Python Regex: Replacing Strings in a Code File

What will you learn?

Discover how to efficiently replace specific strings within code files using Python’s regex module.

Introduction to the Problem and Solution

Imagine having a code file where you need to substitute certain strings accurately. Python’s re module, designed for working with regular expressions, comes to the rescue in such scenarios. By harnessing regex patterns, we can precisely identify and replace target strings within the file.

To tackle this task effectively, we’ll first read the file’s content into memory, perform string substitution using regex patterns, and then save the modified content back to the same file. This approach ensures that our replacements are made accurately while preserving the original structure of the file intact.


import re

# Read from file
with open('', 'r') as file:
    content =

# Perform string substitution using regex
modified_content = re.sub(r'old_string', 'new_string', content)

# Write back to same file
with open('', 'w') as file:

# Copyright PHD


  • Open the code file in read mode and store its content in a variable.
  • Utilize re.sub() function from the re module for replacing occurrences of a specific pattern with another string.
  • Save the modified content back to the original file after performing substitutions.
    How does regular expression (regex) help in replacing strings?

    Regular expressions offer a robust method for defining search patterns within text data efficiently.

    Can I replace multiple different strings at once using regex?

    Yes, you can specify multiple patterns along with their respective replacement texts when employing regex for string substitutions.

    Is there a way to make my search case-insensitive when using regex?

    Certainly! You can enable case-insensitive matching by incorporating re.IGNORECASE flag while working with regex patterns.

    What happens if no matches are found for my specified pattern during substitution?

    In cases where no matches are found during substitution using regex, the original text remains unchanged.

    Can I use complex patterns like wildcards or quantifiers in my replacements?

    Absolutely! Regex supports various metacharacters such as *, +, or \d, allowing you to create intricate search and replace rules effectively.


    Mastering regular expressions in Python empowers you with potent tools essential for advanced string manipulation tasks. Proficiency in utilizing these features enables precise modifications within your codebase swiftly yet reliably. For further insights on optimizing your coding workflows through diverse Python functionalities, explore

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