Regex Tester
Build a regular expression and see exactly what it matches. Matches are highlighted in your test text and listed with their positions, and a broken pattern gets a plain-English error instead of silence.
Test a regular expression
The example loaded finds email addresses — change it and test your own.
Your matches will appear here — matches are highlighted in the text and listed with their positions.
Patterns you can trust
Highlighting, positions and honest errors — the three things regex work actually needs.
Four flags built in
Toggle g, i, m and s without editing the pattern, and see how each flag changes the result set.
Match positions listed
Every match is shown with the index where it starts, plus highlighting in the text itself — two ways to verify the same result.
Honest error reporting
Invalid syntax is reported as an error, never dressed up as “no matches” — the two situations are kept strictly separate.
About the regex tester
A regular expression is only trustworthy once it has been tested against real text, and that is the whole job of this page. Paste a pattern, paste some text, and see the matches — highlighted where they occur and listed with their positions, so a pattern that matches too much or too little is caught before it reaches production.
The tester uses JavaScript regular expressions, the flavour built into browsers and Node.js. The four most useful flags are one click away: global, case-insensitive, multiline and dot-all. The example loaded on the page finds email addresses in a sample paragraph, which makes a good starting point — break it, fix it, and watch the match count change.
One warning worth repeating: regex flavours differ slightly between languages. A pattern that works here will usually work in Python, PHP or .NET, but each has small extras and omissions. Treat this tester as the fast first check, and always run a final test in the system where the pattern will actually live.
Frequently asked questions
Flags, flavours and error messages, answered plainly.
What is a regular expression?
A pattern language for finding text. Instead of searching for one exact word, a regex describes a shape — digits, email addresses, dates, repeated words — and finds everything that fits the shape.
What do the flags g, i, m and s mean?
g (global) finds every match instead of stopping at the first. i makes matching case-insensitive. m makes ^ and $ match at line starts and ends. s lets the dot match newline characters too.
Why does my pattern say invalid?
Something in the syntax is malformed — an unclosed bracket or parenthesis is the usual cause. The tester shows the browser’s exact error so you can find the spot quickly.
Which regex flavour does this tester use?
JavaScript, as used in browsers and Node.js. Most patterns transfer to other languages, but small differences exist, so re-test in your target language before relying on a pattern.
Can it test very large texts?
Yes, within reason. Matching stops after 500 matches to keep the page responsive, and the list shows the first 100 — enough to judge whether a pattern behaves.
Is my test data uploaded?
No. The pattern and the test text are processed in your browser only, so you can safely test against real logs, emails or customer-shaped sample data.