XML comments and "--"
From the standards document:
http://www.w3.org/TR/REC-xml/#sec-comments
[Definition: Comments may appear anywhere in a document outside other markup; in addition, they may appear within the document type declaration at places allowed by the grammar. They are not part of the document's character data; an XML processor may, but need not, make it possible for an application to retrieve the text of comments. For compatibility, the string " -- " (double-hyphen) must not occur within comments.] Parameter entity references must not be recognized within comments.
Maybe it can be helpful for someone. I had a problem, that I wanted to comment out a command line parameter in XML that starts with --:
<arg line="-v --line-break 0" />
so naturally normal way like this
<!-- <arg line="-v --line-break 0" /> -->
didn't work, but I found out, that if the - is replaced by it's UTF-8 equivalent - or - it works and can be tolerated inside comments.
So in my case the string
<arg line="-v --line-break 0" />
is parsed correctly and can be part of comments.
Of course it looks a little ugly, but if someone want to keep a string with -- as comment in his XML - I think it's still better than nothing.
It's one of those stupid rules that's in XML because it was in SGML and people didn't want to break compatibility. Why it's in SGML is anyone's guess: probably because it saved three lines of code in the original parser.
-- is not allowed for compatibility with SGML. From On SGML and HTML:
White space is not permitted between the markup declaration open delimiter("<!") and the comment open delimiter ("--"), but is permitted between the comment close delimiter ("--") and the markup declaration close delimiter (">"). A common error is to include a string of hyphens ("---") within a comment. Authors should avoid putting two or more adjacent hyphens inside comments.
So in SGML <! and > open and close "markup declarations" and -- opens and closes comments.