In this article you'll see how to use Python's .replace() method to perform substring substiution.

You'll also see how to perform case-insensitive substring substitution.

Let's get started!

What does the .replace() Python method do?

When using the .replace() Python method, you are able to replace every instance of one specific character with a new one. You can even replace a whole string of text with a new line of text that you specify.

The .replace() method returns a copy of a string. This means that the old substring remains the same, but a new copy gets created – with all of the old text having been replaced by the new text.

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How does the .replace() Python method work? A Syntax Breakdown

The syntax for the .replace() method looks like this:

string.replace(old_text, new_text, count)

Let's break it down:

  • old_text is the first required parameter that .replace() accepts. It's the old character or text that you want to replace. Enclose this in quotation marks.
  • new_text is the second required parameter that .replace() accepts. It's the new character or text which you want to replace the old character/text with. This parameter also needs to be enclosed in quotation marks.
  • count is the optional third parameter that .replace() accepts. By default, .replace() will replace all instances of the substring. However, you can use count to specify the number of occurrences you want to be replaced.

Python .replace() Method Code Examples

How to Replace All Instances of a Single Character

To change all instances of a single character, you would do the following:

phrase = "I like to learn coding on the go"

# replace all instances of 'o' with 'a'
substituted_phrase = phrase.replace("o", "a" )

print(phrase)
print(substituted_phrase)

#output

#I like to learn coding on the go
#I like ta learn cading an the ga

In the example above, each word that contained the character o is replaced with the character a.

In that example there were four instances of the character o. Specifically, it was found in the words to, coding, on, and go.

What if you only wanted to change two words, like to and coding, to contain a instead of o?

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How to Replace Only a Certain Number of Instances of a Single Character

To change only two instances of a single character, you would use the count parameter and set it to two:

phrase = "I like to learn coding on the go"

# replace only the first two instances of 'o' with 'a'
substituted_phrase = phrase.replace("o", "a", 2 )

print(phrase)
print(substituted_phrase)

#output

#I like to learn coding on the go
#I like ta learn cading on the go

If you only wanted to change the first instance of a single character, you would set the count parameter to one:

phrase = "I like to learn coding on the go"

# replace only the first instance of 'o' with 'a'
substituted_phrase = phrase.replace("o", "a", 1 )

print(phrase)
print(substituted_phrase)

#output

#I like to learn coding on the go
#I like ta learn coding on the go

How to Replace All Instances of a String

To change more than one character, the process looks similar.

phrase = "The sun is strong today. I don't really like sun."

#replace all instances of the word 'sun' with 'wind'
substituted_phrase = phrase.replace("sun", "wind")

print(phrase)
print(substituted_phrase)

#output

#The sun is strong today. I don't really like sun.
#The wind is strong today. I don't really like wind.

In the example above, the word sun was replaced with the word wind.

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How to Replace Only a Certain Number of Instances of a String

If you wanted to change only the first instance of sun to wind, you would use the count parameter and set it to one.

phrase = "The sun is strong today. I don't really like sun."

#replace only the first instance of the word 'sun' with 'wind'
substituted_phrase = phrase.replace("sun", "wind", 1)

print(phrase)
print(substituted_phrase)

#output

#The sun is strong today. I don't really like sun.
#The wind is strong today. I don't really like sun.

How to Perform Case-Insensitive Substring Substitution in Python

Let's take a look at another example.


phrase = "I am learning Ruby. I really enjoy the ruby programming language!"

#replace the text "Ruby" with "Python"
substituted_text = phrase.replace("Ruby", "Python")

print(substituted_text)

#output

#I am learning Python. I really enjoy the ruby programming language!

In this case, what I really wanted to do was to replace all instances of the word Ruby with Python.

However, there was the word ruby with a lowercase r, which I would also like to change.

Because the first letter was in lowercase, and not uppercase as I specified with Ruby, it remained the same and didn't change to Python.

The .replace() method is case-sensitive, and therefore it performs a case-sensitive substring substitution.

In order to perform a case-insensitive substring substitution you would have to do something different.

You would need to use the re.sub() function and use the re.IGNORECASE flag.

To use re.sub() you need to:

  • Use the re module, via import re.
  • Speficy a regular expression pattern.
  • Mention with what you want to replace the pattern.
  • Mention the string you want to perform this operation on.
  • Optionally, specify the count parameter to make the replacement more precise and specify the maximum number of replacements you want to take place.
  • The re.IGNORECASE flag tells the regular expression to perform a case-insensitive match.

So, all together the syntax looks like this:

import re

re.sub(pattern, replace, string, count, flags) 

Taking the example from earlier:

phrase = "I am learning Ruby. I really enjoy the ruby programming language!"

This is how I would replace both Ruby and ruby with Python:

import re

phrase = "I am learning Ruby. I really enjoy the ruby programming language!"

phrase = re.sub("Ruby","Python", phrase, flags=re.IGNORECASE)

print(phrase)

#output

#I am learning Python. I really enjoy the Python programming language!
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Wrapping up

And there you have it - you now know the basics of substring substitution. Hopefully you found this guide helpful.

To learn more about Python, check out freeCodeCamp's Scientific Computing with Python Certification.

You'll start from the basics and learn in an interacitve and beginner-friendly way. You'll also build five projects at the end to put into practice and help reinforce what you learned.

Thanks for reading and happy coding!