Base64 Encoded String Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching base64 encoded string, ported and verified for Python. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# Base64 Encoded String
# ReDoS-safe | RegexVault — Web & Network > Misc

import re

base64_encoded_string_pattern = re.compile(r'^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$')

def validate_base64_encoded_string(value: str) -> bool:
    return bool(base64_encoded_string_pattern.fullmatch(value))

# Example
print(validate_base64_encoded_string("SGVsbG8sIFdvcmxkIQ=="))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > Misc category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Empty string is valid base64 (encodes empty byte string). This pattern validates standard base64 — for URL-safe base64, replace + with \- and / with _.