Python Regex sub()

Summary: in this tutorial, you’ll learn about the Python regex sub() function that returns a string after replacing the matched pattern in a string with a replacement.

Introduction to the Python regex sub function

The sub() is a function in the built-in re module that handles regular expressions. The sub() function has the following syntax:

re.sub(pattern, repl, string, count=0, flags=0)Code language: Python (python)

In this syntax:

• pattern is a regular expression that you want to match. Besides a regular expression, the pattern can be Pattern object.
• repl is the replacement
• string is the input string
• count parameter specifies the maximum number of matches that the sub() function should replace. If you pass zero to the count parameter or completely skip it, the sub() function will replace all the matches.
• flags is one or more regex flags that modify the standard behavior of the pattern.

The sub() function searches for the pattern in the string and replaces the matched strings with the replacement (repl).

If the sub() function couldn’t find a match, it returns the original string. Otherwise, the sub() function returns the string after replacing the matches.

Note that the sub() function replaces the leftmost non-overlapping occurrences of the pattern. And you’ll see it in detail in the following example.

Python regex sub function examples

Let’s take some examples of using the regex sub() function.

1) Using the regex sub() function to return the plain phone number

The following example uses the sub() function to turn the phone number (212)-456-7890 into 2124567890 :

import re

phone_no = '(212)-456-7890'
pattern = '\D'
result = re.sub(pattern, '',phone_no)

print(result)Code language: Python (python)

Output:

2124567890Code language: Python (python)

In this example, the \D is an inverse digit character set that matches any single character which is not a digit. Therefore, the sub() function replaces all non-digit characters with the empty string ''.

2) Using the regex sub() function to replace the leftmost non-overlapping occurrences of a pattern

The following example replaces the 00 with the '' in the string '000000':

import re

pattern = '00'
s = '00000'
result = re.sub(pattern,'',s)

print(result)Code language: Python (python)

Output:

0Code language: Python (python)

In this example, we replace two zeros with empty strings. So the first two are matched and replaced, then the following two zeroes are matches and replaced too, and finally, the last digit remains unchanged.

3) Using the regex sub() with a backreference example

The following example uses the sub() function to replace the text surrounded with (*) (it’s markdown format by the way) with the <b> tag in HTML:

import re

s = 'Make the World a *Better Place*'
pattern = r'\*(.*?)\*'
replacement = r'<b>\1<\\b>'
html = re.sub(pattern, replacement, s)

print(html)Code language: Python (python)

Output:

import re

s = 'Make the World a *Better Place*'
pattern = r'\*(.*?)\*'
replacement = r'<b>\1<\\b>'
html = re.sub(pattern, replacement, s)

print(html)Code language: Python (python)

Output:

Make the World a <b>Better Place<\b>Code language: Python (python)

In this example, the pattern r'\*(.*?)\*' find the text that begins and ends with the asterisk (*). It has a capturing group that captures the text between asterisks (*).

The replacement is a regular expression with a backreference. The backreference \1 refers to the first group in the pattern, which is the text between the asterisks (*).

4) Using the regex sub() function with the replacement as a function

Suppose you have a list of strings where each element contain both alphabet and number:

l = ['A1','A2','A3']Code language: Python (python)

And you want to square the number in each list element. For example, A1 becomes A1, A2 becomes A4, and A3 becomes A9. To do this, you can use the sub() function.

The second argument of the sub() function (repl) can be a function. In this case, the sub() function will call this function for every non-overlapping occurrence of the pattern.

This function (repl) takes a single Match object argument and returns the replacement string.

The following illustrates how to use the second argument as a function:

import re

def square(match):
    num = int(match.group())
    return str(num*num)

l = ['A1','A2','A3']
pattern = r'\d+'
new_l = [re.sub(pattern, square, s) for s in l]

print(new_l)Code language: Python (python)

Output:

['A1', 'A4', 'A9']Code language: Python (python)

How it works.

First, define a list of strings:

l = ['A1','A2','A3']Code language: Python (python)

Second, define a pattern \d+ that match one or more digits:

pattern = r'\d+'Code language: Python (python)

Third, replace the digits with their squares by calling the sub() function and passing the square() function:

new_l = [re.sub(pattern, square, s) for s in l]Code language: Python (python)

Finally, define the square() function that squares the matched digit and returns it:

def square(match):
    num = int(match.group())
    return str(num*num)Code language: Python (python)

Summary

• Use the Python regex sub() function to replace the occurrences of matches of a pattern with a replacement.