Topic 13818 min readStructured Tutorial

Python Substitution: Mastering String Replacement Techniques

Introduction

Substitution in Python primarily refers to replacing parts of strings with new content. This is a common task in programming for data cleaning, formatting, and manipulation.

In this tutorial, you will learn various substitution methods in Python, including simple string replacement and advanced regex-based substitution.

The power of programming lies in transforming data efficiently.

Basic String Replacement with str.replace()

Python's built-in string method str.replace() allows you to replace occurrences of a substring with another substring.

It is straightforward and useful for simple substitutions without the need for pattern matching.

Syntax: str.replace(old, new, count)
old: substring to be replaced
new: substring to replace with
count (optional): number of replacements to perform

Example of str.replace()

Here is a simple example replacing 'cat' with 'dog' in a string.

Advanced Substitution with Regular Expressions

For more complex substitution needs, Python's re module provides the sub() function.

This allows pattern-based replacements using regular expressions, enabling powerful text processing.

Syntax: re.sub(pattern, repl, string, count=0, flags=0)
pattern: regex pattern to match
repl: replacement string or function
string: input string to process
count: maximum number of replacements (default 0 means replace all)

Example of re.sub() for Digit Replacement

This example replaces all digits in a string with the '#' character.

Using Functions for Dynamic Substitution

The re.sub() function can accept a function as the replacement argument.

This allows dynamic computation of the replacement string based on the matched content.

The function receives a match object and returns the replacement string.
Useful for conditional or computed substitutions.

Example: Incrementing Numbers in a String

This example increments every number found in a string by 1.

Examples

Simple str.replace() Example

text = 'The cat sat on the cat mat.'
new_text = text.replace('cat', 'dog')
print(new_text)

This code replaces every occurrence of 'cat' with 'dog' in the string.

Regex re.sub() Example

import re
text = 'My phone number is 123-456-7890.'
new_text = re.sub(r'\d', '#', text)
print(new_text)

This code replaces all digits in the string with '#'.

Dynamic Replacement with Function

import re

def increment(match):
    return str(int(match.group()) + 1)

text = 'Version 1.2.3'
new_text = re.sub(r'\d+', increment, text)
print(new_text)

This code finds all numbers in the string and increments each by 1.

Best Practices

Use str.replace() for simple, literal substring replacements.
Use re.sub() when pattern matching is required for substitution.
When using regex, test your patterns carefully to avoid unexpected replacements.
Use functions with re.sub() for dynamic or conditional replacements.
Always handle edge cases such as empty strings or no matches gracefully.

Common Mistakes

Confusing str.replace() with regex substitution; str.replace() does not support patterns.
Forgetting to import the re module when using regex functions.
Using greedy regex patterns that replace more than intended.
Not specifying the count parameter when only limited replacements are desired.
Assuming re.sub() modifies the original string; strings are immutable in Python.

Hands-on Exercise

Replace Vowels with Asterisk

Write a Python program that replaces all vowels in a given string with '*'. Use both str.replace() and re.sub() methods.

Expected output: Input: 'Hello World' -> Output: 'H*ll* W*rld'

Hint: For str.replace(), chain multiple calls; for re.sub(), use a regex pattern matching vowels.

Mask Email Addresses

Use regex substitution to mask email addresses in a text by replacing the username part with '***'.

Expected output: Input: 'Contact me at user@example.com' -> Output: 'Contact me at ***@example.com'

Hint: Use a regex pattern to capture the username before '@' and replace it.

Interview Questions

What is the difference between str.replace() and re.sub() in Python?

Interview

str.replace() performs simple literal substring replacements, while re.sub() allows pattern-based replacements using regular expressions.

How can you perform conditional substitution in Python strings?

Interview

By passing a function as the replacement argument to re.sub(), which computes the replacement string dynamically based on the match.

Summary

Substitution in Python is a fundamental technique for string manipulation.

The str.replace() method is ideal for simple direct replacements, while the re.sub() function offers powerful pattern-based substitution.

Using functions with re.sub() enables dynamic and context-aware replacements.

Mastering these methods will help you handle a wide range of text processing tasks efficiently.

FAQ

Can str.replace() use regular expressions?

No, str.replace() only replaces literal substrings. For regex-based replacements, use re.sub() from the re module.

Are strings mutable in Python when using substitution methods?

No, strings are immutable in Python. Substitution methods return new strings and do not modify the original.

How do I replace only the first occurrence of a substring?

Use the count parameter in str.replace(old, new, count) or re.sub(pattern, repl, string, count=1) to limit replacements.