How to use the Regex Tester & Extractor
Build, test, debug, and explain JavaScript regular expressions against your sample text. The Regex Tester highlights matches, lists capture and named groups, lets you preview replacements safely, splits on a pattern, flags ReDoS risk in your expression, and exports matches as CSV or JSON. Useful for writing log parsers, extracting fields, validating inputs, and learning regex — and it runs entirely in your browser.
What it does
- Tests a regex against sample text with live highlighting.
- Supports JavaScript flags:
g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), y (sticky), d (indices).
- Inspects capture groups, named groups (
(?<name>...)), and per-match line / column positions.
- Extract, Replace (with preview diff), and Split modes.
- ReDoS / performance safety check for catastrophic-backtracking patterns.
- Imports patterns in
/pattern/flags form copied from other languages.
- Exports matches as CSV or JSON and hands off to Line Tools, JSON Formatter, PII Detector, and more.
When to use it
- Build a log-parsing regex and confirm it captures the right fields.
- Extract emails, URLs, IDs, or IPs from a block of text.
- Validate an input pattern (email, phone, postcode) before shipping it.
- Preview a replace operation safely before running it against a real file.
- Debug a regex copied from a config or a third-party rule (Datadog, Logstash, fail2ban).
- Learn regex by stepping through groups and explanations.
How to use it
- Paste your sample text into the input.
- Enter the regex pattern — either raw, or
/pattern/flags from another language.
- Toggle flags (
g, i, m, s, etc.) and click Run — matches are highlighted in the text.
- Inspect the Groups / Matches tab to see every match, capture group, and position.
- Switch to Extract for a clean list, Replace for a preview diff, or Split to break the text on the pattern.
- Open the Performance / Safety tab to see ReDoS warnings before deploying the regex.
- Export matches as CSV or JSON for downstream work.
Capture & named groups
Use plain groups (...), named groups (?<id>...), and non-capturing groups (?:...). The Groups / Matches tab shows every group's value, start/end, and whether it matched. With the d flag, group start/end indices are returned by the engine.
Tips & pitfalls
- Forgotten
g flag is the #1 reason "find all" returns only the first match.
. does not match newlines unless you add the s (dotAll) flag.- Escape regex metacharacters (
. + * ? ( ) [ ] { } | \) when matching them literally.
- Avoid catastrophic backtracking:
(a+)+, (.*)+, and similar nested quantifiers — the Safety tab flags these.
- For structured data (JSON, XML), use JSONPath or XPath — regex is fragile against structure changes.
- JavaScript regex differs from PCRE / Python / Java around lookbehind, recursion, and certain shorthands.
ReDoS & performance safety
Some regex shapes — nested quantifiers like (a+)+, repeated groups like ([a-z]+)*, ambiguous alternation, leading .* with backtracking — can cause exponential matching time. The Performance / Safety tab flags these heuristically. The engine also caps matches and execution time so a risky pattern cannot freeze the browser.
FAQ
- Does this upload my text? No — JavaScript
RegExp runs entirely in your browser.
- Which engine is used? Your browser's JavaScript
RegExp. Behavior may differ from PCRE, Python, Java, .NET, or RE2 — especially around lookbehind, named groups, and backtracking.
- How do I extract all matches? Add the
g flag and run, or use the Extract tab.
- How do I preview replacements safely? The Replace tab shows replaced output and a colored before/after diff before you copy it out.
- Why does dot not match newlines? JavaScript's
. excludes line terminators unless you set the s (dotAll) flag.
- Can I export matches as CSV or JSON? Yes — use the Exports tab or the Extract tab.
- Why does my pattern not match the expected substring? Common causes: missing
g, missing i, missing s for multi-line matching, and forgotten escaping of literal characters.
- When should I use JSONPath / XPath instead? For structured documents (JSON / XML), prefer JSONPath or XPath — regex is fragile against structure changes.
- What is ReDoS? Regular-expression Denial of Service — input crafted to trigger exponential backtracking. The Safety tab flags patterns vulnerable to it.
Runs locally in your browser. No uploads. Sample text, pattern, and outputs are never sent to a server.
