Developer Reference: Status Codes, Cron, Regex, Colours and More
Look something up, then act on it without changing tabs. Every page in this section explains one specific thing and embeds the tool that operates on it, so the answer and the working version of it sit on the same screen. 575 pages across 12 sections, all free and with nothing to sign up for.
Sections
HTTP status codes
What every HTTP response status code means, when a server should send it, and what to do when you get one. With a request builder to try them.
62 pages · built around the HTTP Request Builder
HTTP headers
One page per HTTP header: what it does, which spec defines it, what happens when it is missing, and a literal example. With a header checker on every page.
69 pages · built around the HTTP Headers Checker
JWT claims and algorithms
What every JWT claim, signing algorithm and JOSE header parameter means, which specification registers it, and how to check one against a real token.
48 pages · built around the JWT Decoder
cron expressions
What each cron expression actually runs, how many times a day it fires, and what every field and operator means. With a parser on every page.
70 pages · built around the Cron Expression Parser
Regex syntax
What each regular expression token, flag, quantifier and group form actually does, with worked examples and a tester on every page.
62 pages · built around the Regex Tester
Hash algorithms
What each hash algorithm outputs, which standard defines it, whether it is broken and in what way, and where it is wrong to use. Generator on every page.
13 pages · built around the Hash Generator
File permissions
What each chmod mode grants, the ls -l rendering, who can read, write and traverse, and the consequence when it is wrong. Calculator on every page.
33 pages · built around the Chmod Calculator
IPv4 subnets
What each CIDR prefix means: subnet mask, wildcard mask, host count, address range, and what the block is actually used for in real networks.
21 pages · built around the IP Subnet Calculator
.gitignore templates
One page per .gitignore template, with the exact patterns the generator produces, what each ignored path really is, and the rules that quietly misfire.
54 pages · built around the .gitignore Generator
HTML entities
What each HTML entity means, its named, decimal and hex forms, the Unicode code point behind it, and when you actually have to escape it in markup.
65 pages · built around the HTML Entity Reference
Image and canvas sizes
Exact pixel dimensions for social, video, device and print canvases, the ratio each one works out to, and what gets cropped when you supply the wrong shape.
42 pages · built around the Aspect Ratio Calculator
Working without uploading
Merge a PDF, strip EXIF, decode a token or check a .env without the file leaving your machine. One page per job, with the real limits.
36 pages · built around the Hash Generator