class Unescaper

class Unescaper {

  // Parser and Inline assume UTF-8 encoding, so escaped Unicode characters
  // must be converted to that encoding.
  const ENCODING = 'UTF-8';

  // Regex fragment that matches an escaped character in a double quoted
  // string.
  const REGEX_ESCAPED_CHARACTER = "\\\\([0abt\tnvfre \\\"\\/\\\\N_LP]|x[0-9a-fA-F]{2}|u[0-9a-fA-F]{4}|U[0-9a-fA-F]{8})";

  /**
   * Unescapes a single quoted string.
   *
   * @param string $value A single quoted string.
   *
   * @return string The unescaped string.
   */
  public function unescapeSingleQuotedString($value) {
    return str_replace('\'\'', '\'', $value);
  }

  /**
   * Unescapes a double quoted string.
   *
   * @param string $value A double quoted string.
   *
   * @return string The unescaped string.
   */
  public function unescapeDoubleQuotedString($value) {
    $self = $this;
    $callback = function ($match) use ($self) {
      return $self
        ->unescapeCharacter($match[0]);
    };

    // evaluate the string
    return preg_replace_callback('/' . self::REGEX_ESCAPED_CHARACTER . '/u', $callback, $value);
  }

  /**
   * Unescapes a character that was found in a double-quoted string
   *
   * @param string $value An escaped character
   *
   * @return string The unescaped character
   */
  public function unescapeCharacter($value) {
    switch ($value[1]) {
      case '0':
        return "\0";
      case 'a':
        return "\7";
      case 'b':
        return "\10";
      case 't':
        return "\t";
      case "\t":
        return "\t";
      case 'n':
        return "\n";
      case 'v':
        return "\v";
      case 'f':
        return "\f";
      case 'r':
        return "\r";
      case 'e':
        return "\33";
      case ' ':
        return ' ';
      case '"':
        return '"';
      case '/':
        return '/';
      case '\\':
        return '\\';
      case 'N':

        // U+0085 NEXT LINE
        return $this
          ->convertEncoding("", self::ENCODING, 'UCS-2BE');
      case '_':

        // U+00A0 NO-BREAK SPACE
        return $this
          ->convertEncoding("", self::ENCODING, 'UCS-2BE');
      case 'L':

        // U+2028 LINE SEPARATOR
        return $this
          ->convertEncoding(" (", self::ENCODING, 'UCS-2BE');
      case 'P':

        // U+2029 PARAGRAPH SEPARATOR
        return $this
          ->convertEncoding(" )", self::ENCODING, 'UCS-2BE');
      case 'x':
        $char = pack('n', hexdec(substr($value, 2, 2)));
        return $this
          ->convertEncoding($char, self::ENCODING, 'UCS-2BE');
      case 'u':
        $char = pack('n', hexdec(substr($value, 2, 4)));
        return $this
          ->convertEncoding($char, self::ENCODING, 'UCS-2BE');
      case 'U':
        $char = pack('N', hexdec(substr($value, 2, 8)));
        return $this
          ->convertEncoding($char, self::ENCODING, 'UCS-4BE');
    }
  }

  /**
   * Convert a string from one encoding to another.
   *
   * @param string $value The string to convert
   * @param string $to    The input encoding
   * @param string $from  The output encoding
   *
   * @return string The string with the new encoding
   *
   * @throws \RuntimeException if no suitable encoding function is found (iconv or mbstring)
   */
  private function convertEncoding($value, $to, $from) {
    if (function_exists('mb_convert_encoding')) {
      return mb_convert_encoding($value, $to, $from);
    }
    elseif (function_exists('iconv')) {
      return iconv($from, $to, $value);
    }
    throw new \RuntimeException('No suitable convert encoding function (install the iconv or mbstring extension).');
  }

}