Developer Cheat Sheets

Dense, scannable reference sheets for SQL, regex, cron syntax, cURL flags, JWT claims and Mermaid diagrams, each paired with a tool that runs in your browser.

• Mermaid Cheat Sheet
  Copy-ready Mermaid syntax for flowcharts, sequence diagrams, class diagrams, ER diagrams, state diagrams, and Gantt charts.
  Use it when a diagram will not render, or you need the syntax for a diagram type you rarely write.
• SQL Cheat Sheet
  A quick reference for SQL clause order, JOIN types, aggregate functions, and common string/date functions.
  Use it when you need the logical order a query runs in, or which JOIN keeps unmatched rows.
• Regex Cheat Sheet
  A quick reference for regex character classes, anchors, quantifiers, groups, flags, and a handful of common patterns.
  Use it when a pattern matches too much, too little, or behaves differently between flavors.
• Cron Syntax Cheat Sheet
  A quick reference for cron field order and ranges, special characters, shortcut expressions, and common example schedules.
  Use it when you need to write a schedule such as every weekday at 09:00 and be sure it means that.
• cURL Flags Cheat Sheet
  A quick reference for the most commonly used cURL flags, each with a one-line meaning and a small usage example.
  Use it when you are turning an API example into a command, or a command into code.
• JWT Claims Cheat Sheet
  A quick reference for the standard registered JWT claims, common custom claims, and why JWT payloads are readable by anyone with the token.
  Use it when you are reading a token and need to know what iss, aud, exp and the rest mean.

What these sheets are for

Most developers do not forget how SQL or regex works; they forget the one detail they need right now: whether the cron day-of-week field starts at 0 or 1, which cURL flag sends a JSON body, or what the "exp" claim in a JWT is measured in. These sheets are written for that moment. Each one is a single page of short tables and examples you can scan in under a minute, with the common cases first and the sharp edges called out instead of buried.

They are references, not tutorials. If you are learning a topic from scratch, a longer course will serve you better. If you already know the topic and need the exact syntax, the exact order of clauses or the one gotcha that breaks your pipeline, this is the faster route.

How to use them with the tools

Every sheet links to the tool that does the same job interactively, so you can read a pattern and then try it on your own data. Check a cron schedule against the Cron Expression tool to see the next run times, paste a cURL command into the cURL Converter to get the equivalent fetch or Python code, or open a token in the JWT Debugger to see which of the claims on the sheet it actually contains. Everything runs in your browser, so tokens, queries and commands you paste are not sent anywhere.

The sheets are plain pages, so Ctrl+F works. Each one has its own URL, which makes them easy to link from a pull request, a runbook or a team wiki.

What is covered

The SQL sheet covers clause order, JOIN types, aggregates and common string and date functions. The regex sheet covers character classes, anchors, quantifiers, groups, flags and a few common patterns with their caveats. The cron sheet lists the five fields and their ranges, special characters, shortcut expressions and example schedules. The cURL sheet explains the flags you reach for most, with one combined example. The JWT sheet lists the registered claims, common custom claims and a security note about why a JWT payload is readable by anyone who holds the token. The Mermaid sheet gives copy-ready syntax for flowcharts, sequence, class, ER and state diagrams and Gantt charts, plus the mistakes that stop a diagram from rendering.

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