code: public String parse(String expression) throws InvalidExpressionException {
  
          this.expression = expression;
  
          try {
              var = check();
          } catch (InvalidExpressionException e) {
              e.printStackTrace();
          }
  
          this.expression = formatString(expression);
          System.out.println(this.expression);
          tokens = tokenize(this.expression);
  
          return this.expression;
      }
docs: Creates tokens in polish notation for and re-formats the expression, unless an expression without valid syntax
  is passed in.
  @param expression a mathematical expression.
  @return a modified version of the expression.