Regex Tester

What does this regex tester do?

It runs your regular expression against your text live and shows you three things most testers make you hunt for: what matched, what each part of your pattern means, and what a find-and-replace would produce.

• Live match highlighting as you type, with every capture group listed.
• A plain-English explanation that breaks your pattern into pieces and labels each one.
• Replace preview so you can see find-and-replace before running it in code.
• A built-in cheat sheet and ready-made patterns for email, URLs, IPv4, dates, hex colours and more.
• A share link that reopens your exact pattern, flags, text and replacement.

Everything runs in your browser on the native regex engine. Nothing you type is uploaded.

How do I use it?

Write the pattern in the top field. The slashes and the flag list around it show you the full expression, exactly as you would write it in code.

Toggle the flags you need with the buttons. Hover any of them for a one-line reminder.

Paste your text below. Matches highlight instantly and the capture groups appear underneath.

Read the explanation to see what each token does. This is the part that turns "it doesn't work and I don't know why" into "ah, that quantifier is greedy."

Replace or share when you are done: preview a find-and-replace, or copy a link that reopens the whole thing for a teammate.

Which regex flavour is this, and why it matters

This is a JavaScript regex tester. It runs your pattern on the browser's own engine, which is the same engine your JavaScript and Node code will use, so what you see here is what you get there.

We are deliberate about this. Tools that claim to test Python, PCRE, Go and .NET usually run those engines on a server. Running them in your browser would mean shipping a heavy engine that would slow the page down, and speed is not something we trade away. So this tester is honest about being JavaScript rather than pretending to be seven languages it cannot actually run.

Most regex syntax is shared across languages, but a few things differ, and they are the ones that bite:

• Lookbehind (?<=...) is supported in modern JavaScript but was missing for years and still varies elsewhere.
• Named groups use (?<name>...) in JavaScript and Python, but (?P<name>...) in older Python.
• Escaping rules for things like \d inside character classes differ subtly between engines.

If your target is Python or PHP, use this to get the logic right, then check the flavour-specific syntax in that language's docs.

Greedy vs lazy, the mistake everyone makes

A quantifier like * or + is greedy by default: it matches as much as it possibly can, then hands characters back only if the rest of the pattern would otherwise fail.

Take <.*> against <a> <b>. You might expect it to match <a>, but greedy .* grabs everything up to the last >, so it matches <a> <b> in one go. Add a question mark to make it lazy, <.*?>, and it stops at the first >, matching <a> and <b> separately.

This single distinction is behind a huge share of "my regex matches too much" problems. The explanation panel labels lazy quantifiers explicitly so you can see which behaviour you are getting.

What are capture groups for?

A capture group is parentheses around part of your pattern that saves what it matched, so you can reuse it.

• Numbered: (\d{4})-(\d{2}) captures the year as $1 and the month as $2.
• Named: (?<year>\d{4}) captures the same thing as $<year>, which is easier to read in a long pattern.
• Non-capturing: (?:...) groups for precedence without saving, which keeps your group numbers clean.

In replace mode you reference these as $1 or $<year> to rearrange and reformat text. Reversing First Last to Last, First is just (\w+) (\w+) replaced with $2, $1.

What this tool does not do

• It does not run Python, PCRE, Go or .NET. It is a JavaScript engine, stated plainly. Use it for the logic, then confirm flavour-specific syntax in your language.
• It does not save a personal library of your patterns. There is no account. Use the share link or copy the pattern to keep it.
• It does not generate a regex from a description. You write the pattern; the tool tests and explains it. It is a tester, not an AI generator.
• It does not validate that a pattern is "correct" for your data. A regex that matches your sample can still miss edge cases. Always test against your real inputs.
• The ready-made patterns are starting points, not law. No single regex perfectly validates every real email or URL. Adapt them to your case.

Is my data private?

Yes. The tester runs entirely client-side on the native regex engine. Your pattern and test string are never uploaded, logged, or stored, and they clear when you close the tab. Even the share link carries the data in the address rather than on a server, and the page keeps working offline once it has loaded.