private static final Pattern numberPattern = Pattern.compile("^_*[\\d.,]+$");
  private static final Pattern operatorPattern = Pattern.compile("^[-+*/]$");
  private static final Pattern parenPattern = Pattern.compile("^[()]$");

  /**
   * Inspects the given string and returns an appropriate instance of a
   * subclass of {@link Token}.
   *
   * @param tok the token to evaluate.
   * @return <code>Token</code> representation of <code>tok</code>.
   */
  public static Token toToken(String tok) {
	  if(Assignment2.operatorPattern.matcher(tok).matches()){
		  return new Operator(tok);
	  } else if(Assignment2.numberPattern.matcher(tok).matches()){
		  return new Operand(Double.parseDouble(tok));
	  } else if(Assignment2.parenPattern.matcher(tok).matches()){
		  return new Parenthesis(tok);
	  } else throw new java.lang.RuntimeException();
  }