Create XML Nodes based on XPath?

Question

Does anyone know of an existing means of creating an XML hierarchy programatically from an XPath expression?

For example if I have an XML fragment such as:

Answer 1

This is an improved version of Christian Peeters' solution that supports namespaces in the xpath expression.

public static XNode CreateNodeFromXPath(XElement elem, string xpath)
{
    // Create a new Regex object
    Regex r = new Regex(@"/*([a-zA-Z0-9_\.\-\:]+)(\[@([a-zA-Z0-9_\.\-]+)='([^']*)'\])?|/@([a-zA-Z0-9_\.\-]+)");

    xpath = xpath.Replace("\"", "'");
    // Find matches
    Match m = r.Match(xpath);

    XNode currentNode = elem;
    StringBuilder currentPath = new StringBuilder();
    XPathNavigator XNav = elem.CreateNavigator();

    while (m.Success)
    {
        String currentXPath = m.Groups[0].Value;    // "/ns:configuration" or "/appSettings" or "/add"
        String NamespaceAndElementName = m.Groups[1].Value;     // "ns:configuration" or "appSettings" or "add"
        String filterName = m.Groups[3].Value;      // "" or "key"
        String filterValue = m.Groups[4].Value;     // "" or "name"
        String attributeName = m.Groups[5].Value;   // "" or "value"

        XNamespace nspace = "";
        string elementName;
        int p = NamespaceAndElementName.IndexOf(':');
        if (p >= 0)
        {
            string ns = NamespaceAndElementName.Substring(0, p);
            elementName = NamespaceAndElementName.Substring(p + 1);
            nspace = XNav.GetNamespace(ns);
        }
        else
            elementName = NamespaceAndElementName;


        StringBuilder builder = currentPath.Append(currentXPath);
        String relativePath = builder.ToString();
        XNode newNode = (XNode)elem.XPathSelectElement(relativePath, XNav);

        if (newNode == null)
        {
            if (!string.IsNullOrEmpty(attributeName))
            {
                ((XElement)currentNode).Attribute(attributeName).Value = "";
                newNode = (XNode)elem.XPathEvaluate(relativePath, XNav);
            }
            else if (!string.IsNullOrEmpty(elementName))
            {
                XElement newElem = new XElement(nspace + elementName);
                if (!string.IsNullOrEmpty(filterName))
                {
                    newElem.Add(new XAttribute(filterName, filterValue));
                }

                ((XElement)currentNode).Add(newElem);
                newNode = newElem;
            }
            else
            {
                throw new FormatException("The given xPath is not supported " + relativePath);
            }
        }

        currentNode = newNode;
        m = m.NextMatch();
    }

    // Assure that the node is found or created
    if (elem.XPathEvaluate(xpath, XNav) == null)
    {
        throw new FormatException("The given xPath cannot be created " + xpath);
    }

    return currentNode;
}