ArrayList<Token> lex ( String input ) {
        ArrayList<Token> tl = new ArrayList<>();

        Pattern pattern = null;
        pattern = Pattern.compile("if[^a-zA-Z]|[a-zA-Z]+|[0-9]+|[+]|[-]");

        for (String s: input.split("\\s")) {

            System.out.println(s);
            Matcher matcher = pattern.matcher(s);
            while (matcher.find()) {
                if (matcher.group().matches("if") && s.length() == 2) {
                    tl.add(new T_If());
                }
                else if (matcher.group().matches("then") && s.length() == 4) {
                    tl.add(new T_Then());
                }
                else if (matcher.group().matches("[+]")) {
                    tl.add(new T_Plus());
                }
                else if (matcher.group().matches("[-]")) {
                    tl.add(new T_Minus());
                }
                else if (matcher.group().matches("[a-zA-Z]+(\\w)*")) {
                    tl.add(new T_Identifier(matcher.group()));
                }
                else if (matcher.group().matches("[0-9]+")) {
                    tl.add(new T_Integer(Integer.parseInt(matcher.group())));
                    //System.out.println(matcher.group().toString());
                }
            }
        }

        for (Token t: tl) {
            System.out.println(t.toString());
        }

        return tl;
    }