Cron & Regex

What this does, and where it stops

Both inspectors run in this tab. The page declares connect-src 'none', so it cannot make a network request even if something in it tried to. Nothing you paste — a crontab line from a production box, a pattern you are debugging against real log text — leaves the browser.

The cron parser

Written from scratch against the POSIX crontab format: five fields, with ranges (1-5), steps (*/15), lists (1,15,30) and the three-letter names for months and weekdays (JAN, MON). The six standard macros — @yearly, @annually, @monthly, @weekly, @daily, @hourly — expand to their canonical five-field forms before anything else looks at them.

One rule surprises people and is worth stating: when both day-of-month and day-of-week are set to something other than *, POSIX cron fires when either matches, not both. This parser follows that, which is why 0 0 13 * 5 means "the 13th, and every Friday", not "Friday the 13th".

Next 10 Triggers

The timeline walks forward a minute at a time from now, skipping whole months, days and hours that cannot match, and stops after five years — which is what bounds an expression like 0 0 31 2 * that can never fire at all. A cron expression is wall clock, so the zone picker is not a display setting: changing it re-evaluates the schedule against that clock, including its daylight-saving rules. That is what makes "when does this fire on a server in UTC" answerable.

The regex matcher

Patterns run through the browser's own RegExp, so the dialect is exactly JavaScript's — not PCRE, not RE2, not Python's re. A pattern that works here may still need adjusting for the language you are pasting it into, and one that hangs the page is a catastrophic-backtracking problem in the pattern rather than in this tool.

Limits

• Five fields only. A six- or seven-field Quartz expression is named as such rather than rejected with a field count, but it is not evaluated.
• No @reboot: it has no schedule, so there is nothing to show.
• None of the non-standard extensions — L, W, #, ? — which Quartz and some cloud schedulers accept but a POSIX crontab does not.
• The expression and the pattern are kept in this browser's storage so the page survives a reload. Nothing else is stored, and a share link carries only what is in the fields.