What is RegExp Tester?

Test a regular expression with the browser’s real JavaScript engine, inspect match spans and captures, preview replacement tokens, and isolate runaway work in a replaceable Worker.

What can RegExp Tester do?

RegExp Tester supports Match and Replace. Each mode is available directly in the page workspace.

Does RegExp Tester upload my data?

RegExp Tester processes your input locally in the browser. DevSexy does not add an upload, account, or server-processing step to this workflow.

JavaScript RegExp Tester — Matches, Groups, Indices and Replace Preview

JavaScript engine tape trace · flavor declared

A match is a span, state, and engine decision.

Patterns do not have one universal meaning. This bench follows the running browser’s ECMAScript RegExp semantics and reports indices, captures, flags, and replacement behavior as execution evidence.

ONLY

PATTERN/(?<proto>https?):\/\/(?<host>[^/:\s]+)(?::(?<port>\d+))?/dguINPUTVisit https://example.com:8080/docs next.

5·

10s

11:

12/

13/

14e

15x

16a

17m

18p

19l

20e

21.

22c

23o

24m

25:

268

270

288

290

30/

31d

32o

33c

34s

MATCH 0 · [6, 35)proto · [6, 11)host · [14, 25)port · [26, 30)

Flag state machine

Flags change meaning and iteration—not decoration.

Indices

Adds start/end spans for the match and captures

Global

Repeats and mutates lastIndex

Ignore case

Uses the engine’s case-folding rules

Multiline

^ and $ also see line boundaries

Dot all

. can include line terminators

Unicode

Code-point-aware parsing and escapes

Unicode sets

Set operations and string properties

Sticky

Must match exactly at lastIndex

Capture ledger

Unmatched is different from an empty match.

0 · full[6, 35)https://example.com:8080/docsparticipated

1 · proto[6, 11)httpsparticipated

2 · host[14, 25)example.comparticipated

3 · port[26, 30)8080participated

4 · optional—undefinedunmatched

Native replacement proof

The template has its own language.

ORIGINALVisit https://example.com:8080/docs next.

TEMPLATE$<proto>://$<host>$<path>

RESULTVisit https://example.com next.

$& · full match$1 … $99 · numbered capture$<name> · named capture$$ · literal dollar$` / $' · prefix / suffix

Backtracking isolation chamber

Responsive UI is not proof of a safe pattern.

/^(a+)+$/aaaaaaaaaaaaaaaaaaaaaaaaaaaa!

A run owns a dedicated Worker. Starting another run terminates the previous one, so the main page can keep responding. Production safety still depends on input distribution, engine, pattern structure, and resource policy.

RegExp Tester