Regex Tester

A regular expression describes a pattern of text — not a specific string, but a shape that many strings can match. /\d{4}-\d{2}-\d{2}/ doesn't match one date, it matches the *shape* of a date: four digits, a dash, two digits, a dash, two digits. That's what makes regex powerful for validation and extraction, and also what makes it easy to write a pattern that matches more (or less) than you intended.

Flags change how the pattern is applied, not what it matches: g finds every match instead of stopping at the first, i ignores letter case, m changes what ^ and $ anchor to, s lets . match newlines too. This tool highlights every match directly in your text and lists each match's capture groups, so you can see exactly what a pattern actually caught — including the parts you didn't mean to catch. Everything runs locally in your browser.

• •Turn on the g (global) flag to see every match, not just the first — its absence is one of the most common sources of "why did it stop after one result" confusion.
• •Named capture groups (?<year>\d{4}) are almost always more maintainable than positional ones ($1, $2) — they survive you reordering or adding groups later without breaking every reference to them.
• •A pattern that matches too much is usually missing an anchor (^, $) or is too greedy — try a more specific character class or a lazy quantifier (*?, +?) before reaching for a completely different pattern.
• •Very long test strings combined with certain nested-quantifier patterns (e.g. (a+)+b) can make matching slow due to backtracking — if a test hangs, simplify the pattern or shorten the test string first.