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Regex Tester Online — Test, Debug & Validate Regex Instantly

Free online regex tester that runs 100% in your browser. Test patterns, highlight matches, inspect groups, preview replacements, and run multi-test suites for email, URL, and custom regex — no signup, no data upload.

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What is the Regex Tester Online — Test, Debug & Validate Regex Instantly?

Stop pasting production logs into tools that upload your data. This regex tester online runs entirely in your browser using JavaScript's native engine — no servers, no uploads, no accounts.

Built for developers who need to debug patterns fast, it gives you:

  • Real-time highlighting as you type, just like Regex101
  • Capture group inspector for 1,1,2 and named groups (?<name>)
  • Live replace preview with backreferences
  • Multi-test mode — validate email regex, URLs, or custom rules against dozens of cases at once with PASS/FAIL
  • Plain-English explainer for every token
  • ReDoS warnings for risky nested quantifiers like (a+)+

The tester runs patterns locally in your browser and includes multi-test mode.

Perfect for email regex validation, JavaScript form validation, log parsing, data extraction, and learning regex without risking sensitive data.

How it works

  1. Enter your pattern — Type your regex. It's JavaScript flavor, identical to how Chrome and Node.js evaluate it.
  2. Set flags — Toggle g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), y (sticky), d (indices). The tool validates flags instantly.
  3. Choose your mode
  • Match: Enter test text to view matches. Zero-length matches advance the search position to avoid an infinite result loop.
  • Replace: Enter a replacement like $1@$2. See the result update live — ideal for reformatting dates or logs.
  • Multi-test: Add test strings and mark each "should match" or "should not". Great for email regex suites.
  1. Load examples — Pick Email, URL, IPv4, UUID, Hex color, and more from the dropdown.
  2. Read the breakdown — Each token (\d, +, (?<name>)) gets a plain-English description, inspired by Regex101's explainer but running locally.
  3. Watch for warnings — Some nested-quantifier patterns trigger a warning; the check is heuristic and cannot identify every regex that may run slowly.

All processing happens client-side with a 5,000-match safety limit — addressing the top complaint about online testers: privacy and performance.

Examples

Validate email addresses

Test a common email regex against valid and invalid inputs

Pattern: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ Flags: i
 
Matches support@example.com and user+tag@domain.co.uk, fails test@.com

Find all URLs in text

Use global flag to highlight every link

Pattern: https?://[^\s]+ Flags: gi Test: Visit https://example.com and http://test.org
 
2 matches: https://example.com, http://test.org

Reformat dates with groups

Convert MM/DD/YYYY to YYYY-MM-DD using capture groups

Pattern: (\d{2})/(\d{2})/(\d{4}) Replacement: $3-$1-$2 Test: 12/25/2025
 
2025-12-25

Frequently asked questions

Is this regex tester really private?

The tester runs pattern evaluation in your browser. Avoid entering sensitive data unless your organization permits it; local processing alone is not a guarantee of compliance or overall security.

What regex flavor does it support?

The tester uses JavaScript (ECMAScript) regular expressions. Other languages and regex engines may use different syntax or behavior, so verify patterns in the target environment.

Can I test email regex here?

Absolutely. Load the "Email" preset or use ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ with the i flag. Switch to Multi-test mode to check valid emails (support@example.com) and invalid ones (test@.com) at the same time.

Why does my pattern freeze other testers but not this one?

Patterns such as (a+)+b can cause catastrophic backtracking. The tester warns about some nested-quantifier patterns and limits match results, but these measures do not prevent every slow regex from blocking the browser.

How is this different from Regex101 and RegExr?

  • Local evaluation: Test patterns in your browser.
  • Multi-test: Run a set of pass/fail cases.
  • Replace mode: Preview replacements with backreferences.
  • Warnings: See warnings for some potentially risky patterns.
  • No account required: Start testing without signing up.

How do I test multiple strings at once?

Use "Multi-test" mode. Add each string, toggle "✓ match" if it should pass or "✗ no match" if it should fail. The tool runs all cases instantly and shows your pass rate — ideal for validation rules and edge cases.

Does it support capture groups and backreferences?

Yes. Numbered groups ($1, $2) and named groups (?<year>\d{4}) appear below each match. In Replace mode, use $1 or $<year> in your replacement string.

Can I use it offline?

Yes. After first load, the page works offline because everything runs locally. No installation, no Chrome extension requesting broad permissions.

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