custom-hooks.php

<?php

/* convert state into region */
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'region' == $params['facet_name'] ) {
        $northwest = array( 'WA', 'OR', 'MT');
        $northeast = array( 'ME', 'VT', 'MA', 'NH' );
        $raw_value = $params['facet_value'];
      
        if ( in_array( $raw_value, $northwest ) ) {
          $params['facet_value'] = 'northwest';
          $params['facet_display_value'] = 'North West';
        } elseif (  in_array( $raw_value, $northeast ) ) {
          $params['facet_value'] = 'northeast';
          $params['facet_display_value'] = 'North East';
        }  
    }
    return $params;
}, 10, 2 );