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

RegEx in Python :

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

Example :

import re txt = "The rain in Amazon" x = re.search("^The.*Amazon$",txt) if(x): print("We found a match!") else : print("No match found") input()

Output :

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
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 :

Metacharacters are characters with a special meaning :

Character	Description	Example
[ ]	A set of characters	"[a-m]"
\	Signals a special sequence (can also be used to escape special characters)	"\d"
.	Any character (except newline character)	"he..o"
^	Starts with	"^hello"
$	Ends with	"world$"
*	Zero or more occurrences	"aix*"
+	One or more occurrences	"aix+"
{ }	Excactly the specified number of occurrences	"al{2}"
|	Either or	"al{2}"
( )	Capture and group	"falls|stays"

Special Sequences :

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

Character	Description	Example
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"
\b	Returns a match where the specified characters are at the beginning or at the end of a word	r"\bmazon" r"zon\b"
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word	r"\Bmazon" r"zon\B"
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"
\D	Returns a match where the string DOES NOT contain digits	"\D"
\s	Returns a match where the string contains a white space character	"\s"
\S	Returns a match where the string DOES NOT contain a white space character	"\S"
\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"
\W	Returns a match where the string DOES NOT contain any word characters	"\W"
\Z	Returns a match if the specified characters are at the end of the string	"Amazon\Z"

Set :

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
[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
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[+]	In sets, +, *, ., |, ( ), $,{ } has no special meaning, so [+] means: return a match for any + character in the string

The findall() Function :

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

Example :

#Print a list of all matches import re txt = "The rain in Amazon" x = re.findall("in",txt) print(x) if(x): print("We found a match!") else: print("No match found") input()

Output :

The search() Function :

The search() function searches the string for a match, and returns a Match object if there is a match.

Example :

#Search for the first white-space character in the string import re txt = "The rain in Amazon" x = re.search("\s", txt) print("The first white-space character is located in position:", x.start()) input()

Output :

Note : If no matches are found, the value None is returned

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

Output :

The sub() Function :

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

#Replace every white-space character with the # symbol import re txt = "The rain in Amazon" x = re.sub("\s", "#", txt) print(x) input()

Output :