# Regex Tester

Canonical URL: https://offlineutils.com/regex-tester
Category: Developer
Published: 2026-06-24
Publisher: offlineutils.com
Runs entirely in the browser; nothing entered into the tool is uploaded.

> A Regex Tester evaluates a regular expression against a sample string in real time, highlighting every match and capture group, all using the browser's native RegExp engine so your pattern and test data never leave the page.

Type a pattern in the slash-delimited box, set flags (g, i, m, s, u, y) with toggle buttons, and paste a test string. Matches are highlighted inline and listed with their index and any capture groups in a table. The tester also surfaces the SyntaxError message verbatim when a pattern is invalid so you can fix it immediately.

## Features

- Real-time match highlighting against your test input
- All ECMAScript flags exposed as toggle buttons (g, i, m, s, u, y)
- Capture groups and named groups listed in a sortable table
- Surfaces SyntaxError messages verbatim for broken patterns
- Debounced input for smooth typing on megabyte-scale test strings

## Use cases

- Authoring a validation regex for a form field
- Extracting structured data out of log lines
- Debugging why a production regex never matches
- Translating a regex you found in a Stack Overflow answer into something you understand

## How to use it

1. **Enter your pattern** — Type the regex between the slashes. Toggle any flags (g, i, m, s, u, y) as needed.
2. **Paste a test string** — Paste any text into the test input — matches highlight live as you type.
3. **Inspect captures** — Each match and its capture groups appear in a table you can copy individually or all at once.

## When to use it, and when to use something else

- **vs regex101.com** — use Regex Tester when you want a pattern tested 100% locally with no third-party logging. Use regex101.com when you need PCRE / Python / Go regex flavors or detailed step-by-step explanations.
- **vs Browser console RegExp** — use Regex Tester when you want match highlighting and capture-group tables for free. Use Browser console RegExp when you only need to test one or two simple patterns.

## Frequently asked questions

### Which regex flavor does this support?

ECMAScript / JavaScript regex via the native RegExp constructor. That means PCRE-only features (lookbehind back-references, named groups with PCRE syntax, possessive quantifiers) won't work, but lookahead, lookbehind, named groups, and Unicode property escapes (with the u flag) do.

### Does it work with very long input?

Yes. We use matchAll with the g flag to enumerate matches lazily, and we debounce the input by 150ms so typing stays smooth even with megabyte-scale test strings.

### How do I match across newlines?

Enable the s (dotAll) flag — that makes the dot metacharacter match newline characters. The m flag changes the meaning of ^ and $ to anchor on each line rather than the whole string.

### Can I copy the matches?

Yes. Each match row has a copy button, and you can copy the full match list as JSON.

## Related tools

- https://offlineutils.com/text-diff
- https://offlineutils.com/json-formatter
- https://offlineutils.com/case-converter