Search XML element with xml namespace prefix C#

Question

Update

I want to have an expression (XPath, or Regex Expression, similar) that can match an XML element with a particular namespace. For example, I want to locate the value of the link element (eg I need the http://url within <b:link>http://url</b:link>) shown below. However, the namespace prefix varies depending on different xml files as shown in cases 1-3.

Considering the allowed character for namespace prefix (eg is any character allowed/valid) , could anyone provide the solution (XPath, Regex Expression or similar?

Please note that because the xml file is unknown, thus, the namespace and prefix are unknown until runtime. Does it mean I cannot use this XDocument/XmlDocument, because it requires namespace to be known in the code.

Update

Case 1

<A xmlns:b="link">
<b:link>http://url
</b:link>
</A>

Case 2

<A xmlns="link">
<link>http://url
</link>
</A>

Case 3

<A xmlns:a123="link">
<a123:link>http://url
</a123:link>
</A>

Please note that the url within the link element could be any http url, and unknown until runtime.

Update

Please mark up my question.

Answer 1

You need to know the namespaces you will be dealing with and register them with an XmlNamespaceManager. Here is an example:

    XmlDocument doc = new XmlDocument();
    doc.LoadXml("<A xmlns:b='link'><b:Books /></A>");
    XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
    nsmgr.AddNamespace("b", "link");

    XmlNodeList books = doc.SelectNodes("//b:Books", nsmgr);

And if you want to do this using XDocument, which I would recommend for its brevity, here is how:

    XDocument xDoc = XDocument.Parse("<A xmlns:b='link'><b:Books /></A>");
    XNamespace ns = "link";
    var books = xDoc.Descendants(ns + "Books");

If you do not know the namespace(s) ahead of time, see this post which shows how to query across an XDocument using only the local name. Here's an example:

XDocument xDoc = XDocument.Parse("<A xmlns:b='link'><b:Books /></A>");
var books = xDoc.Descendants().Where(e => e.Name.LocalName.ToLower() == "books");

Answer 2

Use an XML parser, not a regex.

That being said, you could use:

<(?:(.+?):)?Books />

And the namespace would be in captured group 1.

In fact, I'd more strongly recommend you use

<(?:([^<>]+?):)?Books />

To prevent mistakes like matching over another set of XML tags (who would use <> in a namespace anyway?!)