Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

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

When you have imported the re module, you can start using regular expressions:

Example

Search the string to see if it starts with "The" and ends with "Spain":

import re

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

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

Metacharacters are characters with a special meaning:

Character	Description	Example
[]	A set of characters	"[a-m]"
\	Signals a special sequence	"\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 occurrences	"he.?o"
	Exactly specified number of occurrences	"he{2}o"
\|	Either or	"falls\|stays"

A special sequence is a \ followed by one of the characters below:

\A	Returns match if characters are at start of string
\b	Returns match where specified characters are at start/end of a word
\d	Returns match where string contains digits (0-9)
\s	Returns match where string contains white space character
\w	Returns match containing word characters (a-Z, 0-9, _)

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

import re

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

A Match Object is an object containing information about the search and the result.

Note: If there is no match, the value None will be returned.

import re

txt = "The rain in Spain"
x = re.search("ai", txt)
print(x) #this will print an object

The Match object has properties and methods used to retrieve information: