api.URLPattern - Firefox changed the regex engine

[eight04]

What type of issue is this?

Incorrect support data (example: BrowserX says "86" but support was added in "40")

What information was incorrect, unhelpful, or incomplete?

In Firefox<149, URLPattern uses rust regex, therefore some syntax e.g. look-around will throw:
https://bugzilla.mozilla.org/show_bug.cgi?id=1998871

This information is not documented.

What browsers does this problem apply to, if applicable?

Firefox

What did you expect to see?

I think the lack of look-around syntax should be documented, or raise the support version from 142 to 149, that Firefox changed the regex engine for better compatibility: https://bugzilla.mozilla.org/show_bug.cgi?id=1984171

Otherwise, the example code will throw in Firefox<149.

Did you test this? If so, how?

Here is a jsbin for testing: https://jsbin.com/tocafidoqa/3/edit?js,console

You should see

true
false
true
false

in fully supported browsers.

Can you link to any release notes, bugs, pull requests, or MDN pages related to this?

• 1998871 - URLPattern doesn't support lookahead?
• 1984171 - Use spidermonkey regex in URLPattern

Do you have anything more you want to share?

No response

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/URLPattern

MDN metadata

MDN page report details

• Query: api.URLPattern
• Report started: 2026-02-20T15:21:50.902Z

[caugner]

Thanks for reporting.

Can you confirm whether Chrome and Safari support lookahead/lookbehind?

Since the URL Pattern Standard itself doesn't mention lookahead/lookbehind, is there any evidence that lookahead/lookbehind is mandatory behavior?

• If Chrome and Safari support this, then we can add a note to the Firefox support statement.
• If the behavior is mandatory, we can either mark Firefox as partial, or create a subfeature (but only if Chrome and Safari did not already support this with the initial support for URLPattern).

[eight04]

Tested with Chrome 95 and 147 on Windows 10, both supports look around.

Can't test safari.