using System;

using System.Xml;

using System.Linq;

using HtmlAgilityPack;

public class Program

{

    public static void Main()

    {

        var oldScriptRaw = @"

<script for=""thisForm"" event=""onsubmit()"" language=""JScript"">

​

var Checked = false

var Counter = 0

​

for (;Counter < this.choice.length; Counter++)

{

    if (this.choice[Counter].checked)

    {

        Checked = true

        this.action = this.choice[Counter].value

    }

}

​

if (!Checked)

{

    alert (""Please make a selection"")

    return false

}

</script>";

            var newScriptRaw = @"<script ftype=""text\JScript"">

function thisForm_onsubmit(el)

{

    var Checked = false

    var Counter = 0

​

    for (;Counter < el.choice.length; counter++)

    {

        if (el.choice[counter].checked)

        {

            checked = true

            el.action = el.choice[counter].value

        }

    }

​

    if (!checked)

    {

        alert (""please make a selection"")

        return false

    }

}

</script>";

​

        var doc = new HtmlDocument();

        doc.LoadHtml(oldScriptRaw);

        var script = doc.DocumentNode.SelectSingleNode("//script");

        var newDoc = new HtmlDocument();

        newDoc.LoadHtml(newScriptRaw);

        var newScript = newDoc.DocumentNode.SelectSingleNode("//script");

        script.ParentNode.ReplaceChild(newScript, script);

        Console.WriteLine(doc.DocumentNode.OuterHtml);

    }

}