Python Regex

RegEx Module

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

Import the re module:

import re

regex functions

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

Function	Description
findall	Returns a list containing all matches
split	Returns a list where the string has been split at each match
search	Returns a Match object if there is a match anywhere in the string
sub	Replaces one or many matches with a string

The findall() Function

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

Example

Print a list of all matches:

import re

txt = "We are the 'Vikings' from the north."
x = re.findall("the", txt)
print(x)

The search() Function

The 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 match will be returned:

Example

Search for the position in the string:

import re

txt = "We are the 'Vikings' from the north."
x = re.search("\'V", txt)
print("The position of the search string is:", x.start())

The split() Function

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

Example

Split at each white-space character:

import re

txt = "We are the 'Vikings' from the north."
x = re.split("\s", txt)
print(x)

The sub() Function

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

Example

Replace every white-space character with “_”:

import re

txt = "We are the 'Vikings' from the north."
x = re.sub("\s", "_", txt)
print(x)

Meta Characters

Metacharacters are characters with a special meaning:

Character	Description
[]	A set of characters
\	Signals a special sequence (can also be used to escape special characters)
.	Any character (except newline character)
^	Starts with
$	Ends with
*	Zero or more occurrences
+	One or more occurrences
{}	Exactly the specified number of occurrences
|	Either or
()	Capture and group

Special Sequences

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

Character	Description
\A	Returns a match if the specified characters are at the beginning of the string
\b	Returns a match where the specified characters are at the beginning or at the end of a word
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word
\d	Returns a match where the string contains digits (numbers from 0-9)
\D	Returns a match where the string DOES NOT contain digits
\s	Returns a match where the string contains a white space character
\S	Returns a match where the string DOES NOT contain a white space character
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)
\W	Returns a match where the string DOES NOT contain any word characters
\Z	Returns a match if the specified characters are at the end of the string

Sets

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

Set	Description
[arn]	Returns a match where one of the specified characters (a, r, or n) are present
[a-n]	Returns a match for any lower case character, alphabetically between a and n
[^arn]	Returns a match for any character EXCEPT a, r, and n
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case
[+]	In sets, +, *, .,	, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[0-9]	Returns a match for any digit between 0 and 9
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59