checks for a match only at the beginning of the string.re.search() checks for a match anywhere in the string.import re
text = 'Hello World'
# match looks at the start
match1 = re.match(r'Hello', text)
print('Match Hello:', match1.group() if match1 else 'No match')
# search finds it anywhere
match2 = re.search(r'World', text)
print('Search World:', match2.group() if match2 else 'No match')The re.findall() function returns all non-overlapping matches of a pattern in a string. If the pattern has no capture groups, it returns a list of strings.
import re
matches = re.findall(r'\d+', 'There are 3 apples and 15 oranges.')
print(matches)['3', '15']
You can use parentheses () to create capture groups, allowing you to extract specific parts of a match. If the pattern has multiple capture groups, re.findall() returns a list of tuples containing the captured strings. For a single match object, you can access groups using .group().
import re
match = re.search(r'(\w+)@(\w+\.\w+)', 'Email: [email protected]')
if match:
print(match.group(1))
print(match.group(2))user example.com
The re.sub() function returns the string obtained by replacing the leftmost non-overlapping occurrences of the pattern in the string by a replacement string.
import re
text = 'I like cats. Cats are great.'
result = re.sub(r'[Cc]ats', 'dogs', text)
print(result)I like dogs. dogs are great.
What is the difference between re.match() and re.search()?
re.match() only looks at the very beginning of the string, while re.search() looks anywhere in the string. — re.match() only looks at the very beginning of the string, while re.search() checks anywhere.
Why should you use raw strings (e.g., r'...') for regular expression patterns in Python?
Because standard string literals consume backslashes, so raw strings pass backslashes directly to the regex engine. — Standard string literals consume backslashes (e.g. \n), so you must use raw strings to pass them to the engine.
What does re.findall() return if the pattern contains multiple capture groups?
It returns a list of tuples containing the captured strings. — If the pattern has multiple capture groups, re.findall() returns a list of tuples containing the captured strings.
Use re.findall() to extract all phone numbers in the format XXX-XXX-XXXX from the given text.
import re
def get_phones(text):
return re.findall(r'\d{3}-\d{3}-\d{4}', text)
print(get_phones('Call 123-456-7890 or 987-654-3210.'))Use re.sub() to mask all vowels in the string with an asterisk (*).
import re
def mask_vowels(text):
return re.sub(r'[aeiouAEIOU]', '*', text)
print(mask_vowels('Regular Expressions are powerful.'))