Regex Tester

A regex tester that lets you test JavaScript regular expressions against sample text to see matches, indices, and capture groups in real-time.

This tester runs your pattern against any text and reports every match: what matched, at which character index, and what each capture group contains. Regular expressions are the power tool of text processing — and famously easy to get subtly wrong. The difference between a pattern that works and one that corrupts data in production is usually one escape or one greedy quantifier, which is why experienced developers never ship a regex they haven't tested against realistic samples.

It uses JavaScript's regex engine, so what works here works in Node.js and every browser (and mostly everywhere else — the core syntax is shared across languages). All the standard flags are supported: i for case-insensitive, m to make ^ and $ match line boundaries, s to let the dot cross newlines, u for proper Unicode handling. Matches are always listed globally so you see every occurrence, and capture groups — the parenthesised sub-patterns you'll use to extract or reorder text — are printed per match, which is where extraction bugs actually hide.

A sound testing habit: paste text that includes both strings that should match and near-misses that shouldn't. A pattern for 6-digit Indian PINs should match 110001 but reject 11000 and 1100011 — testing only the happy case is how catastrophic replacements happen. Invalid patterns return the engine's actual error message rather than a shrug, and everything runs locally, so testing against production log excerpts is safe.