code: public static function escapeJsRegularExpression($regexp)
      {
          $pattern = preg_replace('/\\\\x\{?([0-9a-fA-F]+)\}?/', '\u$1', $regexp);
          $deliminator = substr($pattern, 0, 1);
          $pos = strrpos($pattern, $deliminator, 1);
          $flag = substr($pattern, $pos + 1);
          if ($deliminator !== '/') {
              $pattern = '/' . str_replace('/', '\\/', substr($pattern, 1, $pos - 1)) . '/';
          } else {
              $pattern = substr($pattern, 0, $pos + 1);
          }
          if (!empty($flag)) {
              $pattern .= preg_replace('/[^igmu]/', '', $flag);
          }
  
          return $pattern;
      }
docs: Escapes regular expression to use in JavaScript.
  @param string $regexp the regular expression to be escaped.
  @return string the escaped result.
  @since 2.0.6