Zellio.io
Library · 281 entries

Reference you can run, not just read.

Patterns, commands, prompts and fixes — each explained, edge-cased and tested before release. Open any entry in its live tool.

ErrorsPort already in use (EADDRINUSE)
Likely causes1. Another instance of your own app (a previous run, a crashed process, or a second npm run dev) is still bound to the port — check with lsof -i :3000 (macOS/Linux) or…2. A different service (another dev server, a database like Postgres on 5432, or a proxy) happens to be using the same port — check the process name behind that PID before…
Check it withnpm run dev
Read the full fix

10 collections

Patterns86Regex, cron and SQL that hold up — each explained, edge-cased and testable in a live tool.Snippets16Copy-paste code that explains itself — JavaScript, Python, bash and CSS you can trust.Commands20Copy-paste CLI recipes that explain themselves — ffmpeg, git, curl, docker and more.Prompts54Fill-in-the-blank AI prompts for real tasks. Model-agnostic, assembled privately on the page.Templates38Emails and documents ready to adapt — fill the blanks, copy, and send.Formulas16Excel & Sheets formulas with fill-in blanks — set your cells, copy the result.Errors16Paste an error, get the plain-English meaning, the likely causes ranked, and the fix.Comparisons14The trade-off that actually decides it — JSON vs YAML, PUT vs POST, SQL vs NoSQL.Checklists8Tick them off as you go — code review, launch, SEO, security. Progress stays on your device.Cheat sheets13Reference tables you can filter and print — HTTP codes, Git commands and more.

Recently added

ErrorWeb/CORS
413 Payload Too Large
413 Payload Too Large (nginx shows: "413 Request Entity Too Large"; Express/body-parser shows: "PayloadTooLargeError: request entity too large")
Causes & fixes
Errorgit
Automatic merge failed; fix conflicts
Auto-merging src/app.js
Causes & fixes
ErrorLinux
bash: permission denied
bash: ./script.sh: Permission denied
Causes & fixes
ErrorWeb/CORS
blocked by CORS policy: No Access-Control-Allow-Origin
Access to fetch at 'https://api.example.com/data' from origin 'https://app.example.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.
Causes & fixes
ErrorDocker
Cannot connect to the Docker daemon
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
Causes & fixes
ErrorNode.js
Cannot find module
Error: Cannot find module 'express'
Causes & fixes

Browse the library

Search and filter every entry across all 10 collections.

281 results

PatternRegex
Match an email address
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}
Open in Regex Tester & Builder
PatternRegex
Match a web URL
https?:\/\/[\w.-]+(?:\/[\w./?%&=-]*)?
Open in Regex Tester & Builder
PatternRegex
Match an IPv4 address
\b(?:\d{1,3}\.){3}\d{1,3}\b
Open in Regex Tester & Builder
PatternRegex
Match a date in YYYY-MM-DD
\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])
Open in Regex Tester & Builder
PatternRegex
Match a UUID v4
[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-4[0-9a-fA-F]{3}-[89abAB][0-9a-fA-F]{3}-[0-9a-fA-F]{12}
Open in Regex Tester & Builder
PatternRegex
Enforce a strong password policy
(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[^\w\s]).{8,}
Open in Regex Tester & Builder
PatternRegex
Match a URL slug
^[a-z0-9]+(?:-[a-z0-9]+)*$
Open in Regex Tester & Builder
PatternCron
Every weekday at 9:00
0 9 * * 1-5
Open in Crontab Schedule Creator
PatternCron
Every 15 minutes
*/15 * * * *
Open in Crontab Schedule Creator
PatternCron
Nightly at 02:30
30 2 * * *
Open in Crontab Schedule Creator
PatternCron
First day of every month
0 9 1 * *
Open in Crontab Schedule Creator
PatternRegex
Match a phone number
\+?\d[\d\s().-]{7,}\d
Open in Regex Tester & Builder
Showing 12 of 281