String Replace in Python | Python String Replace [2021]

Replacing characters and strings in Python is a crucial task when it comes to Data Cleaning or Text Processing. Your data might have formatting issues with garbage characters that need to be removed, the categories might be having spelling issues, etc. Also while text preprocessing for NLP based problems, string replacement is the most basic and important step while preparing the textual data.

In this tutorial, we will be going over multiple ways to replace different types of strings. By the end of this tutorial, you will have the knowledge of the following:

• Python replace() method
• Regex sub() method
• join() and filter()
• Replacing numeric data in strings

Python replace()

The replace(old_str, new_str, count) method consists of 3 arguments:

• old_str: The string or a part of the string that needs to be replaced
• new_str: The string with which the old string needs to be replaced
• count: The count of times the particular string needs to be replaced

Let’s go over a few examples to understand the working.

Single replace

If you recall, Strings in Python are immutable. So when we call the replace method, it essentially makes another string object with the modified data. Moreover, we didn’t specify the count parameter in the above example. If not specified, the replace method will replace all the occurrences of the string.

Multiple replace

Multiple replace first n occurrences

If you only want first N occurrences, 

Multiple strings replace

In the above examples, we replaced one string a different number of times. Now what if you want to replace different strings in the same big string. We can write an effective function for it and get it done using the same method.

Consider the example as above, but now we want to replace “h”, “is” and “ng” with “X”.

Read: Python Tutorial

Replacing with regex

Python’s regex is a module specifically for dealing with text data – be it finding substrings, replacing strings or anything. Regex has the sub() function to find and replace/substitute substrings easily. Let’s go over its syntax and a few use cases. 

The regex.sub(pattern, replacement, original_string) function takes 3 arguments:

• pattern: the substring that needs to be matched and replaced.
• replacement: can be a string which needs to be put in place, or a callable function which returns the value that needs to be put in place.
• original_string: the main string in which the substring has to be replaced.

Same as the replace method, regex also creates another string object with the modified string. Let’s go over a few working examples.

Removing whitespace

Whitespaces can be treated as special characters and replaced with other characters. In the below example, we intend to replace whitespaces with “X”.

As we see, all the whitespaces were replaced. The pattern is given by r”\s+” which means all the whitespace characters. 

Removing all special characters

To remove all the special characters, we will pass a pattern which matches with all the special characters.

Removing substring as case insensitive

In real life data, there might be cases where there might be many versions of the same word with different upper and lower case characters. To remove them all, putting all the words separately as the pattern wouldn’t be effective. The regex sub() function takes the flag re.IGNORECASE to ignore the cases. Let’s see how it works.

Removing multiple characters using regex

The regex function can easily remove multiple characters from a string. Below is an example.

Replacing using join()

Another way to remove or replace characters is to iterate through the string and check them against some condition.

Replacing using join() and filter()

Above example can also be done by using the filter function.

Must Read: Fascinating Python Applications in Real World

Replacing numbers

Many times the numerical data is also present in the strings that might need to be removed and processed separately as a different feature. Let’s go over a few examples to see how these can be implemented.

Using regex

Consider the below string from which we need to remove the numeric data.

In the above code, we use the matching pattern r'[0-9]’ to match for all the digits. 

Using join() function

We can also iterate upon the string and filter out the digits using the isdigit() method which returns False for alphabets. 

Using join() and filter()

Similarly, we can also put the filtering condition in the filter function to only return the characters which return True.

Before you go

We covered a lot of examples showing different ways to remove or replace characters/whitespaces/numbers from a string. We highly recommend you to try out more examples and different ways to do the above examples and also more examples of your own.

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