public static HtmlNodeCollection SelectNodes(this HtmlNode @this, XPathExpression xpath)
{
    // Reflection
    var _ownerdocument = (HtmlDocument)typeof(HtmlNode)
        .GetField("_ownerdocument", BindingFlags.Instance | BindingFlags.NonPublic)
        .GetValue(@this);

    var nav = (HtmlNodeNavigator)typeof(HtmlNodeNavigator)
        .GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new Type[] { typeof(HtmlDocument), typeof(HtmlNode) }, null)
        .Invoke(new object[] { _ownerdocument, @this });
    // End

    HtmlNodeCollection list = new HtmlNodeCollection(null);

    XPathNodeIterator it = nav.Select(xpath);
    while (it.MoveNext())
    {
        HtmlNodeNavigator n = (HtmlNodeNavigator)it.Current;
        list.Add(n.CurrentNode);
    }

    if (list.Count == 0)
    {
        return null;
    }

    return list;
}

class MyContext : XPathContext
{
    public override string DefaultNamespace => "http://uri";

    [XPathFunction("match", XPathResultType.NodeSet, XPathResultType.String)]
    public bool RegexMatch(string prop, string regex, XPathNavigator docContext)
    {
        return new Regex(regex).Match(prop).Success;
    }
}

static void Main(string[] args)
{
    var doc = new HtmlDocument();
    doc.LoadHtml(@"<div id=""info"">info</div>
<div id=""category_1"">category_1</div>
<div id=""category_2"">category_2</div>
<div id=""output"">output</div>");

    var context = new MyContext();
    context.AddNamespace("fn", context.DefaultNamespace);
    context.AddParam("regex", context.DefaultNamespace, @"category_\d+");

    var xpath = context.Compile(@"//div[fn:match(@id, $fn:regex)]");
    var nodes = doc.DocumentNode.SelectNodes(xpath);

    foreach (var node in nodes)
    {
        Console.WriteLine(HttpUtility.HtmlDecode(node.InnerHtml));
    }
}

class MyContext : XPathContext
{
    public MyContext() { }
    public MyContext(string prefix) : base(prefix) { }

    [XPathFunction("match")]
    public bool RegexMatch1(string content, string regex)
    {
        Console.WriteLine($"  * Invoke: {nameof(RegexMatch1)}");
        return new Regex(regex).Match(content).Success;
    }

    [XPathFunction("match")]
    public bool RegexMatch2(string regex, XPathNavigator docContext)
    {
        Console.WriteLine($"  * Invoke: {nameof(RegexMatch2)}");
        return new Regex(regex).Match(docContext.InnerXml).Success;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var html = @"
<div id=""info"">hello</div>
<div class=""category1"">category_1</div>
<div class=""category2"">category_2</div>
<div id=""output"">bye</div>";

        var doc = new HtmlDocument();
        doc.LoadHtml(html);
        var ctx = new MyContext("re");

        Console.WriteLine($"HTML:{html}");
        Console.WriteLine();

        // Samples
        var xpaths = new[]
        {
            @"//div[re:match(@class, 'category\d+')]",
            @"//div[re:match('(hello|bye)')]",
        };
        foreach (var xpath in xpaths)
        {
            Console.WriteLine($"XPath: {xpath}");
            var nodes = doc.DocumentNode.SelectNodes(ctx[xpath]);
            foreach (var node in nodes)
                Console.WriteLine(node.InnerHtml);
            Console.WriteLine();
        }
    }
}

HTML:
<div id="info">hello</div>
<div class="category1">category_1</div>
<div class="category2">category_2</div>
<div id="output">bye</div>

XPath: //div[re:match(@class, 'category\d+')]
  * Invoke: RegexMatch1
  * Invoke: RegexMatch1
  * Invoke: RegexMatch1
  * Invoke: RegexMatch1
category_1
category_2

XPath: //div[re:match('(hello|bye)')]
  * Invoke: RegexMatch2
  * Invoke: RegexMatch2
  * Invoke: RegexMatch2
  * Invoke: RegexMatch2
hello
bye