Regex Tester & Builder

Test regex patterns locally with live match highlighting, capture groups, and a replace preview. Runs safely in a sandboxed worker.

Regex tester & match highlighter: runs safely, entirely on your device Paste a pattern and some test text into this regex tester and see, instantly: every match highlighted in the text, numbered AND named capture groups per match, and a live replace preview supporting $1, $<name>, $& (the whole match), and $$ (a literal dollar sign). Toggle the g/i/m/s/u/y flags with checkboxes to test regular expressions exactly the way your target language will run them. No account, no upload, no CDN: a private alternative to pasting a pattern into an online regex tester such as regex101. SAFE BY DESIGN: THE PART OTHER REGEX TESTERS GET WRONG A pasted pattern can accidentally (or maliciously) be catastrophic. A classic "evil regex" like nested repetition can make a browser tab hang for minutes against ordinary-looking text. This extension NEVER runs your pattern on the page you're looking at. Every single match/replace/batch-test run happens inside an isolated, sandboxed Web Worker with a hard time budget. If a pattern ever runs too long, the worker is terminated automatically and you get a clear "pattern too slow" message. The popup itself never freezes, no matter what you paste. PRIVATE BY DESIGN The pattern and the text you're testing are often sensitive: a log line with a token, an internal hostname, a customer record. This extension is a paste-mode popup: it never reads the active tab or any page content, requests no host permissions, and sends nothing anywhere except a purchase-verification check with ExtensionPay if you choose to upgrade. This extension collects no data of any kind: no analytics, no tracking, no telemetry. No account is ever required to test a pattern. WHAT IT DOES (FREE) - LIVE MATCH HIGHLIGHTING: every match in your test text highlighted as you type, with alternating colors so adjacent matches are easy to tell apart. - CAPTURE GROUPS: numbered ($1, $2, ...) and named ((?<name>...)) groups shown per match, including a clear marker for a group that didn't participate. - REPLACE PREVIEW: a template field with $1 / $<name> / $& / $$ support, showing the full resulting text live without touching your original input. - FLAGS UI: toggle g, i, m, s, u, and y with simple checkboxes. - CLEAR ERRORS: an invalid pattern shows the exact syntax error instead of a blank result. HOW TO USE 1. Paste (or type) your pattern into the pattern field. 2. Paste (or type) the text you want to test it against. 3. Toggle the g/i/m/s/u/y flags with the checkboxes to match how your target language will actually run the pattern. 4. Read the highlighted matches and capture groups live, and, if you set a replace template, the replace preview (all updating as you type). WHO THIS IS FOR Built for developers debugging a pattern before it ships, QA engineers writing validation rules, and anyone checking whether a log line, an input, or a URL actually matches a pattern, without pasting it into a random website first. FAQ Does this send my pattern or test text anywhere? No. Matching runs inside an isolated, sandboxed Web Worker on your device; the pattern and text you test are never uploaded. Which flags are supported? g, i, m, s, u, and y. Toggle each with a checkbox. What happens with a catastrophic pattern (e.g. nested repetition)? The worker is terminated at a hard time budget and you get a clear "pattern too slow" message naming possible catastrophic backtracking as the likely cause; the popup itself never freezes. Do I need an account? No. There is no account for the free tier, and premium is a one-time purchase, not a subscription. PREMIUM ($5 one-time) - Saved pattern library: name and save patterns you use often, and reload them instantly instead of retyping them. - "Explain this pattern": a plain-English, piece-by-piece breakdown of what your regex actually does, covering anchors, character classes, quantifiers (greedy vs. lazy), groups, lookaround, and escapes. - Batch / multi-line test sets: paste many test lines at once and see, per line, whether the pattern matched and what it caught first. No subscription, no account required. Pay once, keep the features.