/**
     * tokenize xml file like format.
     * read character one by one and feed it to state machine.
     * if token is found, add to a list. Otherwise terminate
     * the process and return null.
     * for example, <name>david</name> => [<name>, david, </name>]
     *
     * @param strChar xxx
     *
     * @return list of tokens if all tokens are valid, otherwise return null
     */
    public static List<String> tokenize(String strChar){
        List<String> list = new ArrayList<String>();
        int state = 0;
        int curr = state;
        String str = "";
        boolean isValid = true;
        for(int i=0; i<strChar.length() && isValid; i++){
            if(curr == 0){
                if(str.length() > 0){
                    list.add(str);
                    str = "";
                }

                if(strChar.charAt(i) == '<'){
                    str += strChar.charAt(i);
                    curr = 1;
                }else if('a' <= strChar.charAt(i) && strChar.charAt(i) <= 'z'){
                    str += strChar.charAt(i);
                    curr = 2;
                    if(i + 1 < strChar.length() && lookAhead(strChar, i) == '<'){
                        curr = 0;
                    }
                }else{
                    isValid = false;
                }
            }else if(curr == 2){
                if('a' <= strChar.charAt(i) && strChar.charAt(i) <= 'z'){
                    str += strChar.charAt(i);
                    curr = 2;
                    if(i + 1 < strChar.length() && lookAhead(strChar, i) == '<'){
                        curr = 0;
                    }
                }else{
                    isValid = false;
                }
            }else if(curr == 1){
                if('a' <= strChar.charAt(i) && strChar.charAt(i) <= 'z'){
                    str += strChar.charAt(i);
                    curr = 3;
                }
                else if(strChar.charAt(i) == '/'){
                    str += strChar.charAt(i);
                    curr = 4;
                }else{
                    isValid = false;
                }
            }else if(curr == 3){
                if(strChar.charAt(i) == '>'){
                    str += strChar.charAt(i);
                    curr = 5; // final
                    curr = 0;
                }else if('a' <= strChar.charAt(i) && strChar.charAt(i) <= 'z'){
                    str += strChar.charAt(i);
                    curr = 3;
                }else{
                    isValid = false;
                }
            }else if(curr == 4){
                if('a' <= strChar.charAt(i) && strChar.charAt(i) <= 'z'){
                    str += strChar.charAt(i);
                    curr = 7;
                }else{
                    isValid = false;
                }

            }else if(curr == 7){
                if('a' <= strChar.charAt(i) && strChar.charAt(i) <= 'z'){
                    str += strChar.charAt(i);
                }else if(strChar.charAt(i) == '>'){
                    str += strChar.charAt(i);
                    curr = 8; // final
                    list.add(str);
                    str = "";
                    curr = 0;
                }else{
                    isValid = false;
                }
            }
        }

        if(curr == 2){
            if(str.length() > 0){
                list.add(str);
                str = "";
            }
        }

        return isValid ? list : null;
    }