Cron Month Field Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching cron 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 Month Field
// ReDoS-safe | RegexVault — Dev & Systems > Cron

const cronMonthFieldRegex = /^(?:\*(?:\/[1-9]|\/1[0-2])?|(?:[1-9]|1[0-2])(?:-(?:[1-9]|1[0-2]))?(?:\/[1-9]|\/1[0-2])?(?:,(?:[1-9]|1[0-2]))*)$/;

function validateCronMonthField(input: string): boolean {
  return cronMonthFieldRegex.test(input);
}

// Example
console.log(validateCronMonthField("*")); // 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

Month is 1-indexed (1=January, 12=December). Some cron dialects accept named months (JAN-DEC) — add case-insensitive alternations if required.