Get internal path from external URL
Dull Ax....

$path='';
$with_trailing_slash=$base_url . '/';

if (strpos($user_input, $with_trailing_slash)===0) {
  $user_input=str_replace($with_trailing_slash, '', $user_input);
  if (strpos($user_input, '?q=')===0) {
    $path=substr($user_input, 3);
  } else {
    $path=$user_input;
  }
} else {
  // this isn't on the local system so there is no way to figure it out
}

if ($path) {
  $normal=drupal_get_normal_path($path);
}