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Regex Tester

Test regular expressions against sample text.

Flags

Highlighted matches

3 matches
Order 123 costs $45.67
  • "123"index 6
  • "45"index 17
  • "67"index 20

Regular expressions extract, validate, and transform text patterns, but they are easy to get wrong. Utilitoo's Regex Tester lets you write a pattern, paste sample text, and see matches highlighted in real time so you can refine your expression before shipping it to production code.

Use it when validating email formats, parsing log lines, building search-and-replace rules, or debugging grep commands. QA engineers test edge cases, and backend developers verify input validation rules against tricky unicode and whitespace samples from real user submissions.

Iterate quickly: adjust flags, tweak capture groups, and watch matches update live. When your pattern works, copy it into your application. Pair with Diff Checker to compare pattern versions or JSON Validator when extracting structured data from API responses.

Testing runs entirely in your browser on Utilitoo. Your sample text and patterns are never uploaded. Free, private, and built for developers, data engineers, and power users who work with text patterns daily. Technical writers documenting validation rules and support agents testing customer input patterns both benefit from a sandbox that shows matches without deploying code changes. Saving a working pattern from Utilitoo directly into your codebase reduces trial-and-error cycles that slow down form validation and log parsing tasks.

Regex Tester highlights JavaScript matches in your browser so you can refine patterns and flags on real samples without uploading logs or form dumps.

Common use cases

  • Debugging a validation or log-parse pattern against must-match and must-not samples before shipping.
  • Checking flag behavior (g, i, m, s, u) when a pattern works in one environment and fails in another.
  • Catching greedy captures on quoted or delimited text with two regions in the sample.
  • Smoke-testing a long near-miss string for catastrophic backtracking before it hits a request handler.
  • Prototyping extract patterns, then switching to Extract Emails or Extract URLs for the common case.

Tips & common mistakes

Toggle flags one at a time and keep pattern + flags paired in code review. Prefer negated classes over greedy .* between delimiters. Test a large near-miss sample for backtracking hangs. JavaScript flavor matches browsers and Node - paste the same source string your RegExp constructor will use. Flag, greediness, and ReDoS pitfalls are covered in the blog guide Regex Tester Mistakes That Break Production Patterns.

Common errors

  • No matches - check flags, escaping, and whether the pattern assumes multiline input.
  • Catastrophic backtracking - nested quantifiers can freeze the tab on large inputs; simplify or test a smaller sample.
  • Only the first hit shown - enable the g flag when you expect every match.

How to use

  1. Enter a regex pattern and choose flags such as g, i, or m.
  2. Paste sample text - matches highlight as you type.
  3. Inspect match indexes and capture groups in the list below.

Frequently Asked Questions

How do I test a regular expression online?▼

Enter your regex pattern and flags, paste sample text, and Utilitoo highlights all matches instantly in your browser. For flag, greediness, and backtracking pitfalls before shipping, see the blog guide Regex Tester Mistakes That Break Production Patterns.

What regex flavor does this tool use?▼

Utilitoo uses JavaScript regular expressions, the same engine used in browsers and Node.js, so patterns transfer directly to your code.

Why is my regex not matching anything?▼

Check for missing flags like i for case-insensitive, unescaped special characters, or anchors ^ and $ that limit where matches can start. Common flag and sample mistakes are covered in the blog guide Regex Tester Mistakes That Break Production Patterns.

Can I test regex for email validation?▼

Yes. Paste sample emails and refine your pattern until valid addresses match and invalid ones do not. Prefer a short policy pattern over a giant RFC regex - see the blog guide Regex Tester Mistakes That Break Production Patterns.

Is it safe to test regex against production log data?▼

On Utilitoo, yes. All matching runs locally. Your text and patterns are not sent to any server.

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