- From: David Carlisle <d.p.carlisle@g...>
- To: Roger L Costello <costello@m...>
- Date: Wed, 23 Mar 2022 12:05:36 +0000
On Wed, 23 Mar 2022 at 11:39, Roger L Costello < costello@m...> wrote: Hi Folks,
I did some testing to see what characters can go into a CDATA section and a comment. Here's what I found:
Recall the CDATA syntax: <![CDATA[ ... ]]>
Question: Can the content of a CDATA section contain the ] character?
<![CDATA[foo]bar]]>
Answer: Yes
Question: Can the content of a CDATA section contain ]]?
<test><![CDATA[foo]]bar]]></test>
Answer: Yes.
Question: Can the content of a CDATA section contain more than two ] characters at the end?
<![CDATA[foo]]]]]>
Answer: Yes.
Question: Can the content of a CDATA section contain more than one [ character at the start?
<![CDATA[[[foo]]>
Answer: Yes.
The perhaps more surprising answer related to this is for the question
Can element content contain the string ]]> without escaping one of the characters?
Recall the comment syntax: <!-- ... -->
Question: Can the content of a comment contain a dash character?
<!-- foo - bar -->
Answer: Yes.
Question: Can the content of a comment contain two dashes?
<!-- foo -- bar -->
Answer: No.
Question: Can the content of a comment end with more than two dashes?
<!-- foo----->
Answer: No.
Notice the inconsistency: A CDATA section can end with more than two ] characters but a comment cannot end with more than two dashes:
<![CDATA[foo]]]]]> Okay
<!-- foo-----> Error
Also, a CDATA section can contain ]] but a comment cannot contain --
<![CDATA[foo]]bar]]>
<!-- foo -- bar -->
Why is there this inconsistency?
It's not inconsistency, it's historical legacy. XML comment syntax is a simplified but compatible subset of SGML comment syntax which allows multiple comments within a declaration, marked with -- so
<!ENTITY -- this is a comment -- foo "bar" -- so is this -- >
so if you parsed xml with an sgml parser a -- would end the comment so in xml it's only allowed at the end.
Isn't inconsistency in a data format language bad?
A world without history would be so dull.
/Roger
David
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