A Quick Note on Python RegEx

A Regular Expression, or RegEx, is a sequence of characters that forms a search pattern. RegEx can be used to check if a string contains the specified search pattern. Python has a built-in package called re, which can be used to work with Regular Expressions.

The re.search() Function

The re.search() function searches the string for a match, and returns a match object if there is a match. If there is more than one match, only the first occurrence of the a match will be returned.

import re

txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)

if x:
    print("YES! We have a match!")
else:
    print("No match")

The re.findall() Function

The re.findall() function returns a list containing all matches.

import re

txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)  # ['ai', 'ai']

The re.split() Function

The re.split() function returns a list where the string has been split at each match.

import re

txt = "The rain in Spain"
x = re.split("\s", txt)
print(x)  # ['The', 'rain', 'in', 'Spain']

The re.sub() Function

The re.sub() function replaces the matches with the text of your choice.

import re

txt = "The rain in Spain"
x = re.sub("\s", "-", txt)
print(x)  # The-rain-in-Spain

Metacharacters and Special Sequences

RegEx uses metacharacters (e.g., [], ., ^, $, *, +) and special sequences (e.g., \d, \s, \w) to define search patterns. These are what make RegEx so powerful.

• []: A set of characters
• \d: Returns a match where the string contains digits (numbers from 0-9)
• *: Zero or more occurrences

import re

txt = "The rain in Spain falls mainly in the plain."

#Find all lower case characters alphabetically between "a" and "m":
x = re.findall("[a-m]", txt)
print(x)

Conclusion

Regular Expressions are an incredibly powerful tool for text processing, and Python’s re module provides a comprehensive set of functions for working with them. While the syntax can be a bit intimidating at first, mastering RegEx will open up a whole new world of possibilities for searching, validating, and manipulating strings.