1024414- How to removes unsupported HTML tags and renders the remaining valid HTML without causing runtime exceptions

Text/Remove-unsupported-HTML-tags/.NET/Remove-unsupported-HTML-tags.slnx

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+<Solution>
+  <Project Path="Remove-unsupported-HTML-tags/Remove-unsupported-HTML-tags.csproj" />
+</Solution>

Text/Remove-unsupported-HTML-tags/.NET/Remove-unsupported-HTML-tags/Program.cs

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+using System.Text.RegularExpressions;
+
+
+HashSet<string> allowedTags = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
+        {
+            "font", "b", "i", "u", "sub", "br", "p"
+        };
+string htmlText = "\r\n<div><p>This is supported text with <span>inline span</span>.</p></div>\r\n<b>Bold text</b> and <i>italic text</i> are supported.\r\n<marquee>This tag is deprecated and unsupported in modern HTML.</marquee>\r\n<blink>Blink tag is unsupported and ignored by browsers.</blink>\r\n<script>alert('Script tag is supported but may be restricted');</script>\r\n";
+Console.WriteLine("Input Text: " + htmlText + "\n");
+string text1 = FilterSupportedHtml(htmlText);
+Console.WriteLine("Sanitize tags: " + text1 + "\n");
+
+string FilterSupportedHtml(string input)
+{
+    if (string.IsNullOrEmpty(input))
+        return input;
+
+    // Step 1: Decode HTML entities (&lt; &gt; etc.)
+    string decodedHtml = System.Net.WebUtility.HtmlDecode(input);
+
+    // Step 2: Remove script tags completely (security)
+    decodedHtml = Regex.Replace(
+        decodedHtml,
+        @"<script\b[^>]*>.*?</script>",
+        string.Empty,
+        RegexOptions.IgnoreCase | RegexOptions.Singleline
+    );
+
+    string output = Regex.Replace(
+        decodedHtml,
+        @"</?([a-zA-Z0-9]+)(\s[^>]*)?>",
+        match =>
+        {
+            string tagName = match.Groups[1].Value;
+
+            // If supported tag → keep as is
+            if (allowedTags.Contains(tagName))
+            {
+                return match.Value;
+            }
+
+            // Else → remove tag
+            return string.Empty;
+        },
+        RegexOptions.IgnoreCase
+    );
+
+    return output;
+}

Text/Remove-unsupported-HTML-tags/.NET/Remove-unsupported-HTML-tags/Remove-unsupported-HTML-tags.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <RootNamespace>Remove_unsupported_HTML_tags</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Syncfusion.Pdf.Net.Core" Version="*" />
+  </ItemGroup>
+</Project>