# Regex Tester

> Free online regex tester. Write JavaScript regular expressions and see live highlighted matches, capture groups, flags and a replace preview — instant, no signup.

Test and debug regular expressions live with highlighted matches, capture groups, a replace preview, flag toggles and ready-made patterns for email, URL, IP and more.

- **Live tool:** https://www.toolnova.org/dev-tools/regex-tester
- **Category:** Developer Tools (https://www.toolnova.org/dev-tools)
- **Price:** Free
- **Requirements:** Runs in a modern web browser · No signup · No install

## What this tool does

- Live match highlighting as you type
- All flags: g, i, m, s, u, y
- Capture-group and match-index breakdown
- Replace preview with $1 group substitution
- Preset patterns: email, URL, IPv4, hex, phone
- Built-in cheatsheet, runs 100% locally

## How to use Regex Tester

1. **Enter a pattern** — Type a regex or pick a preset like Email or URL.
2. **Toggle flags** — Click g, i, m, s, u or y to change matching behaviour.
3. **Add a test string** — Paste text and watch matches highlight live.
4. **Replace or copy** — Preview a replacement or copy the finished /pattern/flags.

## Common use cases

- Validate an email or URL pattern before shipping it
- Debug why a regex isn't matching your data
- Extract capture groups from log lines
- Craft a find-and-replace with group references

## About

### See exactly what your regex matches

Regular expressions are hard to debug in your head. This tester runs your JavaScript regex against a test string and highlights every match in place, counts them, and lists each match's index and capture groups. Toggle flags like global, case-insensitive and multiline with a click, and get instant, readable feedback the moment your pattern changes.

### Build, replace and reuse

Start from a preset for common patterns — email, URL, IPv4, hex colour, phone or date — then adapt it to your data. The replace panel previews the result of a substitution with $1, $2 group references, so you can craft find-and-replace rules with confidence. A quick cheatsheet keeps the syntax handy, and everything runs in your browser so your test data stays private.

## FAQ

### What regex flavour does this tester use?

It uses the JavaScript (ECMAScript) regular expression engine, the same one that runs in browsers and Node.js, including flags g, i, m, s, u and y.

### Can I test capture groups?

Yes. Every match lists its index and the value of each capture group, so you can confirm your groups pull out exactly the right substrings.

### How does the replace preview work?

Type a replacement string using $1, $2 to reference capture groups (or $& for the whole match) and the tool shows the replaced text live.

### Is my test data private?

Yes. The regex runs entirely in your browser — your patterns and test strings are never uploaded.

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ToolNova — free, privacy-first browser tools. Most tools process data locally on your device. See https://www.toolnova.org/llms.txt for the full directory.