Regex Tester
Test and debug regular expressions with real-time matching, capture groups, and replacement preview.
About Regex Tester
Regex Tester is a free, browser-based tool that tests and debugs JavaScript regular expressions with real-time matching, capture groups, and replacement preview. It runs entirely in your browser using the native JavaScript RegExp engine, so your patterns and test text never leave your device.
Enter a pattern and some test text, and matches are highlighted instantly as you type. A match-detail table lists each match with its position index and any captured groups, including named groups, which helps when you are building or debugging an expression.
Toggle the g, i, m, s, u, and d flags to control global, case-insensitive, multiline, dotAll, Unicode, and indices behavior. Switch to Replace mode to preview substitutions using $1, $& and similar tokens, and load a built-in preset for common patterns like email, URL, IPv4, ISO date, hex color, HTML tags, Korean phone numbers, or Hangul.
Because it uses the browser's own RegExp engine, behavior matches JavaScript exactly and may differ from PCRE, Python, or Java regex. There is no server processing, no history, and no account; refreshing the page clears your input.
Frequently asked questions
- Is my pattern or test text sent to a server?
- No. All matching and replacement runs locally in your browser with the native JavaScript RegExp engine, and nothing is uploaded or stored on a server.
- Which regex syntax does it support?
- It uses the JavaScript RegExp engine, so syntax and flags follow the JavaScript standard. Patterns written for PCRE, Python, or Java may behave differently.
- How do I use capture groups in a replacement?
- Switch to Replace mode and reference groups with $1, $2, and so on, or use $& for the full match. The result panel shows a live preview of the replaced text.