Overview

This PR adds a new built-in string format validator z.cron() for validating standard 5-field cron expressions (minute, hour, day-of-month, month, day-of-week).

Why this is useful

Cron expressions are widely used in task scheduling, CI/CD pipelines, cloud functions (AWS EventBridge, GCP Cloud Scheduler, etc.), and configuration files. Validating them at the schema layer catches malformed expressions before they reach the scheduler, preventing silent failures or unexpected behavior at runtime.

Before this PR, users had to reach for a custom .refine() with a hand-written regex or a third-party package:

// Before: verbose and error-prone
const schema = z.string().refine(val => /^(\*|[0-5]?\d).../.test(val), {
  message: "Invalid cron expression",
});

With z.cron(), this is a single, well-tested, semantically clear call.

Usage

Standalone schema

import { z } from "zod";

const CronJob = z.object({
  name: z.string(),
  schedule: z.cron(),
});

CronJob.parse({ name: "cleanup", schedule: "0 2 * * *" }); // OK
CronJob.parse({ name: "cleanup", schedule: "60 * * * *" }); // NG. minute out of range

As a string check

z.string().cron();

Supported syntax

z.cron().parse("* * * * *");        // wildcard
z.cron().parse("0 9-17 * * 1-5");   // ranges (weekdays 9am–5pm)
z.cron().parse("*/15 * * * *");     // steps (every 15 min)
z.cron().parse("0 0 1,15 * *");     // lists (1st and 15th)
z.cron().parse("0-30/5 * * * *");   // combined (every 5 min for first 30 min)