Python regex find and replace

One of the most common scenarios you will ever encounter as you are using the Python re module is the find-and-replace scenario. Here, you will have to search and find a string and then replace it with a desired string. The method for this task is the re.sub() method.

Basic Syntax for the re.sub() Method

Understanding the syntax of any method is an essential skill for any developer. In this section, we will get to understand the syntax for the re.sub() method. Below is the syntax:


re.sub(pattern, replacement, text, count=0)

The table below explains every argument in detail:

Argument	Description
pattern	This is the regular expression pattern that you want to search for in the input text
replacement	This is the string that will replace the matched pattern in the input text
text	This is the input text in which you want to perform the find and replace operation
count(optional)	Specifies the maximum number of replacements to be made in the text

arguments for the re.sub() method

Using the re.sub() Method

Having looked at the re.sub() method syntax, let us now look at some practical applications of the method.

Literal Find and Replace

This is the first practical application of the re.sub() method. Here you might want to do a find-and-replace operation where the arguments pattern and replacement are being taken as normal literal strings without any interpretation associated with them. In such a case, the way around that is the re.escape() method, it ensures that any special characters are treated as literal strings during the find and replace operation. Let us look at the following example:


import re

text = "Any course on SparkByExamples.com website costs $100."

# escaping the pattern
pattern = re.escape("$100")

# declaring the string for replacement
replacement = "€50"

# finding and replacing the string
new_text = re.sub(pattern, replacement, text)
print(new_text)

In the code above, we have a text variable that holds the original string we want to modify. The pattern we want to search for is “$100”, which represents the cost of a course. Since “$” is a special character in regular expressions, we need to escape it using re.escape() to treat it as a literal character.

The replacement variable holds the string we want to replace “$100” with, which is “€50” representing a different currency.

Using re.sub(), we perform the find and replace operation by passing the pattern, replacement, and text as arguments. The resulting updated text is assigned to the new_text variable.

Output:


#Output
Any course on SparkByExamples.com website costs €50.

Replacing with Literal Strings

In situations where the pattern has no special characters, you can just go ahead and use a normal literal as in the example below:


import re

text = "I love learning programming on SparkByExamples.com"

# pattern to search for 
pattern = "SparkByExamples.com"

# replacement string
replacement = "SparkByExamples.edu"

# performing the find and replace operation
new_text = re.sub(pattern, replacement, text)

# printing the modified text
print(new_text)

In the code above, we have a string text containing the original text where we want to perform the find and replace operation.

We define the pattern variable to store the literal string we want to search for, in this case, "SparkByExamples.com". Since we are searching for a literal string, we don’t need to use regular expression syntax or escape any special characters.

The replacement variable contains the literal string that will replace the matched pattern. In this example, we are replacing "SparkByExamples.com" with "SparkByExamples.edu".

Using the re.sub() method, we perform the find and replace operation.

Output:


#Output
I love learning programming on SparkByExamples.edu

Replacing All Occurrences

You might also want to replace all occurrences of a pattern in a string. To do that simply include the count argument and set it to 0. Here is an example:


import re

text = "I love programming. I speak programming. I learn programming on SparkByExamples.com and programming is my passion."

# Pattern to search for
pattern = "programming"

# Replacement string
replacement = "coding"

# Perform the find and replace operation, replacing all occurrences
new_text = re.sub(pattern, replacement, text, count=0)

# Print the modified text
print(new_text)

In this example, the text is "I love programming. I speak programming. I learn programming on SparkByExamples.com and programming is my passion." The pattern to search for is "programming", and the replacement string is "coding".

By calling re.sub(pattern, replacement, text, count=0), the code performs the find and replace operation, replacing all occurrences of the pattern "programming" with the replacement string "coding". Since the count parameter is set to 0, all occurrences are replaced.

The modified text is then stored in the new_text variable.

Output:


#Output
I love coding. I speak coding. I learn coding on SparkByExamples.com and coding is my passion.

Limiting the Number of Replacements

To limit the number of replacements just set the count argument to any positive integer. Here is an example:


import re

text = "I love programming. I speak programming. I learn programming on SparkByExamples.com and programming is my passion."

# pattern to search for
pattern = "programming"

# replacement string
replacement = "coding"

# perform the find and replace operation, replacing all occurrences
new_text = re.sub(pattern, replacement, text, count=2)

# print the modified text
print(new_text)

The example is the same one we used when replacing all occurrences, but here the count argument is set to 2, limiting the replacements to only two occurrences. Therefore, the first two occurrences of "programming" in the text will be replaced with "coding".

Output:


#Output
I love coding. I speak coding. I learn programming on SparkByExamples.com and programming is my passion.

Conclusion

That’s it from this tutorial, we hope there is so much that you have learned about the find and replace using the re.sub() method in re module.