Interest Rate (APR / APY) Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching interest rate (apr / apy), ported and verified for Python. Financial data validation has zero tolerance for false negatives — a missed invalid entry can corrupt downstream calculations. 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

# Interest Rate (APR / APY)
# ReDoS-safe | RegexVault — Finance > Currency & Money

import re

interest_rate_apr_apy_pattern = re.compile(r'^(?:0(?:\.[0-9]{1,4})?|[1-9][0-9]{0,4}(?:\.[0-9]{1,4})?)%?$')

def validate_interest_rate_apr_apy(value: str) -> bool:
    return bool(interest_rate_apr_apy_pattern.fullmatch(value))

# Example
print(validate_interest_rate_apr_apy("0"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Finance > Currency & Money 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

Does not impose an upper bound to accommodate DeFi yields, payday loan rates, or penalty rates that can exceed 100%. Add a domain-specific cap at the application level. APR and APY differ when compounding is applied.