Using RegEx to redact PDFs on Android

Nutrient Android SDK enables you to redact text in a PDF document using regular expression patterns. This guide shows how do this using the existing text extraction and redaction APIs.

In this example, you’re redacting URLs from a document. First, you need to create the regular expression with the URL pattern:

val urlPattern = """[-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_+.~#?&/=]*)"""
val urlRegularExpression = Regex(urlPattern)

Then, you’ll have to loop through all the pages in the document to find and mark the matching URLs for redaction:

val fileUri = ... // File Uri
val document: PdfDocument = PdfDocumentLoader.openDocument(context, fileUri)
val pageCount = document.pageCount


for (i in 0 until pageCount) {
    val text = document.getPageText(i)
    val matchResult: MatchResult = urlRegularExpression.matchEntire(text) ?: continue
    for (matchGroup in matchResult.groups.filterNotNull()) {
        val range = matchGroup.component2()
        val startIndex: Int = range.first
        val length: Int = startIndex + range.last
        val textRects = document.getPageTextRects(i, startIndex, length)
        val redaction = RedactionAnnotation(i, textRects).apply {
            color = Color.BLACK
            fillColor = Color.BLACK
            outlineColor = Color.YELLOW
            overlayText = "REDACTED"
        }
        document.annotationProvider.addAnnotationToPage(redaction)
    }
}

Finally, create a new PDF file for the redacted document by applying redactions.

You can do so using the PdfProcessor API:

val outputFile: File
val document: PdfDocument
val processorTask = PdfProcessorTask.fromDocument(document)
    .applyRedactions()


PdfProcessor.processDocument(processorTask, outputFile)
val redactedDocument = PdfDocumentLoader.openDocument(context, Uri.fromFile(outputFile))

Or, you can use the DocumentSaveOptions#setApplyRedactions() option when saving the document via any of the save methods on PdfDocument.

Bear in mind that this will overwrite the existing document, removing content irreversibly:

val documentSaveOptions = DocumentSaveOptions(null, null, true, null);
documentSaveOptions.setApplyRedactions(true)
document.saveIfModified(documentSaveOptions)