How to Use Python Regex for String Replacement

Key Takeaways

1. Python's re.sub() allows flexible pattern-based string replacement.
2. Grouping and backreferences help restructure matched content.
3. Functions can be used for dynamic, custom replacements.

Regular expressions (regex) are powerful tools for searching and manipulating strings. In Python, the re module allows developers to perform regex-based search and replace operations efficiently. This article explains how to use Python regex to replace substrings with re.sub().

Importing the re Module

To use regex in Python, first import the built-in re module:

import re

Basic Usage of re.sub()

The main function used for regex replacement is re.sub(pattern, repl, string). It searches for all non-overlapping matches of the pattern in the string and replaces them with the string repl.

Example 1: Replacing Digits with a Symbol

import re

text = "Order number: 12345"
result = re.sub(r'\d', '*', text)
print(result)

Output:

Order number: *****

Explanation: The pattern \d matches any digit, and all digits are replaced with *.

Using Groups in Replacement

You can capture parts of the match using parentheses () and refer to them in the replacement string using \1, \2, etc.

Example 2: Reformatting Dates

text = "Today is 2025-04-09"
result = re.sub(r'(\d{4})-(\d{2})-(\d{2})', r'\2/\3/\1', text)
print(result)

Output:

Today is 04/09/2025

Explanation: The regex captures the year, month, and day separately and rearranges them in MM/DD/YYYY format.

Using a Function for Dynamic Replacement

If you want more control over how matches are replaced, you can pass a function as the repl argument.

Example 3: Incrementing Numbers

def add_one(match):
    number = int(match.group())
    return str(number + 1)

text = "Levels: 1, 2, 3"
result = re.sub(r'\d+', add_one, text)
print(result)

Output:

Levels: 2, 3, 4

Explanation: The function takes each number and increments it by 1.

Flags and Case-Insensitive Replacement

The re.sub() function accepts a flags parameter for modifiers like case insensitivity.

Example 4: Case-Insensitive Replacement

text = "Hello hello HeLLo"
result = re.sub(r'hello', 'hi', text, flags=re.IGNORECASE)
print(result)

Output:

hi hi hi

Conclusion

Python's re.sub() is a versatile function that supports simple replacements, group-based reordering, and even dynamic substitution using functions. With proper use of regex patterns, you can perform powerful text processing tasks in just a few lines of code.

FAQs

What is the primary method for regex replacement in Python?

The re.sub() function is used to perform regex-based replacements.

Can I modify matched values dynamically during replacement?

Yes, by passing a function to re.sub(), you can transform each match programmatically.

How do I change the order of matched groups in the replacement?

Use capturing groups and refer to them in the replacement string using \1, \2, etc.

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