The Html Agility Pack uses the underlying .NET XPATH implementation for its XPATH support. Fortunately XPATH in .NET is fully extensible (BTW: it's a shame Microsoft doesn't invest any more in this superb technology...).

So, let's suppose I have this html:

<div>hello</div>
<div>hallo</div>

Here is a sample code that will select both node because it compares the nodes with the 'h.llo' regex expression:

HtmlNodeNavigator nav = new HtmlNodeNavigator("mypage.htm");
foreach (var node in SelectNodes(nav, "//div[regex-is-match(text(), 'h.llo')]"))
{
    Console.WriteLine(node.OuterHtml); // should dump both div elements
}

It works because I use a special Xslt/XPath context where I have defined a new XPATH function called "regex-is-match". Here is the SelectNodes utility code:

public static IEnumerable<HtmlNode> SelectNodes(HtmlNodeNavigator navigator, string xpath)
{
    if (navigator == null)
        throw new ArgumentNullException("navigator");

    XPathExpression expr = navigator.Compile(xpath);
    expr.SetContext(new HtmlXsltContext());

    object eval = navigator.Evaluate(expr);
    XPathNodeIterator it = eval as XPathNodeIterator;
    if (it != null)
    {
        while (it.MoveNext())
        {
            HtmlNodeNavigator n = it.Current as HtmlNodeNavigator;
            if (n != null && n.CurrentNode != null)
            {
                yield return n.CurrentNode;
            }
        }
    }
}

And here is the support code:

    public class HtmlXsltContext : XsltContext
    {
        public HtmlXsltContext()
            : base(new NameTable())
        {
        }

        public override int CompareDocument(string baseUri, string nextbaseUri)
        {
            throw new NotImplementedException();
        }

        public override bool PreserveWhitespace(XPathNavigator node)
        {
            throw new NotImplementedException();
        }

        protected virtual IXsltContextFunction CreateHtmlXsltFunction(string prefix, string name, XPathResultType[] ArgTypes)
        {
            return HtmlXsltFunction.GetBuiltIn(this, prefix, name, ArgTypes);
        }

        public override IXsltContextFunction ResolveFunction(string prefix, string name, XPathResultType[] ArgTypes)
        {
            return CreateHtmlXsltFunction(prefix, name, ArgTypes);
        }

        public override IXsltContextVariable ResolveVariable(string prefix, string name)
        {
            throw new NotImplementedException();
        }

        public override bool Whitespace
        {
            get { return true; }
        }
    }

    public abstract class HtmlXsltFunction : IXsltContextFunction
    {
        protected HtmlXsltFunction(HtmlXsltContext context, string prefix, string name, XPathResultType[] argTypes)
        {
            Context = context;
            Prefix = prefix;
            Name = name;
            ArgTypes = argTypes;
        }

        public HtmlXsltContext Context { get; private set; }
        public string Prefix { get; private set; }
        public string Name { get; private set; }
        public XPathResultType[] ArgTypes { get; private set; }

        public virtual int Maxargs
        {
            get { return Minargs; }
        }

        public virtual int Minargs
        {
            get { return 1; }
        }

        public virtual XPathResultType ReturnType
        {
            get { return XPathResultType.String; }
        }

        public abstract object Invoke(XsltContext xsltContext, object[] args, XPathNavigator docContext);

        public static IXsltContextFunction GetBuiltIn(HtmlXsltContext context, string prefix, string name, XPathResultType[] argTypes)
        {
            if (name == "regex-is-match")
                return new RegexIsMatch(context, name);

            // TODO: create other functions here
            return null;
        }

        public static string ConvertToString(object argument, bool outer, string separator)
        {
            if (argument == null)
                return null;

            string s = argument as string;
            if (s != null)
                return s;

            XPathNodeIterator it = argument as XPathNodeIterator;
            if (it != null)
            {
                if (!it.MoveNext())
                    return null;

                StringBuilder sb = new StringBuilder();
                do
                {
                    HtmlNodeNavigator n = it.Current as HtmlNodeNavigator;
                    if (n != null && n.CurrentNode != null)
                    {
                        if (sb.Length > 0 && separator != null)
                        {
                            sb.Append(separator);
                        }

                        sb.Append(outer ? n.CurrentNode.OuterHtml : n.CurrentNode.InnerHtml);
                    }
                }
                while (it.MoveNext());
                return sb.ToString();
            }

            IEnumerable enumerable = argument as IEnumerable;
            if (enumerable != null)
            {
                StringBuilder sb = null;
                foreach (object arg in enumerable)
                {
                    if (sb == null)
                    {
                        sb = new StringBuilder();
                    }

                    if (sb.Length > 0 && separator != null)
                    {
                        sb.Append(separator);
                    }

                    string s2 = ConvertToString(arg, outer, separator);
                    if (s2 != null)
                    {
                        sb.Append(s2);
                    }
                }
                return sb != null ? sb.ToString() : null;
            }

            return string.Format("{0}", argument);
        }

        public class RegexIsMatch : HtmlXsltFunction
        {
            public RegexIsMatch(HtmlXsltContext context, string name)
                : base(context, null, name, null)
            {
            }

            public override XPathResultType ReturnType { get { return XPathResultType.Boolean; } }
            public override int Minargs { get { return 2; } }

            public override object Invoke(XsltContext xsltContext, object[] args, XPathNavigator docContext)
            {
                if (args.Length < 2)
                    return false;

                return Regex.IsMatch(ConvertToString(args[0], false, null), ConvertToString(args[1], false, null));
            }
        }
    }

The regex function is implemented in a class called RegexIsMatch at the end. It's not super complicated. Note there is a utility function ConvertToString that tries to coerce any xpath "thing" into a string that's very useful.

Of course, with this technology, you can define whatever XPATH function you need with very little code (I use this all the time to do upper/lower case conversions...).