php - Nginx root inside location not working

I am using nginx 1.8 and knowing about the alias/try_files bug i wrote the following nginx conf:

root /var/www/
location  /foo/ {
   root /var/www/projects/bar/public/;
    try_files $uri $uri/ index.php;

}
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in which i get a 404, but if i write the following way (which should point to the same location)

  root /var/www/
  location   ~ ^/foo/(.*)$  {
    try_files /projects/bar/public/$uri /projects/bar/public/$uri/ /projects/bar/public/index.php;

 }
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i get the contents. Why the other root seems not to be applyied?