Choosing just the alphanumeric words with regex

Try this regular expression:

\b([a-z]+[0-9]+[a-z0-9]*|[0-9]+[a-z]+[a-z0-9]*)\b

Or more compact:

\b([a-z]+[0-9]+|[0-9]+[a-z]+)[a-z0-9]*\b

This matches all words (note the word boundaries \b) that either start with one or more letters followed by one or more digits or vice versa that may be followed by one or more letters or digits. So the condition of at least one letter and at least one digit is always fulfilled.

With lookaheads:

'/\b(?![0-9]+\b)(?![a-z]+\b)[0-9a-z]+\b/i'

A quick test that also shows example usage:

$str = 'foo bar F0O 8ar';
$arr = array();
preg_match_all('/\b(?![0-9]+\b)(?![a-z]+\b)[0-9a-z]+\b/i', $str, $arr);
print_r($arr);

Output:

F0O
8ar