Regex Tester & Debugger Online Free

About Regex Tester

Test, debug, and build regular expressions in real time with our free online regex tester. Pattern and test string are evaluated live as you type, with matches highlighted and capture groups extracted. Works with full JavaScript regex syntax including lookaheads, lookbehinds, and named captures. **What this tool gives you** - **Live match highlighting**: every match in your test string is highlighted as you type - **Capture group extraction**: numbered groups are listed per-match - **Flag toggles**: click buttons for global (g), case-insensitive (i), multiline (m), dotAll (s) - **Regex syntax errors**: invalid patterns show the exact error message - **Match position reporting**: each match includes its character index - **Non-blocking**: handles huge test strings without freezing **Supported regex flags** - **g (global)**: find all matches, not just the first - **i (case-insensitive)**: ignore case when matching - **m (multiline)**: ^ and $ match line breaks, not just start/end of string - **s (dotAll)**: the `.` character matches newlines too - **u (unicode)**: interpret pattern as Unicode code points - **y (sticky)**: match only from the exact `lastIndex` position **Common regex patterns** - **Email:** `[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}` - **URL:** `https?:\/\/[\w.-]+(?:\/[\w./?=-]*)?` - **Phone (US):** `\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}` - **IPv4:** `\b(?:\d{1,3}\.){3}\d{1,3}\b` - **Hex color:** `#(?:[0-9a-fA-F]{3}){1,2}\b` - **ISO date:** `\d{4}-\d{2}-\d{2}` - **Whitespace collapse:** `\s+` **Why regex is everywhere** Regular expressions are in validation libraries, search engines, log parsers, code editors, and every modern language's standard library. Good regex skills save hours every week. Bad regex. Unbounded backtracking, catastrophic complexity. Can freeze production. This tool helps you iterate fast and spot problems before they ship. **How this compares to regex101 / regexr** Both are excellent tools. 3STF's regex tester is smaller, faster, and has no ads. It uses the browser's native RegExp engine, so the behaviour is identical to what your JavaScript code will see. No server round-trip, no login, no tracking. **Gotchas** - **Escape backslashes**: in a regex literal, `\d` is a digit, but when typing into a form field, you only type `\d` once - **Avoid catastrophic backtracking**: patterns like `(a+)+` can hang on pathological inputs - **Lookbehind support**: modern JavaScript supports `(?<=...)` but older browsers may not - **Named captures** use `(?<name>...)`, accessible via `match.groups.name` **Privacy** Your pattern and test string are processed via the browser's built-in RegExp engine. No server involved. Nothing logged. Nothing transmitted.