Regex Tester & Debugger

The tester evaluates your regex with every keystroke, highlighting matches in the test string and listing capture groups separately. Each of JavaScript's six regex flags has distinct behavior: the `g` flag makes a pattern stateful (lastIndex advances between matches, which matters when you use the same RegExp object for multiple tests), the `i` flag makes matching case-insensitive using Unicode case folding when the `u` flag is also set, the `m` flag changes `^` and `$` to match at line boundaries rather than only at string boundaries, the `s` flag makes `.` match newlines (without `s``.` matches everything except `\n`), the `u` flag enables proper Unicode handling including surrogate pair support for characters outside the BMP, and the `y` flag performs sticky matching anchored at lastIndex.

Capture groups come in three forms. Numbered groups `(...)` are the classic form, referenced by `$1``$2`etc. in replacements. Named groups `(?<name>...)` are referenced by `$<name>` and make complex patterns dramatically more readable, a date-parsing regex using named groups like `(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})` is far clearer than one relying on position alone. Non-capturing groups `(?:...)` group atoms for quantifiers without allocating a capture slot, which matters for performance on patterns used in tight loops. The tester displays each capture group separately with the captured substring and its position, making it easy to verify that group boundaries are where you expect.

Common patterns that trip up regex writers: matching email addresses is famously harder than it looks (the RFC 5322 grammar for valid email is about 6,400 characters as a regex, so most validators use a simplified approximation that accepts some invalid addresses and rejects some valid ones). Matching URLs has the same problem plus the complication of internationalized domain names. Matching HTML with regex is actively harmful because HTML is not a regular language, use a real parser. For CSV parsing, regex can handle simple cases but falls over on embedded commas, quoted strings, and multiline fields, so use a CSV library. The tester is best used for validating specific field formats where the grammar is actually regular: phone numbers, postal codes, semver strings, hex color codes, and log-line patterns where the source format guarantees regularity.