Regex Tester
Paste your pattern and test string below, and this regular expression tester highlights every match live as you refine your syntax.
About
About Regex Tester
A regex tester gives you a live feedback loop while writing a pattern, instead of running your code repeatedly against sample data just to see if a match actually works. This tool highlights exactly what matched, what didn't, and what's inside each capture group, which turns debugging a pattern from guesswork into something you can actually watch happen.
Certain characters mean something structural in regex rather than representing themselves literally, a period matches any character, a question mark makes the preceding token optional, parentheses create a capture group. Forgetting to escape one of these when you actually want it to match literally, a period in an email address, for instance, is one of the most common reasons a pattern matches more or less than intended.
Form validation is where a sloppy regex becomes an actual security concern, not just a bug. A pattern that's too permissive can let malformed or malicious input through a field meant to catch it. Testing a wide range of valid and deliberately invalid sample strings against your pattern here, before it ships, catches that gap while it's still cheap to fix.
Enter your pattern, add flags like g for global or i for case-insensitive matching, then paste your test string. Every match highlights immediately, and if your pattern includes parentheses to capture specific parts of the match, like the area code inside a phone number, those groups break out separately so you can see exactly what each one captured.
This runs on the same regex engine JavaScript uses natively, which closely mirrors what Python and PHP support too, though lookbehind assertions in particular can behave slightly differently across languages, worth double-checking if your pattern is headed for a different backend. Everything matches locally, so a log file or data sample with real user information in it never leaves your browser.
FAQ
Frequently asked questions
What exactly is a regular expression?
It's a sequence of characters that defines a search pattern, used in code to find, extract, or replace specific pieces of text within a larger string or document, far more flexible than a plain text search.
Why isn't my pattern matching anything at all?
Usually a syntax issue or a missing flag. A common mistake is forgetting to escape a special character, like a literal period or question mark, that regex otherwise interprets as a structural symbol. Build your pattern piece by piece in the tester and add complexity gradually until the match breaks.
Does this tool support capture groups?
Yes, any parentheses in your pattern create a capture group, and the tester highlights and lists each one separately, which makes parsing something structured, like a phone number or a URL, much easier to verify.
Which regex flavor does this tester actually use?
It uses the JavaScript regex engine, the standard across modern browsers. It's very close to what Python or PHP use, with the main differences showing up in more advanced features like lookbehind assertions, worth a quick check if your final pattern is going into a different language.
What does the global flag actually change?
Without it, a regex stops the moment it finds its first match. The global flag tells the engine to keep scanning and return every match in the string instead of stopping at the first one.
Is my test data or pattern saved anywhere?
No, matching happens entirely through client-side JavaScript in your browser. Nothing you paste, including sample data that might contain real user information, gets stored or transmitted to a server.
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