Regular expression tester
Test your JavaScript regular expressions live: matches highlighted, captured groups (including named ones), flags and a preview of the replacement, with a syntax reminder. The g, i, m and s flags each change what matches, and a named group reads far better than $1 in a replacement. The expression runs on your device, so a pattern you are testing never leaves your machine.
- 100% local
- Free · no account
- Instant result
This tool runs entirely in your browser — nothing is sent to a server.
Regular expressions are the most powerful tool for searching, validating or transforming text — and the least readable. A few symbols (., *, +, ?, ( ), [ ]) allow you to describe whole families of strings, from an e-mail address to a complete JSON.
This tester highlights all matches in your text live, lists each result with its position and its captured groups — including named groups — and previews the replacement. Syntax errors are flagged as you type, and a cheat sheet recalls the common symbols.
How it works
Type the figures or drop the file you want to process — the tool checks them as you go.
Everything is computed directly in your browser. Nothing is sent to a server.
The result appears instantly, with the detailed breakdown ready to copy, download or keep.
Why use this tool
No account, no download, no limit: the result appears as soon as you enter your data.
Every rule, algorithm and check reflects what we use ourselves every day at Flexina.
The calculations follow the official standards, and the step-by-step detail lets you double-check the outcome.
What does running it locally bring you?
Files, figures and documents never leave your device: nothing is stored, nothing is sent.
Once the page is loaded, you can cut the network: the tool keeps working.
No upload, no account, no third-party server in between: results stay fast and fully private.
Frequently asked questions
What are the g, i and m flags for?
The g (global) flag searches all occurrences, not only the first; i makes the search case-insensitive; m makes ^ and $ match the start and end of each line rather than those of the whole text. The s flag additionally lets the dot match line breaks.
How do capture groups work?
Parentheses capture a portion of the text, reusable in the replacement with $1, $2, etc. You can also name them with the (?<name>…) syntax and reuse them via $<name>, which makes the expression much more readable.
Is the syntax the same in all languages?
The basics are common, but the details differ. This tool uses your browser’s JavaScript engine: the expressions tested here will work identically in JavaScript, and most often in PHP or Python, with a few particularities.