Regex Tester & Debugger - Free & No Account Required

Test JavaScript and Python regular expressions live in your browser. Highlight matches, inspect capture groups, preview replacements, and generate multi-language code snippets.

Open Regex Tester How it works
Worker-isolated execution Captures and named groups Replacement preview Eight language snippets

Multi-Language Regex Tester

Choose a live engine, enter a pattern and test text, configure flags, and inspect every result. JavaScript and Python use different syntax and are intentionally tested separately.

Local worker execution
0 / 10,000
Enter only the pattern, or import a JavaScript literal such as /hello/gi.
JavaScript mode uses the browser’s native RegExp engine in a disposable worker.
JavaScript replacement references use $1 and $<name>.
0 / 2,000,000
Ready to test
01
Real live enginesNative JavaScript and Python re
02
Worker isolationTimeout protects the main page
03
Private testingPattern and text remain local
04
Multi-language codeEight integration examples
Accurate regex debugging

Test regular expressions without confusing language flavors

This multi-language Regex Tester separates JavaScript and Python into genuine live execution modes. Patterns are not silently translated from one flavor to another, helping you identify syntax and behavior differences accurately.

Each run reports complete matches, numbered captures, named captures, source ranges, line and column locations, replacement output, and execution duration. Matching text is rendered with escaped DOM text nodes so user input cannot inject HTML.

JavaScript and Python execution

JavaScript mode uses the browser’s native RegExp implementation. Python mode loads CPython through Pyodide and executes the standard-library re engine inside a separate Web Worker.

Code for additional languages

The code generator creates starting examples for PHP, Java, C#, Ruby, Go, and Rust. Because these engines have different feature sets, generated code should be validated in the target runtime.

Developer-focused features

Everything needed to inspect a regex

Move from pattern construction to matching, capture debugging, replacement validation, and implementation code in one workflow.

Safe match highlighting

Matched ranges are rendered with escaped text nodes rather than inserting raw user HTML.

Capture inspection

Review full matches, numbered groups, named groups, positions, lines, and columns.

Replacement testing

Compare JavaScript dollar references with Python backslash and named-group replacement syntax.

Timeout protection

Potentially expensive expressions run away from the UI and are terminated after a strict limit.

Reusable code snippets

Generate implementation examples for JavaScript, Python, PHP, Java, C#, Ruby, Go, and Rust.

Portable reports

Download structured JSON or readable TXT reports containing flags, matches, captures, and output.

Three-step workflow

How to test a regular expression

Step 1

Choose an engine

Select JavaScript or Python so the expression uses the intended syntax and flags.

Step 2

Add pattern and text

Enter a pattern, configure flags, paste test text, and optionally add a replacement.

Step 3

Inspect and export

Review highlighted matches and captures, then copy code or download a complete report.

Engine differences

JavaScript, Python, and generated flavors

No universal regex flavor exists. Always validate patterns in the runtime that will execute them.

Capability
JavaScript
Python
Live runtime
Native browser RegExp
CPython re through Pyodide
Named group
(?<name>...)
(?P<name>...)
Named backreference
\k<name>
(?P=name)
Replacement group
$1 or $<name>
\1 or \g<name>
Special flags
u, v, y, d
x and ASCII mode
Regex best practices

Build safer and more maintainable patterns

1

Test in the target engine

Do not assume JavaScript, Python, PCRE, Java, .NET, RE2, and Rust support identical syntax.

2

Avoid uncontrolled backtracking

Nested ambiguous quantifiers can cause catastrophic runtime growth on adversarial text.

3

Use anchors intentionally

Understand how multiline mode changes the behavior of start and end anchors.

4

Include negative test cases

Test malformed, empty, Unicode, multiline, long, and near-matching input before deployment.

Common questions

Regex Tester FAQ

What regex engines can I run in this tester?

The tester executes JavaScript RegExp and Python re patterns using their real runtimes. It also generates integration examples for PHP, Java, C#, Ruby, Go, and Rust.

Are my regular expressions and test text uploaded?

No. Patterns and test text stay in your browser. Python mode downloads the Pyodide runtime, but your regex input is processed locally inside a browser worker.

Why can the same regex behave differently in JavaScript and Python?

Regular expression engines use different syntax, flags, replacement rules, Unicode behavior, and feature support. A valid expression in one language may be invalid or behave differently in another.

Does the tester support capture groups and named groups?

Yes. Match details include numbered captures, named captures, source ranges, line numbers, and columns when the selected engine provides that information.

Can I test regex replacements?

Yes. Enable replacement output and enter a replacement expression. JavaScript uses dollar-style references such as $1, while Python uses backslash references such as \1 or named group syntax.

Can a dangerous regex freeze the page?

Regex execution runs in a disposable Web Worker with a hard timeout. If a pattern takes too long, the worker is terminated so the main page remains responsive.

What can I export from the Regex Tester?

You can copy generated code, copy individual matches, copy replacement output, and download complete regex test reports as JSON or plain text.

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Ready to debug your regular expression?

Test JavaScript or Python patterns with isolated execution, highlighted matches, complete captures, replacement output, exports, and multi-language code examples.

Start Testing Regex