Wikimedia Bugzilla is closed!

Due to the way we do anchor encoding, there's no way to reliably reverse it.  ".3F", for instance, is translated to ".3F", but so is "?".  And both "_" and " " become "_".  It would be nice if anchor encoding were made reversible to avoid unintended conflicts and permit anchor decoding facilities.

Currently we do, roughly

$id = str_replace( ' ', '_', $id );
$id = Sanitizer::decodeCharReferences( $id );
$id = urlencode( $id );
$id = str_replace( '%3A', ':', $id );
$id = str_replace( '%', '.', $id );

This should be

$id = Sanitizer::decodeCharReferences( $id );
$id = urlencode( $id );
$id = str_replace( '_', '%5F', $id );
$id = str_replace( '.', '%2E', $id );
$id = str_replace( '%20', '_', $id );
$id = str_replace( '%3A', ':', $id );
$id = str_replace( '%', '.', $id );

That could then be reversed reliably (to within entity encoding) with

$id = str_replace( '.', '%', $id );
$id = str_replace( '_', ' ', $id );
$id = urldecode( $id );