Java regular expression for masked alphanumeric codes

I need a help to use or build a regular expression to mask alphanumeric with *.

I tried it with this expression, but it doesn't work correctly when it has zeros in the middle of the string:

(?<=[^0].{3})\w+(?=\w{4})

Live samples: https://www.regexplanet.com/share/index.html?share=yyyyf47wp3r

The rules are:

1. The first 4 that are not zeros, and the last 4 a must be displayed.
2. Leading zeros are ignored, but not removed or replaced.

You can capture initial zeros and 3 alhpanumeric chars right after them in one group, the middle part into a second group, and then the last 4 alphanumeric chars into a third group, then only replace each char in the second group.

Here is an example (Java 11 compliant):

String text = "0001113033AA55608981";
Matcher mr = Pattern.compile("^(0*\\w{4})(.*)(\\w{4})$").matcher(text);
text = mr.replaceFirst(m -> m.group(1) + "*".repeat(m.group(2).length()) + m.group(3));
System.out.println(text); // => 0001113**********8981

See the Java demo.

The regex matches

• ^ - start of string
• (0*\w{4}) - Group 1: zero or more 0 chars and then any four alphnumeric/underscore chars
• (.*) - Group 2: any zero or more chars other than line break chars (replace with \w* if you only allow "word" chars)
• (\w{4}) - Group 3: four "word" chars
• $ - end of string.

Java 8 compliant version:

String text = "0001113033AA55608981";
Matcher m = Pattern.compile("^(0*\\w{4})(.*)(\\w{4})$").matcher(text);
StringBuffer result = new StringBuffer();
while (m.find()) {
    m.appendReplacement(result, m.group(1) + String.join("", Collections.nCopies(m.group(2).length(), "*")) + m.group(3));
}
m.appendTail(result);
System.out.println(result.toString());

See the Java code demo online.

But you may just use

String text = "0001113033AA55608981";
Matcher m = Pattern.compile("^(0*\\w{4})(.*)(\\w{4})$").matcher(text);
String result = "";
if (m.matches()) {
    result = m.group(1) + String.join("", Collections.nCopies(m.group(2).length(), "*")) + m.group(3);
}
System.out.println(result);