Large Number with Thousands Separator Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching large number with thousands separator, 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

# Large Number with Thousands Separator
# ReDoS-safe | RegexVault — Finance > Currency & Money

import re

large_number_with_thousands_separator_pattern = re.compile(r'^(?:[1-9][0-9]{0,2}(?:,[0-9]{3})*|0)$')

def validate_large_number_with_thousands_separator(value: str) -> bool:
    return bool(large_number_with_thousands_separator_pattern.fullmatch(value))

# Example
print(validate_large_number_with_thousands_separator("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

Enforces proper comma grouping — 1000 is rejected (missing comma), 1,00 is rejected (wrong grouping). For European dot-separated format (1.000.000), swap comma/dot in the pattern.