Quick reference guides and cheatsheets for developers — bookmark them, print them, use them daily.
These reference guides cover the topics developers look up most frequently: regex validation patterns, HTTP status code meanings, CSS color format conversions, JWT RFC 7519 claims, cron expression syntax, and WCAG-compliant color palettes. Each guide is written to be accurate, concise, and copy-paste ready.
Every guide is available as a downloadable A4 PDF — enter your email and click "Download PDF" (or skip the email and download anyway). Each PDF is generated from the same source as the page you are reading, and carries the full text, code samples and tables, with page numbers and a link back to the guide on every page.
Battle-tested regular expression patterns for email, URL, UUID, IPv4, semantic version, password, hex color, date, and data extraction. Includes JavaScript, Python, and Go equivalents.
Complete reference for all HTTP status codes — 2xx success, 3xx redirect, 4xx client error, 5xx server error — with REST API design guidance and caching behavior notes.
Quick reference for HEX, RGB, RGBA, HSL, HSLA, HWB, OKLCH, and named colors — with conversion code, browser support, and guidance on which format to use for design tokens and gradients.
Complete reference for all JWT registered claims from RFC 7519, including claim names, types, validation requirements, and usage in OAuth 2.0 and OpenID Connect.
A reference of 30 practical cron expressions for common scheduling scenarios: every N minutes, specific times, weekdays, weekends, monthly schedules, and cloud variants.
Curated color palettes for web development with WCAG AA contrast ratios pre-verified. Includes neutral gray scales, semantic status colors, and dark mode token mappings.
Dev Tools ! provides six reference guides covering the topics developers look up most often: regex patterns (email, URL, UUID, IPv4), HTTP status codes for REST API design, CSS color formats (HEX, RGB, HSL, OKLCH), JWT RFC 7519 registered claims, cron expression examples, and accessible color palettes with pre-verified WCAG AA contrast ratios.
Yes. Every reference guide has a "Download PDF" button that saves a real A4 PDF file to your device — no print dialog, no browser extension. Each PDF is typeset from the same source as the web page, with the full text, code samples and tables, plus page numbers and a source link on every page.
All reference guides on Dev Tools ! are completely free to use, bookmark, print, and share. They are published by Cosyslabs under an open content license. No registration or API key is required.
Reference guides are reviewed and updated when relevant standards change — for example when a new HTTP status code is ratified, or when a new CSS color format gains broad browser support. Each guide displays its last-reviewed date.