WP search exclude last letter from string

You can use the get_search_query filter.

function mySearchFunction($search_term){

    $search_term = substr($search_term, 0, -1);

    return $search_term;

}

add_filter('get_search_query','mySearchFunction');

EDIT

Action to filter query results:

function mySearchFilterFunction($query) {

 if ( !is_admin() && $query->is_main_query() && $query->is_search) {

    $search_term = $query->get('s');
    $query->set('s', substr($search_term, 0, -1));

 }

}
add_action( 'pre_get_posts', 'mySearchFilterFunction');

get_search_query filter is not needed because it uses get_query_var( 's' ) which is modified by our pre_get_posts action.