Given a string representing a code snippet, implement a tag validator to parse the code and return whether it is valid.

A code snippet is valid if all the following rules hold:

1. The code must be wrapped in a valid closed tag. Otherwise, the code is invalid.
2. A closed tag (not necessarily valid) has exactly the following format : `<TAG_NAME>TAG_CONTENT</TAG_NAME>`. Among them, `<TAG_NAME>` is the start tag, and `</TAG_NAME>` is the end tag. The TAG_NAME in start and end tags should be the same. A closed tag is valid if and only if the TAG_NAME and TAG_CONTENT are valid.
3. A valid `TAG_NAME` only contain upper-case letters, and has length in range [1,9]. Otherwise, the `TAG_NAME` is invalid.
4. A valid `TAG_CONTENT` may contain other valid closed tags, cdata and any characters (see note1) EXCEPT unmatched `<`, unmatched start and end tag, and unmatched or closed tags with invalid TAG_NAME. Otherwise, the `TAG_CONTENT` is invalid.
5. A start tag is unmatched if no end tag exists with the same TAG_NAME, and vice versa. However, you also need to consider the issue of unbalanced when tags are nested.
6. A `<` is unmatched if you cannot find a subsequent `>`. And when you find a `<` or `</`, all the subsequent characters until the next `>` should be parsed as TAG_NAME (not necessarily valid).
7. The cdata has the following format : `<![CDATA[CDATA_CONTENT]]>`. The range of `CDATA_CONTENT` is defined as the characters between `<![CDATA[` and the first subsequent `]]>`.
8. `CDATA_CONTENT` may contain any characters. The function of cdata is to forbid the validator to parse `CDATA_CONTENT`, so even it has some characters that can be parsed as tag (no matter valid or invalid), you should treat it as regular characters.

Example 1:

    Input: code = "<DIV>This is the first line <![CDATA[<div>]]></DIV>"
    Output: true
    Explanation:
    The code is wrapped in a closed tag : <DIV> and </DIV>.
    The TAG_NAME is valid, the TAG_CONTENT consists of some characters and cdata.
    Although CDATA_CONTENT has an unmatched start tag with invalid TAG_NAME, it should be considered as plain text, not parsed as a tag.
    So TAG_CONTENT is valid, and then the code is valid. Thus return true.

Example 2:

    Input: code = "<DIV>>>  ![cdata[]] <![CDATA[<div>]>]]>]]>>]</DIV>"
    Output: true
    Explanation:
    We first separate the code into : start_tag|tag_content|end_tag.
    start_tag -> "<DIV>"
    end_tag -> "</DIV>"
    tag_content could also be separated into : text1|cdata|text2.
    text1 -> ">>  ![cdata[]] "
    cdata -> "<![CDATA[<div>]>]]>", where the CDATA_CONTENT is "<div>]>"
    text2 -> "]]>>]"
    The reason why start_tag is NOT "<DIV>>>" is because of the rule 6.
    The reason why cdata is NOT "<![CDATA[<div>]>]]>]]>" is because of the rule 7.

Example 3:

    Input: code = "<A>  <B> </A>   </B>"
    Output: false
    Explanation: Unbalanced. If "<A>" is closed, then "<B>" must be unmatched, and vice versa.

Constraints:

- `1 <= code.length <= 500`
- `code` consists of English letters, digits, `'<'`, `'>'`, `'/'`, `'!'`, `'['`, `']'`, `'.'`, and `' '`.