TSQL - remove illegal character from malformed XML string - sql

Question

Inspired by convert-string-to-xml-illegal-characters
I wonder if there is way in pure T-SQL to convert malformed XML string to well-formed version.
I have NVARCHAR like:
DECLARE #string NVARCHAR(MAX) =
N'<root>
<stuff attrib="Ooop,bad character<">
<test>Here I get &, and "<" or ">>>>" </test>
<test2>And even more <<<>><><<<><> </test2>
</stuff>
</root>';
SELECT CONVERT(XML, #string);
Of course this will fail because & should be replaced by &, this is easy.
But how to replace < and > when they are in element text or attribute without knowing structure in advance?

Answer 1

There is not a magic method for changing a string into valid XML. You have to be sure that you build your XML string in a way that ensures that it is syntactically correct. Even your simple method of replacing all & with & does not work in all cases. Consider this XML string:
<root>
<stuff>
<test>Here I get &</test>
</stuff>
</root>';
The simple replacement will result in:
<root>
<stuff>
<test>Here I get &amp;</test>
</stuff>
</root>';
Unless you want to write a lot of code to parse strings into XML, you should either:
Use the XML methods to build your XML
Use other standard methods such as the FOR XML clause in select
statements.
Ensure that as you build the string you ensure that any variable part (tags, attributes, or data) conform to the XML standards in conformance to what that variable part represents. For example: wrapping data variables in <![CDATA[ ]]> or replacing invalid characters in variable tags and attributes.