Cron Day-of-Month Field Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching cron day-of-month field, ported and verified for JavaScript. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your JavaScript project — whether you're validating in an Express middleware, a Next.js API route, or a client-side form.

Javascript Implementation

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

const cronDayofmonthFieldRegex = /^(?:[?]|\*(?:\/[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]))*)$/;

function validateCronDayofmonthField(input: string): boolean {
  return cronDayofmonthFieldRegex.test(input);
}

// Example
console.log(validateCronDayofmonthField("*")); // 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 JavaScript developers because especially critical in long-running Node.js event loops where a ReDoS vulnerability can block the entire process. 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.