Cron Day-of-Month Field Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching cron day-of-month field, 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

# Cron Day-of-Month Field
# ReDoS-safe | RegexVault — Dev & Systems > Cron

import re

cron_dayofmonth_field_pattern = re.compile(r'^(?:[?]|\*(?:/[1-9]|/[12][0-9]|/3[01])?|(?:[1-9]|[12][0-9]|3[01])(?:-(?:[1-9]|[12][0-9]|3[01]))?(?:/[1-9]|/[12][0-9]|/3[01])?(?:,(?:[1-9]|[12][0-9]|3[01]))*)$')

def validate_cron_dayofmonth_field(value: str) -> bool:
    return bool(cron_dayofmonth_field_pattern.fullmatch(value))

# Example
print(validate_cron_dayofmonth_field("*"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Dev & Systems > Cron 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

? is used in Quartz scheduler and AWS EventBridge to indicate no specific value. Standard Unix cron does not support ?. Day 31 is structurally valid but semantically invalid for short months.