Python Regular Expressions (RegEx) Tutorial

Learn how to use Python's built-in re module for working with Regular Expressions (RegEx). Discover how to define and search for patterns in strings with this comprehensive guide.

Python RegEx

A Regular Expression, or RegEx, is a sequence of characters that defines a search pattern. It is commonly used to check if a string contains a specific pattern.

RegEx Module

Python includes a built-in package called re, which you can use to work with Regular Expressions.

Importing the re Module

First, you need to import the re module:

Syntax

import re

Using RegEx in Python

Once you've imported the re module, you can start using regular expressions. Here’s an example:

Example


import re

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

RegEx Functions

The re module provides a set of functions to search a string for a match:

findall: Returns a list of all matches
search: Returns a Match object if there's a match
split: Splits the string at each match and returns a list
sub: Replaces one or many matches with a string

Metacharacters

Metacharacters are characters with special meanings in RegEx:

Character	Description	Example
[]	A set of characters	"[a-m]"
\	Signals a special sequence (or escapes special characters)	"\d"
.	Any character (except newline)	"he..o"
^	Starts with	"^hello"
$	Ends with	"planet$"
*	Zero or more occurrences	"he.*o"
+	One or more occurrences	"he.+o"
?	Zero or one occurrence	"he.?o"
{}	Exactly the specified number of occurrences	"he.{2}o"
\|	Either or	"falls\|stays"
()	Capture and group

Special Sequences

Special sequences are denoted by a backslash (\) followed by a character and have a special meaning:

Character	Description	Example
\A	Matches if specified characters are at the beginning of the string	"\AThe"
\b	Matches if specified characters are at the beginning or end of a word	r"\bain", r"ain\b"
\B	Matches if specified characters are not at the beginning or end of a word	r"\Bain", r"ain\B"
\d	Matches any digit (0-9)	"\d"
\D	Matches any non-digit	"\D"
\s	Matches any whitespace character	"\s"
\S	Matches any non-whitespace character	"\S"
\w	Matches any word character (letters, digits, and underscore)	"\w"
\W	Matches any non-word character	"\W"
\Z	Matches if specified characters are at the end of the string	"Spain\Z"

Sets

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

Set	Description
[arn]	Matches any one of the specified characters (a, r, or n)
[a-n]	Matches any character alphabetically between a and n
[^arn]	Matches any character except a, r, and n
[0123]	Matches any of the specified digits (0, 1, 2, or 3)
[0-9]	Matches any digit between 0 and 9
[0-5][0-9]	Matches any two-digit numbers from 00 to 59
[a-zA-Z]	Matches any character between a and z (case insensitive)
[+]	Matches any + character (in sets, +, *, ., \|, (), $, and {} have no special meaning)

The findall() Function

The findall() function returns a list of all matches:

Example


import re

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

Output

['ai', 'ai']

The search() Function

The search() function searches the string for a match and returns a Match object if there's a match. Only the first occurrence is returned:

Example


import re

txt = "The rain in Spain"
match = re.search("\s", txt)

print("The first white-space character is located in position:", match.start())

Output

The first white-space character is located in position: 3

The split() Function

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

Example


import re

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

Output

['The', 'rain', 'in', 'Spain']

The sub() Function

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

Example


import re

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

Output

The9rain9in9Spain

Match Object

A Match Object contains information about the search and the result. If there is no match, None is returned.

Example


import re

txt = "The rain in Spain"
match = re.search("ai", txt)
print(match)

Output

The Match object has methods and properties to extract details about the search result:

.span(): Returns a tuple with the start and end positions of the match
.string: Returns the string passed into the function
.group(): Returns the part of the string where there was a match

Example


import re

txt = "The rain in Spain"
match = re.search(r"\bS\w+", txt)
print(match.span())

Output

(12, 17)

Example


import re

txt = "The rain in Spain"
match = re.search(r"\bS\w+", txt)
print(match.string)

Output

The rain in Spain

Example


import re

txt = "The rain in Spain"
match = re.search(r"\bS\w+", txt)
print(match.group())

Output

Spain