Regex Tester & Playground

Live JavaScript regular expression sandbox: edit your pattern and flags, see matches with highlighting, inspect capture groups, and try find-and-replace — all in your browser.

Free · No signup
Flags

Current flags: gi — engine is ECMAScript RegExp (same family as JavaScript in Chrome, Firefox, Safari, Node).

102 / 120,000 chars

Quick examples

Live highlight

4 matches
The quick brown fox jumps over the lazy dog. DOG repeated: dog vs Dog — pick case-insensitive matches.

Match details

  • #1 dog
    index 40
  • #2 DOG
    index 45
  • #3 dog
    index 59
  • #4 Dog
    index 66

Works great with these tools

After extracting JSON fragments from logs, validate structure with our free JSON formatter.

Regex helps slice CSV rows — convert clean extracts to JSON with our free CSV to JSON converter.

Encode decoded captures for transport testing with our free Base64 encoder.

Scheduling jobs that grep logs? Pair patterns with timing via our free cron expression parser.

Last updated:

How to use Regex Tester & Playground

Use this Regex Tester for a fast regex check — or as a regex playground and regex sandbox for everyday developer work. Edit patterns without wrapping slashes, toggle ECMAScript flags, paste logs or markup into the test string, and instantly see matches highlighted in context. Inspect each match like a lightweight regex debugger — including numbered groups and named captures — then switch to replace mode to validate substitution templates before you paste into JavaScript, Node, VS Code search, or CI scripts.

Step 1: Enter the pattern

Write your regular expression and choose flags (global, case-insensitive, multiline) that match your language runtime.

Step 2: Add sample text

Paste realistic strings you expect to match and reject so you can see capture groups and edge cases live.

Step 3: Tune until matches look right

Adjust the pattern, review highlights, then copy the final expression into your code or validation rules.

Features

  • ECMAScript RegExp engine (same family as JavaScript and Node in your browser)
  • Interactive flags: g i m s u y d v with plain-language hints
  • Match highlighting with alternating colors for scanability
  • Per-match list with indices, numbered groups, and named groups
  • Replace preview using JavaScript replacement substitution rules
  • Quick examples (email, URL, digits, named groups) for fast onboarding
  • Privacy-first: patterns and sample text stay in your browser session

Common use cases

  • Debug validation patterns for forms and APIs before shipping RegExp literals
  • Highlight tokens in logs, stack traces, or CSV exports while drafting parsers
  • Prototype search-and-replace strings for editors and code mods
  • Compare global vs single-match behavior with the g flag toggled
  • Verify named capture groups for routing and URL parsing templates
  • Teach regex basics with live feedback (regex practice without installing tools)

Why use Regex Tester & Playground?

Regex is easy to get almost right and hard to verify mentally. Live testing against real samples prevents false positives in form validation, log parsing, and search filters.

Related on ToolsRacks: JSON Formatter, and CSV to JSON.

Practical tips

  • Start simple, then add anchors and groups once basic matches work.
  • Test empty strings and unicode characters if your users paste them.
  • Document the pattern’s intent in a code comment before you leave the tool.

Frequently asked questions

Common questions people ask before using this tool.

Is this the best free regex checker online for JavaScript?

It is a focused JavaScript / ECMAScript regex tester: you get instant compile errors, live highlighting, groups, and replace preview without installing software. For Python, Java, or .NET-specific dialect quirks, re-test in those runtimes after prototyping here.

Does this regex tester work like PCRE or Python regex?

No — it uses your browser’s native RegExp engine (ECMAScript). Many features overlap with PCRE, but anchors, lookbehind availability, and flag meanings can differ from Python re or PHP PCRE. Treat this as a JavaScript regex sandbox first.

Can I use this as a regex debugger with match highlighting?

Yes. Matching spans are highlighted in the sample text, and each match lists capture groups and named groups when present — useful for debugging why a pattern matched part of a line.

Is there an AI regex generator built into ToolsRacks?

This page does not call external AI models. You can still draft patterns manually with quick examples and iterate safely. For AI-generated expressions from prose, paste outputs here to validate syntax before production use.

How is this helpful for SEO or marketing copy?

You can extract URLs, emails, SKUs, or tracking codes from pasted exports before rewriting meta descriptions or UTMs — faster than trial-and-error in a spreadsheet formula.

What does the replace tester do?

It runs JavaScript-style replacement using your pattern and replacement string, supporting $1-style numbered references and $<name> named references when your groups are set up correctly.

Why did my pattern hang or freeze?

Pathological regex on large inputs can be expensive. This tool caps sample length and the number of matches collected to keep the UI responsive. Split huge files or simplify overly broad quantifiers.

Does ToolsRacks store my regex or text?

No server upload is required — matching runs locally in your browser. Avoid pasting secrets or personal data you would not put into any web page.

Can beginners practice regex here?

Yes. Start from Quick examples, toggle flags one at a time, and watch how highlights change. Pair with the FAQ answers and developer docs for ECMAScript RegExp for deeper learning.

What makes this different from desktop regex apps?

You get a cross-platform regex tester in the browser with zero install, shareable workflow on any OS, and copy-ready JSON of matches for tickets — ideal for quick QA and pair programming.

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