DEV-TRAFFIC-084 · Custom Tool

Regex Tester & Debugger

Run a JavaScript regular expression against test text and inspect match positions, numbered capture groups, and named groups.

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Test text71 characters

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No matches yet.Run the tester to inspect match ranges and capture groups.

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WORKED EXAMPLE

Regex Tester & Debugger example

Offsets are JavaScript UTF-16 string indexes.

Example input

Pattern: (?<word>\b[A-Z][a-z]+\b)
Flags: g
Text: Ada builds tools. Grace reviews them.

Expected output

2 matches
Ada — offsets 0–3 — named group word: Ada
Grace — offsets 18–23 — named group word: Grace

INSTRUCTIONS

How to use Regex Tester & Debugger

1
Enter a JavaScript regular-expression pattern without slash delimiters.
2
Select global, case-insensitive, multiline, dotAll, or Unicode flags.
3
Paste test text and choose Test expression.
4
Inspect matches and capture groups. Correct any inline syntax error and run again.

PURPOSE

What this tool does

The tester constructs the browser's native JavaScript RegExp with the selected flags and enumerates matches in source order.

Each result reports its start and end offsets, full matched text, numbered captures, and named captures when the expression defines them.

SUPPORTED BEHAVIOR

Features and options

JavaScript RegExp dialect
g, i, m, s, and u flags
Numbered and named captures
Match offsets
Unicode property escapes when the u flag is enabled
Safe progress through zero-length global matches

IMPLEMENTATION NOTES

Technical information

Matching behavior comes from the browser's ECMAScript RegExp implementation. Lookahead and lookbehind support therefore follows the current browser engine.

Without the global or sticky flag, JavaScript returns only the first match. Global results are capped at 500 to keep the interface responsive. UTF-16 code-unit offsets are shown, matching JavaScript's string indexing model.

WHEN IT HELPS

Common use cases

Testing validation patterns
Inspecting capture groups
Debugging anchors and flags
Checking Unicode matching
Finding the location of repeated matches

REVIEW BEFORE USING

Limitations

This is not a PCRE, Python, Java, .NET, Go, or Rust regex engine.
Replacement and substitution are not implemented in this tool.
The tester does not estimate ReDoS risk or guarantee that an expensive expression will complete quickly.
Results stop after 500 matches.

NEXT LOGICAL TASKS

AUTHORITATIVE MATERIAL

References & Sources

tc39.es · SpecificationECMAScript Language Specification — RegExp Objects

Defines the JavaScript RegExp object and normative matching behavior used by this browser-local tester.

Verified 2026-08-13 ↗MDN Web Docs · Official DocumentationMDN JavaScript RegExp Reference

Practical reference for JavaScript regular-expression syntax, flags, methods, and browser behavior.

Verified 2026-08-13 ↗

QUICK ANSWERS

Regex Tester & Debugger FAQ

Which regex flavor is supported?

The JavaScript ECMAScript RegExp dialect provided by your browser.

Why do I see only one match?

Enable the g flag to enumerate all non-overlapping matches.

Are lookbehind and Unicode properties available?

They are available when supported by the current browser. Unicode property escapes normally require the u flag.

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