private static readonly Regex _pbrRegex = new Regex(@"<(?!br|/br|p|/p).+?>",
           RegexOptions.Compiled | RegexOptions.IgnoreCase);
/// <summary>
/// حذف تمامی تگ‌ها منهای دو تگ ذکر شده
/// </summary>
/// <param name="html"></param>
/// <returns></returns>
public static string CleanTagsExceptPbr(string html)
{
    return _pbrRegex.Replace(html, string.Empty);
}

using NUnit.Framework;

namespace testWinForms87
{
   [TestFixture]
   public class CTestRegExHelperPbr
   {
       [Test]
       public void TestCleanTagsExceptPbr1()
       {
           Assert.AreEqual(
               CRegExHelper.CleanTagsExceptPbr("<b>data1</b><br/>data2"),
               "data1<br/>data2");
       }

       [Test]
       public void TestCleanTagsExceptPbr2()
       {
           Assert.AreEqual(
               CRegExHelper.CleanTagsExceptPbr("<b>data1</b><br>data2"),
               "data1<br>data2");
       }

       [Test]
       public void TestCleanTagsExceptPbr3()
       {
           Assert.AreEqual(
               CRegExHelper.CleanTagsExceptPbr("<p><b>data1</b><br/>data2</p>"),
               "<p>data1<br/>data2</p>");
       }

       [Test]
       public void TestCleanTagsExceptPbr4()
       {
           Assert.AreEqual(
               CRegExHelper.CleanTagsExceptPbr("<b>data1</b><p>data2<br />"),
               "data1<p>data2<br />");
       }
   }
}