How it is tested

Every explanation on this site is computed by first-party JavaScript: the same code that runs in your browser runs at build time, against a test suite, before any page is written. The build stops if a single check fails. Latest build: 2026-10-31 16:16 EDT, 19,057 checks, 0 failures (Node v22.23.4, Python 3.14.8).

Results by tool

chmod: 16,433 passed

• stat.filemode, all 4096 modes: 4,096
• round trips: 12,288
• GNU chmod on real files: 26
• umask in a real shell: 9
• input forms: 14

CIDR: 2,054 passed

• ipaddress: 1,976
• RFC 5952 formatting: 14
• containment: 14
• range to CIDR: 7
• special ranges (IANA): 29
• input forms: 14

Cron: 431 passed

• croniter: 372
• hand-verified: 11
• daylight saving: 4
• descriptions: 17
• rejections: 27

Epoch: 39 passed

• datetime: 23
• unit detection: 6
• date parsing: 10

Regex: 100 passed

• token corpus: 14
• explanations: 15
• flavour differences: 19
• syntax errors: 19
• backtracking: 31
• tester: 2

What each check compares against

• Cron. 372 next-run lists (10 runs each, from 5 start instants including month, year and leap-day boundaries) against croniter 6.0.0 (MIT licence), for 62 Unix and Quartz expressions. Where croniter differs from Vixie cron on purpose (a day field such as */2 combined with a weekday: Vixie treats it as "*" for the OR rule, croniter does not), and for Quartz W, L, # and years, the expected dates come from a hand-checked 2026 calendar. Daylight saving is tested in America/New_York on the real 2026 change dates. Descriptions are compared word for word with reviewed text.
• CIDR. 1976 checks against Python's ipaddress module: network, last address, netmask, host mask, size, first and last usable host and a 4-way split, for 41 chosen blocks plus 300 random IPv4 and IPv6 blocks (fixed seed). Address formatting follows RFC 5952 and is checked against Python too. Special-purpose ranges are checked against the IANA registries by hand.
• chmod. All 4,096 modes (0000 to 7777) are converted to rwxr-xr-x form and compared with Python's stat.filemode, then converted back, and the generated symbolic command is applied to the opposite mode to check it lands exactly. Symbolic commands such as go-rx, a+X and +t are run with chmod (GNU coreutils) 9.11 on real files, and umask results come from a real shell creating real files and directories.
• Regex. A corpus of patterns with the expected token sequence and categories, explanation wording for each flag, PCRE-only and JavaScript-only syntax, syntax errors (and agreement with the JavaScript engine's own parser), and the backtracking heuristics on known-exponential, known-quadratic and known-safe patterns. The 159 example strings on the pattern pages are run through JavaScript and, where the syntax allows, Python's re (159 of them); both must agree with the page.
• Epoch. Timestamps from 1 AD to 9999 AD, including the 2038 boundary and leap days, against Python's datetime: ISO date, weekday, ISO week and day of year.

Known limits

• The catastrophic-backtracking check is a heuristic. It catches the common shapes (a repeat inside a repeat, overlapping alternatives under a repeat, adjacent repeats over the same characters, an unanchored repeat before $) and can miss others. The live tester runs your pattern in a background worker and stops it after 1.5 seconds, which catches the rest in practice.
• Regex explanations cover JavaScript and PCRE syntax. Python, Java, .NET and Go are close but not identical; the page notes the differences it knows about.
• Cron next-run times follow Vixie/cronie rules for daylight saving. systemd timers, Kubernetes and cloud schedulers have their own rules.
• chmod operations follow GNU chmod. BSD and BusyBox chmod differ in edge cases (for example how = treats setuid on directories).

Sources

• IANA IPv4 and IPv6 Special-Purpose Address Registries and the IANA HTTP Status Code Registry (facts, restated in our own words).
• IETF RFCs cited on each page (RFC 9110 for HTTP semantics, RFC 1918, 6598, 3021, 4291, 5952 and others for addressing).
• Syntax: POSIX crontab, the Vixie/cronie and Quartz Scheduler documentation, ECMAScript and PCRE2 documentation, GNU coreutils.
• Test oracles: croniter (MIT licence), Python standard library, GNU coreutils. They run at build time only and are not served to visitors.

All explanations, page text, code, icons and illustrations are original to breakitdown. Machine-readable results of the latest build: validation.json.