What is a regex tester and why do you need one?
A regular expression (regex for short) is a tiny language for describing patterns in text. Things like "any email address", "a 16 digit credit card number", "a date written as 2026-05-13" or "the word color, spelled either color or colour". You write the pattern, the regex engine finds every place it shows up in your text.
The problem: regex is famously hard to get right on the first try. One character off and the whole thing breaks, or worse, it silently matches things you did not want. You add a parenthesis, you forget to escape a dot, you switch flags and suddenly nothing matches.
This tester lets you write a pattern on the left, paste your text on the right, and see the matches highlighted live. Switch to Replace mode and watch the new text appear with $1, $2 backrefs. Switch to Split mode and see your text broken into parts. Plus a pattern library with battle-tested expressions for emails, URLs, UUIDs and more, and a cheat sheet for the syntax you keep forgetting.
How to use it
- Type your pattern in the top-left field. Do not type the surrounding slashes (`/.../`), the tester shows them visually around your input.
- Pick flags from the chips: `g` (global, find all), `i` (ignore case), `m` (multiline), `s` (dot matches newlines), `u` (unicode), `y` (sticky). For most use cases you want `g` and often `i`.
- Paste your text in the top-right field, this is what the pattern searches through.
- Watch the matches highlight in the text and appear in the list below, with their start position and any capture groups expanded.
- Switch to Replace mode (segmented bar at the top) to add a Replacement field. Use `$1`, `$2` to refer to capture groups, `$&` for the whole match.
- Switch to Split mode to use the regex as a delimiter, the result is the array of pieces between matches.
- Click any item from the Pattern library to load a ready expression for email, URL, UUID, IPv4 / IPv6, ISO date, US ZIP, credit card, username, strong password and more.
- Stuck on syntax? Open the cheat sheet at the bottom: character classes, quantifiers, anchors, groups, lookarounds, escapes, all in one place.
When this is useful
Six common moments when a live regex tester saves you from copying half-broken patterns off Stack Overflow:
- Validating a form field. You need to accept email addresses in a sign-up form. You grab the Email pattern from the library, paste a few sample inputs to see edge cases (plus-addressing, dotted local-parts), tweak the pattern, then paste it into your code with confidence.
- Cleaning up messy data. A CSV has phone numbers in twelve different formats. You write a pattern that matches all of them, switch to Replace mode, replace with a canonical format using capture groups. One pass, one consistent column.
- Searching a log file. You need every line that looks like an IP plus a 4xx status. You paste a chunk of the log, type the pattern, see exactly which lines match. Faster than scrolling.
- Extracting parts of a string. You have a list of filenames like `2026-05-13-meeting-notes.md` and you want the date and the slug separately. Capture groups: `(\d{4}-\d{2}-\d{2})-(.+?)\.md` gives you both pieces.
- Splitting structured text. You have one big string with items separated by commas, semicolons or pipes (no consistency). Split mode with `[,;|]\s*` gives you a clean array.
- Learning regex without fear. You read about lookaheads in a tutorial, you want to play. You paste sample text, try `(?=\d)` versus `(?!\d)`, see the difference in matches instantly. No setup, no project, just a sandbox.