wp-includes/pomo/translations.php:302Returns the nplurals and plural forms expression from the Plural-Forms header.
$headerstringarray{0: public function nplurals_and_expression_from_header( $header ) { if ( preg_match( '/^\s*nplurals\s*=\s*(\d+)\s*;\s+plural\s*=\s*(.+)$/', $header, $matches ) ) { $nplurals = (int) $matches[1]; $expression = trim( $matches[2] ); return array( $nplurals, $expression ); } else { return array( 2, 'n != 1' ); } }Introduced in 2.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/pomo/translations.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.