code: private function getBestDelimiter($pattern)
      {
          // try do find something that's not used
          $delimiters = [];
          foreach (self::$delimiters as $k => $d) {
              if (false === strpos($pattern, $d)) {
                  return $d;
              }
  
              $delimiters[$d] = [substr_count($pattern, $d), $k];
          }
  
          // return the least used delimiter, using the position in the list as a tie breaker
          uasort($delimiters, static function ($a, $b) {
              if ($a[0] === $b[0]) {
                  return Utils::cmpInt($a, $b);
              }
  
              return $a[0] < $b[0] ? -1 : 1;
          });
  
          return key($delimiters);
      }
docs: Get the delimiter that would require the least escaping in a regular expression.
  
  @param string $pattern the regular expression
  
  @return string the preg delimiter