Regex Tester

• 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.

• 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.

1. Paste your sample text into the input.
2. Enter the regex pattern — either raw, or /pattern/flags from another language.
3. Toggle flags (g, i, m, s, etc.) and click Test Regex — matches are highlighted in the text.
4. Inspect the Groups / Matches tab to see every match, capture group, and position.
5. Switch to Extract for a clean list, Replace for a preview diff, or Split to break the text on the pattern.
6. Open the Performance / Safety tab to see ReDoS warnings before deploying the regex.
7. Export matches as CSV or JSON for downstream work.

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.

• 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.

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.

• Test a regex pattern online

Runs locally in your browser. No uploads. Sample text, pattern, and outputs are never sent to a server.