Python Regular Expressions (RegEx)

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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.

For beginners, imagine RegEx as a much more advanced "Search" or "Find" (Ctrl+F) feature. If regular search feature can only find exact same words, RegEx allows you to find specific patterns, like finding all phone numbers, email addresses, or date formats in a very long document without needing to know specific text content.

Although at first RegEx looks confusing because it is full of unique symbols, mastering this technique will greatly help you in text processing and performing user input validation in your Python application much more efficiently.

Python has a built-in module named re, which can be used to work with Regular Expressions.

Using re Module

To use RegEx in Python, you must import the re module:

import re

Functions in re Module

The re module offers a set of functions that allows us to search a string for a match:

search() Function

The search() function searches the string for a match, and returns a Match object if there is a match. If more than one match is found, only the first occurrence of the match is 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")

findall() Function

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

import re

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

The list contains the matches in the order they are found. If no matches are found, an empty list is returned.

split() Function

The 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)

You can control the number of occurrences by specifying the maxsplit parameter:

import re

txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)

sub() Function

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

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x)

You can control the number of replacements by specifying the count parameter:

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)

Metacharacters

Metacharacters are characters with a special meaning:

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Example

import re

# Find string that contains lowercase letter between a and n
x = re.findall("[a-n]", txt)
print(x)

If no match is found, findall() will return an empty list.

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