How to trim white space in search terms?

I came across the same problem.
In my opinion, this should be the default search behavior in WP.

The solution is to filter the array of parsed query variables.
See the documentation here.

Add this to the functions.php file in your theme directory.

add_filter('request', function ($query_vars) {
    if (!is_admin() && !empty($query_vars['s'])) {
        $query_vars['s'] = trim($query_vars['s']);
    }

    return $query_vars;
});

In WordPress search terms are handled as query variables. This means that the request filter is available to change the variable before the actual database query. Like this:

add_filter ('request', 'wpse308466_filter_search_query');
function wpse308466_filter_search_query ($request) {
  if (!is_admin) { // only do this on the front end
    ltrim ($request['s']); // strip all whitespaces from beginning of string
    rtrim ($request['s']); // strip all whitespaces from end of string
    }
  return $request;
  }