How to change the search action in wordpress search bar?

There's this addition to the SQL WHERE clause on wp-includes/class-wp-query.php:1306:

<?php// wp-includes/class-wp-query.php:~1306foreach ( $q['search_terms'] as $term ) {    //...    $like = $n . $wpdb->esc_like( $term ) . $n;    $search .= $wpdb->prepare( "{$searchand}(({$wpdb->posts}.post_title $like_op %s) $andor_op ({$wpdb->posts}.post_excerpt $like_op %s) $andor_op ({$wpdb->posts}.post_content $like_op %s))", $like, $like, $like );    // ...

Therefore, I'd hook into the pre_get_posts, and supply the words of the query as explicit "search_terms", since they get added into that clause, with the LIKE modifier just as you said were looking for!

So, we might do something like this:

<?php// functions.phpfunction fuzzify_query(\WP_Query $q) {    if (true === $q->is_search()        && true === property_exists($q, 'query')        && true === key_exists('s', $q->query)    ) {        $original_query = $q->query['s'];        $words          = explode(' ', $original_query);        $fuzzy_words    = array_map(            function($word) {                return '%'.$word.'%';            },            $words        );        $q->query_vars['search_terms'] = $fuzzy_words;        return $q;    }    return $q;} add_action('pre_get_posts', 'fuzzify_query', 100); // Or whatever priority your fuzziness requires!