Is this a PCRE or ICU regex tester?

No. The website uses the running browser’s ECMAScript RegExp implementation. PCRE and native ICU syntax or behavior cannot be assumed to match.

Why does only the first match appear?

Without g or y, execution uses first-match semantics. Enable the appropriate flag when you want iteration, and consider zero-width behavior when reviewing ranges.

Are match positions byte offsets?

No. JavaScript string ranges use UTF-16 code units. Emoji and some other characters occupy two units; inspect Unicode text separately if your consumer expects bytes or scalar positions.

Text

RegExp Tester

Test JavaScript regular expressions with groups, indices, and replacement output.

Online tools Pricing

Input0 characters

Reference0 characters

Result0 characters

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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.

Run a reproducible example

Inspect named captures and use the same groups in a replacement under the browser’s ECMAScript engine.

Using an example replaces the current input, reference input, operation, and options. Public demonstration data only.

Find two named capture pairs

ECMAScript with g for iteration and d for capture indices.

Input

(?<name>[A-Za-z]+)=(?<count>\d+)

Reference

Ada=17 Lin=2

The report finds Ada=17 and Lin=2 and exposes the name/count groups.

Matches: 2
<name> "Ada"
<count> "17"

Reorder groups in every match

Use g, Replace all, and the named replacement $<count>:$<name>.

Input

(?<name>[A-Za-z]+)=(?<count>\d+)

Reference

Ada=17 Lin=2

The replacement changes each pair to count:name while preserving the separating space.

17:Ada 2:Lin

Continue with a related task

Text Escapes & Unicode Inspector

Inspect code points, normalization, graphemes, and language escape sequences.

Text Diff

Compare two texts by line, word, or character in a local worker.

JavaScript Beautifier & Minifier

Format or compress JavaScript with parser-backed diagnostics.

Repeat the task in DevSexy for Mac

Use the native app for repeated offline work and direct Option shortcuts.

Direct answer

What is RegExp Tester?

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

What can it do?

Match
Replace

How to use RegExp Tester

01
Prepare the input
Put the expression in the first pane and sample text in the reference pane.
02
Choose the operation
Set ECMAScript flags and choose Match or Replace with an explicit replacement.
03
Review the evidence
Inspect capture ranges and test edge cases; do not assume another regex engine behaves identically.

Frequently asked questions

Is this a PCRE or ICU regex tester?: No. The website uses the running browser’s ECMAScript RegExp implementation. PCRE and native ICU syntax or behavior cannot be assumed to match.
Why does only the first match appear?: Without g or y, execution uses first-match semantics. Enable the appropriate flag when you want iteration, and consider zero-width behavior when reviewing ranges.
Are match positions byte offsets?: No. JavaScript string ranges use UTF-16 code units. Emoji and some other characters occupy two units; inspect Unicode text separately if your consumer expects bytes or scalar positions.