Regex Tester
Paste your text, type a regular expression, and inspect every match — its character positions and the value and span of every capture group, both numbered and named. Toggle case-insensitive, multiline and dot-all modes. Runs in your browser; nothing is uploaded.
About this tool
Regex Tester runs a regular expression against a block of text and gives you a full, structured breakdown of what it matched — the way a regex debugger does. For every match it reports the character start/end position, and within each match it lists the value and span of every capture group, both numbered and named.
- Match positions: each match is reported with its 0-based character offsets, so you can see exactly where in the text it landed (Unicode-aware — positions count characters, not bytes).
- Capture groups: numbered groups
(…)and named groups(?<name>…)are both broken out, with their own value and span. Optional groups that did not participate are shown as (no match) rather than silently dropped. - Match flags: tick Ignore case for case-insensitive matching,
Multiline so
^and$anchor at the start and end of each line, and Dot matches newline so.also spans line breaks.
The syntax is the Rust regex flavour — a clean,
linear-time engine with no catastrophic backtracking, so even pathological
patterns stay fast.
Everything runs locally in your browser via WebAssembly — your text and your pattern are never uploaded.
Handy for
- Debugging why a pattern matches more or less than you expected.
- Checking that named and numbered capture groups grab the right sub-strings.
- Confirming match positions before wiring a regex into code.
Looking to pull matches out instead?
If you just want the list of matches (or a single capture group) rather than the full positional breakdown, use the companion Regex Extract tool.
FAQ
Why is my lookahead or backreference rejected?
The engine is the Rust regex flavour, which deliberately has no
lookahead/lookbehind and no backreferences — that's what guarantees
linear-time matching with no catastrophic backtracking. Patterns using
(?=…), (?!…) or \1 fail with a syntax error; usually you can
restructure with alternation or capture groups instead.
Are the match positions byte offsets or character offsets?
0-based character offsets, Unicode-aware — é or a CJK character counts
as one position, not two or three bytes. That means the numbers line up with
what you'd get from JavaScript's String.prototype.slice on most text, not
with raw UTF-8 byte indices.
What does “(no match)” next to a capture group mean?
The group is optional (e.g. (\d+)? or one arm of an alternation) and did
not participate in that particular match. It's reported explicitly instead of
being silently dropped, so group numbering stays stable across matches.
How do the three flag checkboxes map to regex flags?
Ignore case is i; Multiline is m, making ^/$ anchor at every
line instead of just the whole text; Dot matches newline is s, letting
. cross line breaks. They can be combined freely, and an empty pattern is
rejected rather than matching everywhere.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool regex-tester "Paste the text to test against…" 'pattern=e.g. (?<year>\d{4})-(\d{2})-(\d{2})'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/regex-tester/?text=Paste%20the%20text%20to%20test%20against%E2%80%A6&pattern=e.g.%20%28%3F%3Cyear%3E%5Cd%7B4%7D%29-%28%5Cd%7B2%7D%29-%28%5Cd%7B2%7D%29&ignore_case=true&multiline=true&dotall=trueMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.