Improve node_autocomplete_validate() code to be less confusing

[NormPlum]

Description of the need

Some of the code in node_autocomplete_validate() is confusing and, if people are reading it to understand how something works, can give them the wrong idea.

The code in question is:

$result = preg_match('/\[([0-9]+)\]$/', $string, $matches);
  if ($result > 0) {
    // If $result is nonzero, we found a match and can use it as the index into
    // $matches.
    $nid = $matches[$result];

if ($result > 0) { seems to imply that $result can be 1, 2, 3, etc. Whereas the PHP manual says:

preg_match() returns 1 if the pattern matches given subject, 0 if it does not, or false on failure.

This is also reinforced by $nid = $matches[$result]; where it seems that whatever the $result value is, that's what you need to use as the key to get the value of the match. However the PHP manual says:

If matches is provided, then it is filled with the results of search. $matches[0] will contain the text that matched the full pattern, $matches[1] will have the text that matched the first captured parenthesized subpattern, and so on.

So you may not always want the first parenthesized match, but that's all you'll ever get if you use $result as the key.

Proposed solution

Here's what I propose changing it to:

$result = preg_match('/\[([0-9]+)\]$/', $string, $matches);
  // If $result equals 1, we found a match.
  if ($result == 1) {
    // Get the first parenthesized match.
    $nid = $matches[1];