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👨‍💻 14 free developer tools

Free Developer Tools

The utilities you reach for mid-task: a regex tester with plain-English explanations, a cron expression builder with next-run times, a JWT decoder, SHA hash and checksum generation, JSON ⇄ YAML ⇄ CSV conversion, a WCAG contrast checker, a URL inspector, a Markdown editor, an exact subnet/CIDR calculator, a seeded fake-data generator and JSON-to-code type generation for TypeScript, Go, Rust and more.

14 tools

Regex Tester & Explainer Free online regex tester with live highlighted matches, capture groups, replace preview, a plain-English expla... JWT Decoder Decode and inspect JWTs entirely in your browser — header, payload, expiry. Nothing leaves your device. Cron Expression Builder Build and decode cron schedules in plain English, with the next run times. JSON ⇄ YAML ⇄ CSV Converter Convert between JSON, YAML and CSV instantly, with validation and pretty-print. Subnet Calculator — IPv4, CIDR & Mask Free IPv4 subnet calculator — network & broadcast address, host ranges, wildcard mask, binary breakdown, subne... JSON to Code (TypeScript, Go, Rust…) Paste JSON and get strongly-typed definitions in TypeScript, Go, Rust, Python, Kotlin, C# and JSON Schema — de... WCAG Contrast Checker Check colour contrast for accessibility — AA/AAA pass-fail with live preview. Hash & Checksum Generator Generate SHA-1/256/384/512 hashes of text or files, in your browser. URL Encoder & Inspector Encode/decode URLs and break any URL into its parts and query params. Markdown Editor Write Markdown with a live preview, formatting toolbar and one-click export. Auto-saves. Color Contrast Checker (WCAG) Check WCAG color contrast for any foreground/background pair — live preview and AA/AAA pass/fail badges for no... Fake / Test Data Generator Build a schema and generate realistic mock data in bulk — export JSON, NDJSON, CSV or SQL. Seeded for reproduc... Text Diff Checker Paste two text blocks and instantly see a line-by-line diff — added lines in green, removed in red, and unchan... CSS Box-Shadow Generator Instantly generate CSS box-shadow values with live preview — tweak offset, blur, spread, colour, opacity, and...

Everything runs locally, which is not a nicety here — it is a security requirement. Tokens, connection strings, proprietary payloads and customer data pasted into these tools never leave your machine, so they are safe to use with real production values. No accounts, no telemetry, no “pro tier” hiding the useful half of the tool.

Questions

Is it safe to paste real tokens and payloads here?
Yes. Everything is processed locally in your browser — JWTs, hashes, JSON and regex input are never transmitted, logged or stored, so real secrets and proprietary code stay on your machine.
Do these tools match what my language does?
They follow the standards: PCRE-style regex semantics, standard cron syntax with next-run previews, RFC-compliant JWT decoding, exact CIDR maths and WCAG 2.x contrast ratios — so results transfer directly to your codebase.
Can I use the generated test data and code in my project?
Yes — output from the fake-data generator and JSON-to-code tool is yours to use freely in personal and commercial projects, with deterministic seeds so fixtures are reproducible in CI.

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