Question

I am trying to replace 1234567890123456 with ************3456 with the following code:

Regex.Replace(xml,@"\b\d{13,16}\b", string.Concat(new String('*',12),a.Value.Substring(a.Value.Length - 4)));

Where a.Value is the number, but it just results in:

****************

Complete code:

//Check if value is the credit card
        if(a.Value.Length >= 13 && a.Value.Length <= 16)
                xml = Regex.Replace(xml,@"\b\d{12}(?=\d{4})\b", new String('*',12));
        else //If value is not a credit card, replace it with ***
            xml = Regex.Replace(xml,@"\b\d+\b", "***");

I replaced the first part of the if statement with:

Regex.Replace(xml, @"\b\d{13,16}\b", match => 
    String.Concat(new String('*', match.Length - 4), match.Value.Substring(match.Length - 4)));

but I still get *** for the cardnum.

Here is the XML. Note the card number can be between 13 and 16 digits and I always want to keep the last 4.

<Details>
<CreditCard cardnum='1234567890123456'
ccv='123' 
exp='0212' 
cardType='1' 
name='joe' />
</Details>

long numeric;

    string xml = @"<Details>
<CreditCard cardnum='1234567890123456'
ccv='123' 
exp='0212' 
cardType='1' 
name='joe' />
</Details>";

XElement element = XElement.Parse(xml);
IEnumerable<XElement> elementsWithPossibleCCNumbers = 
        element.Descendants()
               .Where(d => d.Attributes()
                            .Where(a => a.Value.Length >= 13 && a.Value.Length <= 16)
                            .Where(a => long.TryParse(a.Value, out numeric))
                            .Count() == 1).Select(x=>x);


foreach(var x in elementsWithPossibleCCNumbers)
{

   foreach(var a in x.Attributes())
   {

    //Check if the value is a number
    if(long.TryParse(a.Value,out numeric))
    {
        //Check if value is the credit card
        if(a.Value.Length >= 13 && a.Value.Length <= 16)
            Regex.Replace(xml,@"\b\d{12}(?=\d{4}\b)", new String('*',12));
        else //If value is not a credit card, replace it with ***
            xml = Regex.Replace(xml,@"\b\d+\b", "***");
    }
   }
}

Answer 1

You can use lookahead to match the trailing 4 digits but preserve them:

Regex.Replace(xml,@"\b\d{12}(?=\d{4}\b)", new String('*',12))

Edit: This version is closer to your code. However, rather than relying on a pre-set a.Value, it uses the match variable to allow you to apply your transformation on whatever was actually captured in your regex.

Regex.Replace(xml, @"\b\d{13,16}\b", match => 
    new String('*', match.Length - 4) +
    match.Value.Substring(match.Length - 4));

The above code assumes that you want your redacted string to have the same length as the original, and preserve the last 4 digits.

If you want to redact the first 12 digits and preserve whatever remains (1–4 digits), use:

Regex.Replace(xml, @"\b\d{13,16}\b", match => 
    new String('*', 12) +
    match.Value.Substring(12));

Edit: I think you already got it. But probably what you need to do is replace:

xml = Regex.Replace(xml, …

with:

a.Value = Regex.Replace(a.Value, …

Edit: I went through your now-complete code, and I realized that you don’t even need regex for what you’re trying to accomplish, since you’re replacing the attribute value in entirety. Here’s how I would adapt your code (with a number of changes):

string xml = @"
    <Details>
        <CreditCard cardnum='1234567890123456'
                    ccv='123' 
                    exp='0212' 
                    cardType='1' 
                    name='joe' />
    </Details>";

XElement element = XElement.Parse(xml);
IEnumerable<XElement> elementsWithPossibleCCNumbers =
    element.Descendants()
            .Where(d => d.Attributes()
                         .Select(a => a.Value)
                         .Any(v => v.Length >= 13 &&
                                   v.Length <= 16 &&
                                   v.All(Char.IsDigit)));

foreach (var x in elementsWithPossibleCCNumbers)
{
    foreach (var a in x.Attributes())
    {
        //Check if the value is a number
        if (a.Value.All(Char.IsDigit))
        {
            //Check if value is the credit card
            if (a.Value.Length >= 13 && a.Value.Length <= 16)
                a.Value = new String('*', a.Value.Length - 4) + a.Value.Substring(a.Value.Length - 4);
            else //If value is not a credit card, replace it with ***
                a.Value = "***";
        }
    }
}

xml = element.ToString();