PHP explode function

As long as there can't be quotes within quotes (eg. "foo\"bar" isn't allowed), you can do this with a regular expression. Otherwise you need a full parser.

This should do:

function split_words($input) {
  $matches = array();
  if (preg_match_all('/("([^"]+)")|(\w+)/', $input, $reg)) {
    for ($ii=0,$cc=count($reg[0]); $ii < $cc; ++$ii) {
      $matches[] = $reg[2][$ii] ? $reg[2][$ii] : $reg[3][$ii];
    }
  }
  return $matches;
}

Usage:

$input = 'windows linux "mac os x"';
var_dump(split_words($input));

When the user is entering the string "mac os x" you can automatically detect the white space and change to string to "mac-os-x" then you can still explode this way:

$os = "metasys solaris mac-os-x";
$strings = explode(' ', $os);

You can do this using the replace function.

Either have the user separate their tag values with commas as Elzo Valugi suggested, or improve on your UI so that users enter one tag at a time (similar to Google Wave or Wordpress's tagging UI). I suggest the later.

If you really want to stick with your proposed entry format (which I don't suggest), you could maintain a list of multi-word tags (those that aren't supposed to be split). Compare the combined tag string provided by the user against this list and make sure that you don't split those terms. If you're set on sticking to this method, I could go into the details more, but I don't think it's a good idea as the entry format itself is flawed.

I would ask the user to enter the tags commas separated and explode with comma delimiter:

$string = "windows, linux, mac os x";
$pieces = explode(',', $string);

This is they way most tag system work anyway.

otherwise you'll need to construct a parser because explode cannot cope with what you want. Regex is an overkill in my opinion.

You could do a regex. I'm not the best at writing them, but someone else here should be able to match the 'words' breaking them on spaces that aren't in quotes.