Free Regex Tester & Builder Tool Online
Test and debug regular expressions with live match highlighting, flag support, and instant results. Ideal for JavaScript, Python, and PCRE patterns.
IN: Pattern & text · OUT: Highlighted matches · RUNS: 100% in your browser
Frequently asked questions
What regex flavor does this tool use?+
It uses JavaScript's native RegExp engine, the same one used in browsers and Node.js. Syntax may differ slightly from PCRE, Python's re module, or POSIX regex in edge cases like lookbehind support in older engines.
What do the g, i, m, and s flags mean?+
g (global) finds all matches instead of stopping at the first; i (case-insensitive) ignores letter case; m (multiline) makes ^ and $ match line boundaries; s (dotAll) makes . match newline characters too.
Why are my matches not highlighting?+
Check that your pattern doesn't have a syntax error (shown in red above the test string) and that the global flag is effectively applied — the tool automatically searches globally even if you don't select 'g', so every occurrence should highlight.
Can I test capture groups?+
The highlighter shows the full match. For inspecting individual capture groups, use browser DevTools console with the same pattern, since this tool focuses on match highlighting for quick pattern iteration.
Is my test data sent to a server?+
No, pattern matching runs entirely client-side using the browser's built-in regex engine. Your text never leaves your device.
Why does my pattern with a catastrophic backtracking risk seem slow?+
Certain regex patterns (like nested quantifiers) can cause exponential backtracking on some inputs. If a pattern hangs, it's usually the pattern itself, not this tool, and it's worth simplifying it (e.g. avoiding nested `(a+)+`).