Year filter from the search query

A good question was asked in the post Natural-language date queries:

Is it possible to extend this so that I can simply add a the year in the search and only results from that year would show?

The answer is yes! Add these functions to your site:

add_filter( 'relevanssi_modify_wp_query', function( $query ) {
    $m = preg_match( '/\b\d{4}\b/', $query->query_vars['s'], $matches );
    if ( 1 === $m ) {
        $year = $matches[0];
        if ( $year > 1900 && $year < 2100 ) {
            global $relevanssi_original_query;
            $relevanssi_original_query = $query->query_vars['s'];

            $query->query_vars['year'] = $year;
            $query->query_vars['s']    = str_replace( $year, '', $query->query_vars['s'] );
        }
    }
    return $query;
} );

add_filter( 'posts_pre_query', function( $a, $b ) {
    global $wp_query, $relevanssi_original_query;
    if ( ! empty( $relevanssi_original_query ) ) {
	    $wp_query->query_vars['s'] = $relevanssi_original_query;
    }
    return $a;
}, 100, 2 );

The code consists of two functions. The first one checks the search query. If there’s a single four-digit number between 1900 and 2100 (change those numbers to whatever is reasonable for your site), the number is removed from the search query and converted into a year parameter.

The second function restores the search query to what it was before the number was removed, so the user doesn’t have to wonder why their search query changed.

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