Zellio.io

Regex Tester & Builder

Test patterns against text and build common regex from presets.

Runs locallyWorks offlineShare link carries settings, never your data
Send output toBase64 Encode / DecodeCode FormatterSlug / URL EncoderBuild a workflow from thisOutput stays on this page until you send it.

Frequently asked questions

Is my test text uploaded?

No. Everything runs locally in your browser using standard web APIs — your text, files and inputs are never uploaded to a server, so the tool works even offline once the page has loaded.

Which regex flavour does it use?

It uses JavaScript (ECMAScript) regular expressions, including flags like global, case-insensitive and multiline, which closely match most other languages for common patterns. Most common regex syntax (character classes, quantifiers, anchors, alternation) works identically across JavaScript, Python, Perl and most other regex engines, so you can test patterns here and use them in your production code with minimal translation. More advanced features like lookahead/lookbehind and named captures also have wide support in modern languages. If you're writing regex for a different language with different features, test your specific edge cases to ensure compatibility.

Can it explain what a pattern does?

The presets give you working patterns for common tasks, and live highlighting shows exactly what each part matches as you tweak it. By selecting preset patterns for email, URL, phone, or date validation, you get tested regex expressions you can adapt rather than building from scratch. The live highlighting feature color-codes which parts of your sample text match each portion of the regex pattern, giving you immediate visual feedback as you edit. This trial-and-error approach with instant feedback is much faster than reading documentation or guessing at regex syntax.

Pro tips

  • Build regex patterns incrementally by testing against 10+ sample strings simultaneously and seeing matches highlighted in real-time.
  • Use preset patterns for common tasks (email, phone, URL, date formats) as starting points instead of building from scratch.
  • Toggle global, case-insensitive, and multiline flags to debug why patterns aren't matching expected strings.
  • Export matches and non-matches separately to validate patterns against test datasets before deploying to production code.

About Regex Tester & Builder

Matches highlight as you type, so you can watch a pattern narrow or blow up with each character you add. The presets cover the expressions people rewrite constantly, as a starting point to adapt rather than trust blindly.

Test regular expressions against sample text with live match highlighting, or build common patterns — emails, URLs, dates, phone numbers — from ready-made presets. As you edit the pattern and flags, matches update instantly so you can refine without guesswork.

It is the quick way to validate a pattern before dropping it into code, or to work out how an unfamiliar one behaves. Test the awkward cases as well as the obvious ones — an empty string, a trailing space, a doubled separator — because those are what fail in production.

A few habits prevent most regex pain. Anchor with ^ and $ when you mean the whole string, or a validator happily matches a fragment inside garbage. Prefer explicit character classes to the dot, which matches more than people expect. And remember quantifiers are greedy by default: .* runs to the last possible match rather than the first, which is why naive tag-matching patterns swallow an entire document instead of one element.

One failure mode is worth treating as a security issue rather than a bug. Nested quantifiers over overlapping alternatives — the classic shape being (a+)+ — can take exponential time on input that merely fails to match, so a pattern that is instant on your test string can hang a server on a slightly longer one. If a regex will ever run on untrusted input, test it against a long non-matching string as deliberately as you test the matches.

Validating an email address with a regular expression is the classic dead end worth naming. The specification permits far more than any short pattern accepts — quoted strings, comments, addresses with no dot in the domain — so a strict pattern rejects valid addresses while a permissive one accepts nonsense. The only proof an address works is mail arriving at it, which is why the practical approach is a loose shape check followed by a confirmation email.

Common use cases

  • Backend developers validating form input patterns (emails, phone numbers, postal codes) before accepting user data.
  • Log analysis teams building regex patterns to extract error codes, timestamps, and stack traces from server logs.
  • Data engineers cleaning messy datasets by writing patterns to identify and extract phone numbers, addresses, or SKUs.
  • Security teams hunting for indicators of compromise in network logs using regex patterns for suspicious IP addresses and domains.
How it comparesCompared to Regex101.com, our tester includes preset patterns library and faster match highlighting, while offering simpler UX than advanced tools like RegexBuddy that require purchasing licenses, making it ideal for quick pattern validation without learning curve.