How can I allow (or replace) ampersands using .NET's XML library?

Question

Let's say that I have the following XML:

<someRootElement>
  <someTagWithUrl>http://www.google.com/s.php&amp;test=testing&lt;/someTagWithUrl&gt;
</someRootElement>

The ampersand inside the someTagWithUrl is invalid and needs to be escaped (by using &), but suppose I have a single string with the entire above contents.

How can I safely escape the ampersand so it becomes valid XML? Can .NET's XML library ignore this? (currently XElement.Parse will throw an exception)

I've thought about using a regular expression to search for ampersands between tags but I can't quite get the syntax correct. (something like >(\&)\< as a regex replace using & but I can't figure it out).

Answer 1

What you've pasted is invalid XML and any attempt to parse it with the XML libraries will fail. The best way to ensure that it is properly escaped is to use XML/HTML writers to create the data. For example the XmlWriter. They will ensure all strings are properly escaped.

Answer 2

Try this for your regular expression:

&(?!quot;|apos;|amp;|lt;|gt;#x?.*?;)

This will find only the invalid standalone & characters in your text (those that aren't part of an entity). Here's a sample of how you'd do text replacements before processing the source data as XML:

var regex = new Regex("&(?!quot;|apos;|amp;|lt;|gt;#x?.*?;)");
string fixedXml = regex.Replace(input, "&");