code: function(){
                  /*
                   * expression
                   *   : [ [ PLUS | '-' | DIMENSION | NUMBER | STRING | IDENT ] S* ]+
                   *   ;
                   */
  
                  var tokenStream = this._tokenStream,
                      value       = "";
                  while(tokenStream.match([Tokens.PLUS, Tokens.MINUS, Tokens.DIMENSION,
                          Tokens.NUMBER, Tokens.STRING, Tokens.IDENT, Tokens.LENGTH,
                          Tokens.FREQ, Tokens.ANGLE, Tokens.TIME,
                          Tokens.RESOLUTION, Tokens.SLASH])){
  
                      value += tokenStream.token().value;
                      value += this._readWhitespace();
                  }
                  var val = value.length ? value : null
                  return val;
  
              }
docs: CSS3 Selectors