Regex Lookaround

Learn lookahead and lookbehind, and which engines support them.

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Workspace actions
Regular expression
Reference

Regex quick reference

^ · $
Start/end; m = lines.
[abc] · \d
Listed character / digit.
* · + · ?
Zero or more, one or more, optional.
(...)
Capture a numbered group.
(?<name>...)
Named capture.
(?=...)
Ahead; lookbehind varies.
i · m · s
Ignore case / line anchors / dot-newline.
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the RegExp of this browser (version not pinned)

Not available in JavaScript · This browser: x · ignore pattern whitespace; a · ASCII-only classes; U · lazy by default; J · duplicate names. u and v cannot be combined.

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Verified Fixture FX-S06

What it does

Lookahead and lookbehind check what is next to a position without including it in the match.

How to use it

  1. Load the example: (?<=ID=)\d+ matches 42 without ID=.
  2. Check the counterexample.
  3. Switch to Go to see that it rejects lookaround.

Verified example

Example

Text "ID=42 code=9"

JavaScript · This browser
(?<=ID=)\d+

Result: "42"

Python re · Local WASM
(?<=ID=)\d+

Result: "42"

PCRE2-8 · Local WASM
(?<=ID=)\d+

Result: "42"

Counterexample

Text "code=9"

JavaScript · This browser
(?<=ID=)\d+

Result: no match

Python re · Local WASM
(?<=ID=)\d+

Result: no match

PCRE2-8 · Local WASM
(?<=ID=)\d+

Result: no match

Recorded by running fixture FX-S06 on : rp.js@1 on Node v24.21.0 (V8), v24.21.0, CPython 3.14.2 (Pyodide 314.0.7), PCRE2 10.48, go1.27.1. JavaScript results come from the rp.js@1 core on Node’s V8; your browser runs the same core.

Engines

Limits