Rewrite Rules: Only treat a leading www. as optional in url_to_postid()

[youknowriad]

url_to_postid() compares the URL's host to the site's host after removing www. from both. The removal is unanchored, so it strips www. from anywhere in the host, not just the prefix. On a site at example.com:

url_to_postid( 'https://exwww.ample.com/?p=16' )   // 16
url_to_postid( 'https://example.cwww.om/?p=16' )   // 16
url_to_postid( 'https://evil.com/?p=16' )          // 0

Both are ordinary registrations (ample.com; cwww.om under Oman's TLD). Since a post ID from this function is widely treated as proof that a URL is internal, a lookalike domain currently passes that test.

Trac ticket: https://core.trac.wordpress.org/ticket/65016

This surfaced while reviewing #12870 on that ticket, which uses url_to_postid() for a trust decision. Worth fixing on its own regardless of what happens to that PR.

Changes

Anchor the removal so only a leading www. is optional, matching the idiom already used in ms-functions.php and ms-blogs.php:

$url_host = preg_replace( '|^www\.|', '', $url_host );

Testing

A data-provided case in tests/phpunit/tests/rewrite.php covers the site host, the www. prefix, both collisions, an unrelated host, and the site host as an attacker subdomain. Reverting only the source change fails exactly the two collision cases.

npm run test:php -- --filter Tests_Rewrite    # 92 tests, 279 assertions

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Research, implementation, and tests; reviewed and edited by me.

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[johnbillion]

Is this an unintentional change? The parameter name is $_blog_id and doesn't need to change.

[johnbillion]

The switch to a regex looks good, thanks. The unintentional change to the $_blog_id parameter in the test isn't needed though.