AnyTool
Your files never leave your device. All processing happens locally in your browser.

How do I test a regular expression online?

Paste your pattern and test string into AnyTool's Regex Tester. Every match is highlighted live as you type, with capture groups, flags, and a plain-English explanation. Everything runs in your browser — your patterns and text never leave your device.

  • Live match highlighting and color-coded capture groups
  • Toggle flags (g, i, m, s, u, y, d) with one click
  • Plain-English explanation, replace mode, and common-pattern library
  • 100% in your browser — no upload, no signup, works offline

What is

Regex Testing

Regex testing runs a regular expression against sample text to see which substrings match, which capture groups fire, and how flags change the result — used to build and debug patterns before using them in code.

Developer Tools

Related terms

Regular ExpressionCapture GroupPattern MatchingRegex Flags

How to Test a Regular Expression

Test and debug a regex with live highlighting entirely in your browser.

20 secWeb browser
  1. 1

    Enter a pattern

    Type or paste your regular expression and toggle the flags you need.

  2. 2

    Add test text

    Paste the string you want to match against; matches highlight instantly.

  3. 3

    Inspect & refine

    Review capture groups, the plain-English explanation, and live replace preview, then copy the pattern.

Result: A validated regular expression

Frequently Asked Questions

Yes — it is completely free and runs entirely in your browser, so patterns never leave your device.

AnyTool's Regex Tester is free with no signup. Matching runs locally with the native JavaScript engine, so your pattern and test text are never uploaded — unlike tools where saving a pattern makes it public or sends data to a server.

It uses the JavaScript (ECMAScript) regex flavor with full flag support.

The tester runs the native JavaScript RegExp engine, so it matches the ECMAScript flavor with named groups, Unicode property escapes, and the g/i/m/s/u/y/d flags. This is the most common need for web and Node.js development.

Yes — it breaks the pattern down into a human-readable explanation as you type.

As you build a pattern, the tool parses it into an abstract syntax tree and describes each token — anchors, quantifiers, character classes, and groups — in plain English, so you can learn and debug at the same time.

Detailed Explanation

Methodology

How the Regex Tester Works

AnyTool tests regular expressions using the browser's native JavaScript RegExp engine, so matching, flags (g/i/m/s/u/y/d), and capture groups behave exactly as in ECMAScript. The pattern is parsed into an abstract syntax tree to generate a plain-English explanation and color-code tokens, while the d (hasIndices) flag drives precise match and group highlighting. All matching runs client-side — patterns and test text are never uploaded.

  • Native RegExp matching — JavaScript/ECMAScript flavor
  • Live highlighting of matches and color-coded capture groups
  • AST-driven plain-English explanation of the pattern
  • Replace/split modes and a common-pattern library
  • No upload, no signup; works offline
Privacy & Security

Privacy and Flavor Constraints

Because matching uses the in-browser engine, the tool is a JavaScript-flavor regex tester and does not run PCRE, Python, or .NET semantics server-side — which keeps the privacy guarantee honest. Saved or shared patterns are encoded in the URL fragment, never persisted on a server, unlike tools whose pattern libraries are public.