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.