<?php
$test = <<<EOF
"john" 'likes-cake' "ca,ke" but john doesn't like apples
EOF;

var_dump(
  preg_match_all('#(?<!["\']) \b \w+ \b (?!["\'])#', $test, $matches),
  $matches
);

Warning: preg_match_all(): Internal pcre_fullinfo() error -3 on line 7
bool(false)
array(0) {
}