function readString(source, start, line, col, prev) {
  var body = source.body;
  var position = start + 1;
  var chunkStart = position;
  var code = 0;
  var value = '';

  while (position < body.length && (code = charCodeAt.call(body, position)) !== null &&
  // not LineTerminator
  code !== 0x000A && code !== 0x000D &&
  // not Quote (")
  code !== 34) {
    // SourceCharacter
    if (code < 0x0020 && code !== 0x0009) {
      throw (0, _error.syntaxError)(source, position, 'Invalid character within String: ' + printCharCode(code) + '.');
    }

    ++position;
    if (code === 92) {
      // \
      value += slice.call(body, chunkStart, position - 1);
      code = charCodeAt.call(body, position);
      switch (code) {
        case 34:
          value += '"';break;
        case 47:
          value += '/';break;
        case 92:
          value += '\\';break;
        case 98:
          value += '\b';break;
        case 102:
          value += '\f';break;
        case 110:
          value += '\n';break;
        case 114:
          value += '\r';break;
        case 116:
          value += '\t';break;
        case 117:
          // u
          var charCode = uniCharCode(charCodeAt.call(body, position + 1), charCodeAt.call(body, position + 2), charCodeAt.call(body, position + 3), charCodeAt.call(body, position + 4));
          if (charCode < 0) {
            throw (0, _error.syntaxError)(source, position, 'Invalid character escape sequence: ' + ('\\u' + body.slice(position + 1, position + 5) + '.'));
          }
          value += String.fromCharCode(charCode);
          position += 4;
          break;
        default:
          throw (0, _error.syntaxError)(source, position, 'Invalid character escape sequence: \\' + String.fromCharCode(code) + '.');
      }
      ++position;
      chunkStart = position;
    }
  }

  if (code !== 34) {
    // quote (")
    throw (0, _error.syntaxError)(source, position, 'Unterminated string.');
  }

  value += slice.call(body, chunkStart, position);
  return new Tok(STRING, start, position + 1, line, col, prev, value);
}
Reads a string token from the source file.

"([^"\\\u000A\u000D]|(\\(u[0-9a-fA-F]{4}|["\\/bfnrt])))*"