Test a regex without leaving the browser. Regex Tester runs a JavaScript regular expression against sample text with live match highlighting — useful when you need to debug a regular expression against real sample text before using it in code between two other steps of a debugging session. Everything runs inside your browser, so the file never touches a server, which matters when the payload contains tokens, customer records or anything else you would not paste into an unknown server.
How to
Four steps, no account, nothing to install.
Write the expression and pick the flags you need.
Use text that includes both matches and near-misses.
Check the highlights and captured groups.
Preview the substitution before running it in code.
Overview
A regular expression compiles into a matching engine that scans text for a pattern. The power comes from a small vocabulary of character classes, quantifiers, groups and anchors — and the difficulty comes from the fact that a one-character mistake silently changes what matches.
Enter the pattern — Write the expression and pick the flags you need. Paste sample text — Use text that includes both matches and near-misses. Read the matches — Check the highlights and captured groups. Try a replacement — Preview the substitution before running it in code. The whole loop is designed to be repeatable: change the input, read the new result, no re-submit step in between.
The failures people hit are predictable: Hard to read months later; Catastrophic backtracking risks; Flavour differences between languages. Each of those shows up as a specific, fixable symptom rather than a vague error, which is exactly what this tool surfaces.
Input validation, Log parsing, Search and replace refactors — these are the contexts where Regex shows up, and this page is the fast path between "I have a blob of input" and "I understand what it contains". Keep the tab open next to your editor and your terminal. Scope note: uses the javascript regex engine, which differs from pcre in lookbehind and named-group details.
Features
Live highlighting as you type
Capture group inspection
Flag toggles (g, i, m, s, u)
Replacement preview
Reference
| Property | Value |
|---|---|
| Format | Regular expressions |
| Extension | .txt |
| MIME type | text/plain |
| Human readable | No |
| Typical use | Input validation |
Examples
Copy the raw payload out of the network tab, drop it in here, and read the result immediately — no curl round trip, no scratch file.
Paste the snippet from the diff to confirm what it evaluates to before approving the change.
Because processing is local, real payloads with real identifiers can be inspected without violating a data-handling policy.
Benefits
FAQ
1. Enter the pattern: Write the expression and pick the flags you need. 2. Paste sample text: Use text that includes both matches and near-misses. 3. Read the matches: Check the highlights and captured groups. 4. Try a replacement: Preview the substitution before running it in code.
No. Everything runs inside your browser, so the file never touches a server — safe for tokens, staging payloads and customer data.
Yes, with no account, no rate limit and no paid tier.
A pattern language for matching, extracting and replacing text. The power comes from a small vocabulary of character classes, quantifiers, groups and anchors — and the difficulty comes from the fact that a one-character mistake silently changes what matches.
Hard to read months later; Catastrophic backtracking risks; Flavour differences between languages.
Uses the JavaScript regex engine, which differs from PCRE in lookbehind and named-group details.
Once the page has loaded, yes — the processing needs no network access.
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