How to get taxonomy by URL alias?
I know this is a pretty old question but seems unanswered yet.
This Drupal 7 snippet will help you to load a taxonomy term but you need to know the name of the vocabulary it belongs to, lets say it is 'position'.
Ex. www.example.com/foo/6-class/bar

$term_path=drupal_get_normal_path('position/'.arg(1));
$tid = basename($term_url);
$term = taxonomy_term_load($tid);


If you know your url is an alias to your taxonomy term, just use

$term_path=drupal_get_normal_path(current_path());
$tid = basename($term_url);
$term = taxonomy_term_load($tid);


The drupal_lookup_path with source and uri approach did not work for you because probably that path is not an alias, or at least not for your taxonomy.

Hope this helps someone.