Regex Tester - Test Regular Expressions Online

Test regular expressions, flags, and capture groups directly in your browser.

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Local-first
AI optional

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Regex Tester runs your pattern against sample strings and highlights matches, flags, and capture groups before you commit code.

Use it to debug email, URL, and ID validators, or to see how global and case-insensitive flags change results.

Everything evaluates locally. Use synthetic test data instead of production customer records when possible.

Common uses for Regex Tester

Typical tasks this tool is built for.

Validate patterns for emails, URLs, IDs, and phone numbers.
Inspect how flags change regex matching behavior.
Test capture groups before using them in parsing code.
Debug regular expressions during development.

How to use Regex Tester

Step 1. Enter your regular expression pattern in the pattern field.
Step 2. Paste representative sample text — include edge cases like empty lines or URLs.
Step 3. Enable the flags your production code will use (g, i, m, etc.).
Step 4. Review matches and capture groups, then copy the working pattern into your project.

Why use this tool?

Test patterns against sample strings before adding them to application code.
Toggle flags such as global and case-insensitive to see how matches change.
Runs locally — test strings and patterns stay in the browser.

Privacy and formats

Regex matching runs locally in your browser. Patterns and sample text are not uploaded.

Input: TextOutput: Processed text

Best results with Regex Tester

Practical tips before you download or share the output.

Be careful with very complex regular expressions on very large input text, as they can become slow to evaluate.

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Common problems this tool helps with

Situations where this workflow saves time.

Catastrophic backtracking — very nested quantifiers on long strings can freeze the tab; simplify the pattern.

Expecting multiline ^/$ behavior — enable the multiline flag when testing line-by-line rules.

Email validation regexes — use simple checks in apps; full RFC-style patterns are long and brittle.

Different engines — JavaScript regex may differ from PCRE; test in the environment you ship.

Frequently asked questions

Is my test data uploaded?

No. Matching runs locally in your browser.

Why are there no matches?

Check escaping (backslashes), flags, and whether you need ^/$ anchors. A dot does not match newlines unless dotall mode is enabled.

Can I test replace operations?

Use the replace field when available to preview substitutions before coding them.

Should I paste customer PII here?

Use synthetic samples when possible. Local processing helps, but shared devices still pose a risk.

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How to use Regex Tester

Step 1. Enter your regular expression pattern in the pattern field.

Step 2. Paste representative sample text — include edge cases like empty lines or URLs.

Step 3. Enable the flags your production code will use (g, i, m, etc.).

Step 4. Review matches and capture groups, then copy the working pattern into your project.