Redact PDF text with a regular expression in C#

Build it step by step.

1. 1Install the NuGet packages

   SelectPdf.Universal is the library. The native package carries the Chromium engine for one platform: add the one for the platform you run on.

   Windows x64 Linux x64 Linux ARM64 macOS · Apple Silicon
   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.win-x64

   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.linux-x64

   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.linux-arm64

   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.osx-arm64

2. 2Create a console app

   Any .NET 8 or .NET 10 project works. To follow along, create a console app, run the install commands above inside its folder, then replace Program.cs with the code below.

   terminal COPY

   dotnet new console -n PdfSample
   cd PdfSample

3. 3Import the namespaces

   Program.cs · lines 1–2 COPY

   using System.Text.RegularExpressions;
   using SelectPdf.Universal;

   Regex comes from System.Text.RegularExpressions; everything else is in SelectPdf.Universal.

4. 4Create a PDF to redact

   Program.cs · lines 4–12 COPY

   // 1. Make a statement that contains card numbers.
   HtmlToPdf converter = new HtmlToPdf();
   PdfDocument statement = converter.ConvertHtmlString("""
       <h1>Card statement</h1>
       <p>Primary card: 4111-1111-1111-1111</p>
       <p>Backup card: 5500-0000-0000-0004</p>
       """);
   statement.Save("statement.pdf");
   statement.Close();

   To keep the sample self-contained, HtmlToPdf converts a small statement with two card numbers into statement.pdf. In your application this is the PDF you already have — an upload or a report you generated earlier.

5. 5Load it into a PdfRedactionManager

   Program.cs · lines 14–16 COPY

   // 2. Remove every card number from the file.
   var redactor = new PdfRedactionManager();
   redactor.Load("statement.pdf");

   PdfRedactionManager works on an existing PDF. Load opens the file you want to clean.

6. 6Describe what to remove

   Program.cs · lines 17–17 COPY

   redactor.AddRedaction(new Regex(@"\d{4}-\d{4}-\d{4}-\d{4}"));

   AddRedaction with a Regex marks every match in the document. The pattern \d{4}-\d{4}-\d{4}-\d{4} matches four groups of four digits separated by dashes — the format of the two card numbers. Adjust it to your data: national IDs, account numbers, e-mail addresses.

7. 7Mark where content was removed (optional)

   Program.cs · lines 18–18 COPY

   redactor.OverlayText = "REDACTED";

   OverlayText writes a label over each redacted area so readers can see that something was taken out. Leave it unset for a plain box.

8. 8Apply, save and close

   Program.cs · lines 19–21 COPY

   redactor.Redact();
   redactor.Save("statement-redacted.pdf");
   redactor.Close();

   Redact applies all redactions. Save writes the result to a new file, so the original stays untouched, and Close releases the document.

9. 9Run it

   terminal COPY

   dotnet run

   The program writes statement-redacted.pdf to the working folder. Without a license key, every page carries a trial watermark; set GlobalProperties.LicenseKey before the first conversion to remove it.

The complete Program.cs.

This is the exact file our samples pipeline compiled and ran on Linux ARM64 and Linux x64 and macOS ARM64 and Windows x64 to produce the output on this page.

using System.Text.RegularExpressions;
using SelectPdf.Universal;

// 1. Make a statement that contains card numbers.
HtmlToPdf converter = new HtmlToPdf();
PdfDocument statement = converter.ConvertHtmlString("""
    <h1>Card statement</h1>
    <p>Primary card: 4111-1111-1111-1111</p>
    <p>Backup card: 5500-0000-0000-0004</p>
    """);
statement.Save("statement.pdf");
statement.Close();

// 2. Remove every card number from the file.
var redactor = new PdfRedactionManager();
redactor.Load("statement.pdf");
redactor.AddRedaction(new Regex(@"\d{4}-\d{4}-\d{4}-\d{4}"));
redactor.OverlayText = "REDACTED";
redactor.Redact();
redactor.Save("statement-redacted.pdf");
redactor.Close();

Open the project on GitHub → this program and the variations below, ready to dotnet run

The PDF it produces.

Variations you will need next.

Each variation is a separate file in the same project, compiled and run with the program above.

Redact a known name or phrase

AddRedaction(text, caseSensitive, wholeWordsOnly) marks every occurrence of an exact text. FillColor changes the box color from the default black. Redact returns a PdfRedactionResult for each area it removed, with the page and the text, which you can write to an audit log.

using SelectPdf.Universal;

static class RedactKnownText
{
    public static void Run()
    {
        HtmlToPdf converter = new HtmlToPdf();
        PdfDocument contract = converter.ConvertHtmlString("""
            <h1>Supply agreement</h1>
            <p>Between Contoso Ltd and Fabrikam Inc.</p>
            <p>Contoso Ltd delivers 400 units per month.</p>
            """);
        contract.Save("contract.pdf");
        contract.Close();

        var redactor = new PdfRedactionManager();
        redactor.Load("contract.pdf");
        redactor.FillColor = new PdfColor(128, 128, 128);

        // exact text: case-sensitive, whole words only
        redactor.AddRedaction("Contoso Ltd", true, true);

        // Redact returns what was removed, for an audit log
        foreach (PdfRedactionResult r in redactor.Redact())
            Console.WriteLine($"page {r.PageIndex + 1}: {r.Text}");

        redactor.Save("contract-redacted.pdf");
        redactor.Close();
    }
}

Redact a fixed area and work in memory

When the position is known, such as a signature box on a fixed form, AddRedaction(pageIndex, rectangle) removes everything inside that rectangle. Coordinates are in points from the top-left corner of the page. Save() without a file name returns the redacted PDF as bytes.

using SelectPdf.Universal;
using SelectPdf.Universal.Drawing;

static class RedactArea
{
    public static void Run()
    {
        HtmlToPdf converter = new HtmlToPdf();
        PdfDocument form = converter.ConvertHtmlString("""
            <h1>Application form</h1>
            <p>Name: Jane Doe</p>
            <p>Signature: ____________________</p>
            """);
        form.Save("form.pdf");
        form.Close();

        var redactor = new PdfRedactionManager();
        redactor.Load("form.pdf");

        // a fixed area on page 1 (x, y, width, height in points from the top-left corner)
        redactor.AddRedaction(0, new RectangleF(0, 60, 300, 50));
        redactor.Redact();

        // in memory: Save() returns the redacted PDF as bytes
        byte[] pdf = redactor.Save();
        redactor.Close();
        File.WriteAllBytes("form-redacted.pdf", pdf);
    }
}

Common problems and fixes.

Good to know.

• Redaction removes text, images, links and form fields under the redaction from the file; there is nothing left to copy or extract. Source
• Redactions can be found by area, by text or by regular expression, with optional overlay text. Source
• Redaction is part of the commercial SelectPdf.Universal package. Source