Developer

Why "runs in the browser" is the important claim here

On most of this site, doing the work locally is a privacy nicety. In this category it is the whole reason to use these instead of the first result in a search, because the things a developer pastes into a formatter are disproportionately the things that should not leave the machine: a token from a staging environment, a config fragment with a hostname in it, a query with a customer identifier in the WHERE clause, a payload from a bug report.

Every tool here is JavaScript running in your own tab. There is no request carrying your input, no server-side render of your data, nothing stored and nothing to delete afterwards. You can verify it rather than take it on trust: open the network panel, paste a token into the JWT decoder, and watch nothing leave.

Every tool speaks one dialect, and says which

The most expensive kind of wrong answer in this category is a correct answer from the wrong dialect. Regular expressions, cron, semantic versioning and unit prefixes all have several incompatible readings in common use, and a tool that does not name the one it implements will confidently disagree with the system you are actually deploying to.

• The regex tester runs the JavaScript engine. PCRE, Python and Go differ in the details — recursion, possessive quantifiers, \K — so a pattern that behaves here may not behave there, and the honest advice is to test against the engine that will run it.
• The cron parser reads the five-field crontab form: minute, hour, day-of-month, month, day-of-week, with no seconds field and no year field. A six-field expression belongs to a different scheduler, and reading one as though it were crontab shifts every field by one place.
• The file size converter distinguishes the decimal prefixes from the binary ones, because a kilobyte of 1,000 bytes and one of 1,024 differ by 2.4% — and that gap compounds with every step up the ladder, which is the entire explanation for a drive sold as 1 TB showing up as something smaller.
• The semantic version comparator implements the precedence rules from the specification, including the ones people get wrong: a pre-release version sorts below the release it precedes, and build metadata is ignored in comparison altogether.

Parsing rather than evaluating

Several of these tools take a string that could be executable and have to be careful with it. The regex tester passes your pattern to a constructor inside a try/catch rather than evaluating it as a literal, because the shortcut version would run whatever was pasted in as code and report a syntax error only after it had already done so.

The same tester also refuses, by default, patterns whose shape invites catastrophic backtracking — a repetition wrapped around a group that itself repeats or alternates — behind an explicit run-it-anyway. That check is deliberately over-cautious, because the engine cannot be interrupted once it starts: there is no timeout and no cancel, so the alternative to one extra click is a dead tab and lost text.

The reference tools, which are not calculators at all

Three of these answer a question rather than transform an input. The HTTP status code reference explains what each code actually commits a server to, which is more useful than the one-line summary most tables give. The uptime and SLA calculator turns nines into time you can staff for — three nines is 8h 45m 36s of downtime across a 365-day year, four nines is 52m 34s, and each further nine divides the allowance by exactly ten. And the CIDR subnet calculator turns a prefix length into the range, the broadcast address and the usable host count, which is arithmetic nobody should be doing on paper at two in the morning.

The six most people open first

JSON Formatter & Validator

Indent it, validate it, and get told where the error actually is.

JWT Decoder

Read the claims in a token without pasting it into somebody else’s server.

Regex Tester

A pattern, some text, and a group-by-group account of what matched.

Cron Expression Parser

A crontab line read back in English, with the next times it will fire.

CIDR Subnet Calculator

A prefix turned into a range, a mask and a host count.

Code Case Converter

An identifier moved between camel, snake, kebab and pascal case.