Regex replace() Method in Python

In this tutorial, we will look into the use and function of the re.sub() method and look into the example codes. The Python’s re module provides the various functionalities using regular expression for both Unicode and 8-bit strings. The functionalities include string replacement, splitting, and matching, etc.

Regex Replace Using the re.sub() Method in Python

The re.sub(pattern, repl, string, count=0) method takes the string as input and replaces the leftmost occurrences of the pattern with the repl. If no pattern is found in the string argument, the string is returned without any changes.

The pattern argument must be in the form of a regular expression. The repl can be a string or a function. If the repl argument is a string, then the pattern in the string is replaced by the repl string. If a function is passed as the repl argument, then the function will be called whenever an occurrence of the pattern is found. The function takes the matchObject as input and returns the replacement string. The matchObject will have its value equal to True if the match is found and will have its value equal to None otherwise.

The optional count argument represents the maximum occurrences of the pattern we want to replace in the string.

The below example code demonstrates how to use the re.sub() method to replace some pattern from the string using the regular expression:

import re

string = "Hello! How are you?! Where have you been?!"
new_string = re.sub(r"""[!?'".<>(){}@%&*/[/]""", " ", string)
print(new_string)

Output:

Hello  How are you   Where have you been  

The above code example removes the characters specified in the pattern argument with the blank space, which is our repl argument in the above code.

We can also use the function as the repl argument to perform the same task by returning " " whenever an occurrence of the repl argument is found, as shown in the below example code:

import re


def repl_func(match):
    if match == True:
        return " "


string = "Hello! How are you?! Where have you been?!"
new_string = re.sub(r"""[!?'".<>(){}@%&*/[/]""", repl_func, string)
print(new_string)

Output:

Hello How are you Where have you been