Credit card masker. Any digits between 13 - 16 will be masked.

CreditCardMasker.java

  public static String maskCreditCard(String sentenceThatMightContainACreditCard) {
    String maskedSentence = null;

    // Find sequence of numbers between 13-16. 
    // Numbers can have dashes, spaces, or no dashes / no spaces.
    Pattern regex = Pattern.compile("\\b(?:\\d[ -]*?){13,16}\\b");

    Matcher regexMatcher = regex.matcher(sentenceThatMightContainACreditCard);

    if (regexMatcher.find()) {
      // Credit card has been found. e.g Here's an Amex: 3782-8224 6310005
      String creditCard = regexMatcher.group();

      // Strip out spaces and dashes (if any). e.g. 378282246310005
      String strippedCreditCard = creditCard.replaceAll("[ -]+", "");

      // Take a chunk of the creditcard starting at 7th position,
      // ending at the last 4. e.g. 24631
      String subSectionOfCreditCard = strippedCreditCard.substring(6, strippedCreditCard.length() - 4);

      //Get the first 6 chars of the stripped credit card
      String prefix = strippedCreditCard.substring(0, 6);
      
      //Replace the subsection of credit card with 'xxx'
      String middle = String.join("", Collections.nCopies(subSectionOfCreditCard.length(), "x"));
      
      //Get the last 4 chars of the stripped credit card
      String suffix = strippedCreditCard.substring(strippedCreditCard.length() - 4, strippedCreditCard.length());
      
      // Mask the sub section of the credit card with 'x'. e.g 378282xxxxx0005
      String maskedCreditCard = prefix + middle + suffix;
      
      // Take the original text with credit card, and replace the found credit
      // card with a masked credit card. 
      // e.g. 'Market Lane Cafe $4.50 3782-8224 6310005 Large Latte'
      // is turned into 'Market Lane Coffee $4.50 378282xxxxx0005 Large Latte'
      maskedSentence = sentenceThatMightContainACreditCard.replace(creditCard, maskedCreditCard);
    } else {
      // If credit card was not found in the text, let's return the original input.
      maskedSentence = sentenceThatMightContainACreditCard;
    }

    return maskedSentence;
  }