Regex Tester & Explainer

A regular expression that almost works is worse than one that plainly fails, because you ship it. The Regex Tester highlights every match inside your sample text as you type, breaks the pattern into labelled tokens, and lists each capture group by name and by number, so “it matched” becomes “it matched this, here, with these groups”.

All 6 JavaScript flags are individual toggles with a plain-English hint on each: g for every match, i for case, m to make anchors line-aware, s to let the dot cross newlines, u for Unicode code points, and y for sticky matching from the last index.

A replace panel sits below the matches, so you can check a substitution before running it anywhere destructive. Named groups work in the replacement string, which is what makes reordering a date a one-line operation. Everything runs against text in the page.

A Real Example: Reordering a Date

The tool opens on the pattern (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})with a replacement of $<day>/$<month>/$<year>. Against a line listing three ISO dates it finds 3 matches, each with 3 named groups, and the replace preview rewrites every one into day-first order. Six more sample patterns are a click away, covering email addresses, URLs, hex colours, IPv4 addresses and the doubled-word check \b(\w+)\s+\1\b, which is the shortest useful demonstration of a backreference there is.

Limits and What Gets Truncated

• Only the first 50,000 characters of your test text are matched, and a notice says so when the rest is being ignored. Highlighting a larger document interactively is what makes a tab stutter.
• Matching stops after 2,000 hits with Stopped after 2,000 matches — narrow the pattern to see the rest. The count is still accurate up to that point.
• Four stat cards report matches, unique matches, capture groups in the first match, and characters actually tested. Comparing the first two is the quickest way to spot a pattern matching the same token repeatedly.
• An invalid pattern is reported with the engine's own message rather than a generic failure, and the previous highlighting is cleared rather than left stale.
• A pattern that can match the empty string will not spin forever. The matcher advances past a zero-length match, which is the guard that keeps something like a* from hanging the page.
• This is the JavaScript engine, not PCRE. Lookbehind is supported in current browsers, but atomic groups, possessive quantifiers and recursion are not, so a pattern lifted from PHP or Perl may need rewriting.