using System;

using System.Text;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var matches = (new Regex("<form.*?action=(\"[^\\\"]+?\"|'[^\']+?'|[\\S]+?).*?>"

, RegexOptions.Singleline | RegexOptions.IgnoreCase)

).Matches("HTML CODE <form action=1><form action='2'><form action=\"3\">");

        foreach(Match m in matches)

{

            var actionValue = m.Groups[1].Value.Trim(new char[]{'\'','"'});

            Console.WriteLine(actionValue);

} 

    }

}

​

​