About the Regex Tester
A live regular expression tester with highlighting, groups, and replace mode — all in your browser.
What it is
The Computezy Regex Tester is a free, fully client-side tool for writing, debugging, and learning regular expressions. Enter a pattern and a test string, and the tool instantly highlights every match and lists each one with its position, full matched text, and any capture groups — numbered or named. It uses the JavaScript (ECMAScript) regex engine, the same engine that runs in every modern browser and in Node.js.
Everything runs in your browser. No data is sent to a server. Your patterns and test text stay entirely on your device.
Features
Live Highlighting
Every match is highlighted in a preview pane as you type. First match is marked in green; subsequent matches in yellow. Updates in real time with a short debounce.
Flag Toggles
Toggle the six standard JavaScript regex flags — global (g), case-insensitive (i), multiline (m), dotall (s), unicode (u), and sticky (y) — with a single click.
Capture Groups
The match list shows numbered capture groups ($1, $2, …) and named groups (?<name>…) for every match, so you can see exactly what each group captured.
Replace Mode
Switch to Replace mode to preview the result of String.replace with your pattern. Supports $1, $<name>, $& (full match), and $$ (literal dollar sign).
Error Handling
Invalid patterns show a clear inline error with the engine's exact message instead of crashing or freezing. A length guard on the test string prevents runaway patterns from hanging the tab.
Cheat Sheet
A collapsible cheat sheet covers character classes, anchors, quantifiers, groups, sets, lookaround, and replace references — a quick reference without leaving the page.
Privacy
Patterns and test strings can be sensitive — API keys, PII patterns, internal data formats. The Regex Tester is fully client-side: it makes no network requests with your input. Nothing is logged, stored, or sent anywhere. You can verify this by watching the Network tab in your browser's DevTools.