$tokens = [
  'c' => 'C',
  'h' => 'H',
  'm' => 'O',
  'x' => 'Cl|Br|F'
];
$vars = [];
foreach ($tokens as $key => $token) {
  if (!preg_match('/(' . $token . ')([' . '^A-Z]*)([' . 'A-Z]|$)/', [!substance_formula], $matches)) continue;
  $vars[$key] = !$matches[2] ? 1 : (is_numeric($matches[2]) ? intval($matches[2]) : 0);
}
$val = 100 / (1 + 4.773 * ($vars['c'] + ($vars['h'] - $vars['x'] - 2 * $vars['m']) / 4));
return $val > 0 ? $val : NULL;