// Function to resolve references to my custom functions.
        public override IXsltContextFunction ResolveFunction(string prefix,
     string name, XPathResultType[] ArgTypes)
        {
            XPathRegExExtensionFunction func = null;
            // Create an instance of appropriate extension function class.
            switch (name)
            {
                case "Match":
                    // Usage 
                    // myFunctions:Matches(string source, string Regex_pattern) returns Boolean
                    func = new XPathRegExExtensionFunction("Match", 2, 2, new
        XPathResultType[] {XPathResultType.String, XPathResultType.String}
        , XPathResultType.Boolean );
                    break;
                case "Split":
                    // Usage 
                    // myFunctions:Split(string source, string Regex_pattern, int n) returns string
                    func = new XPathRegExExtensionFunction("Split", 3, 3, new
        XPathResultType[] {XPathResultType.String, XPathResultType.String, 
XPathResultType.Number}, XPathResultType.String);
                    break;
                case "Replace":
                    // Usage
                    // myFunctions:Replace(string source, string Regex_pattern, string replacement_string) returns string
                    func = new XPathRegExExtensionFunction("Replace", 3, 3, new
        XPathResultType[] {XPathResultType.String, XPathResultType.String, 
XPathResultType.String}, XPathResultType.String);
                    break;
            }
            return func;
        }

// This method is invoked at run time to execute the user defined function.
        public object Invoke(XsltContext xsltContext, object[] args,
     XPathNavigator docContext)
        {
            Regex r;
            string str = null;
            // The two custom XPath extension functions
            switch (m_FunctionName)
            {
                case "Match":
                    r = new Regex(args[1].ToString());
                    MatchCollection m = r.Matches(args[0].ToString());
                    if (m.Count == 0)
                    {
                        return false;
                    }
                    else
                    {
                        return true;
                    }
                    break;

                case "Split":
                    r = new Regex(args[1].ToString());
                    string[] s1 = r.Split(args[0].ToString());
                    int n = Convert.ToInt32(args[2]);
                    if (s1.Length < n)
                        str = "";
                    else
                        str = s1[n - 1];
                    break;
                case "Replace":
                    r = new Regex(args[1].ToString());
                    string s2 = r.Replace(args[0].ToString(), args[2].ToString());
                    str = s2;
                    break;
            }
            return (object)str;
        }

query = navigator.Compile("xdUtil:Match(9,'\\d')");
            CustomContext cntxt = new CustomContext();
            // Add a namespace definition for myFunctions prefix.
            cntxt.AddNamespace("xdUtil", "http://myXPathExtensionFunctions");
            query.SetContext(cntxt);
            Evaluate(query, navigator);