Regular Expression to mask given number either from begining or end - regex

Question

I have a scenario where have to mask two number in return from my application based on the configured regular expression patterns. I have following two numbers and need to mask as shown below.
20128569 --> 2012****
40953186 --> ****3186
I need two regular expression patterns to achieve this pattern accordingly using the String.replaceAll(...) or some other possible way.
public static void main(String[] args) {
String value = "20128569";
String pattern = "(?<=.{4}).?" ;
String formattedValue = value.replaceAll(pattern, "*");
System.out.println(formattedValue);
}
Note: I need two regular expression patterns in order to mask number as shown above.
However currently i have resolve this issue temporally through the following code. But it is nice if i can resolve this issue through only regular expression.
String maskedAccountNumber = Pattern.compile(aRegexPattern).matcher(aKey).replaceFirst(MASK_CHARACTER);
StringBuilder maskBuffer = new StringBuilder();
for(int i = 0; i <= aKey.length() - maskedAccountNumber.length() ; i++){
maskBuffer.append(MASK_CHARACTER);
}
return maskedAccountNumber.replace(MASK_CHARACTER, maskBuffer.toString());
Below are the two regulare expressions i used so far:
^.\d{1,3}
.\d{1,3}$

Answer 1

That's fairly easy to do, it's still hard to understand under what circumstances do you want which regex.
Either way, (\d*)\d{4} and replace it with $1****, as seen https://regex101.com/r/uI0zJ6/2
Or \d{4}(\d*) and replace with ****$1 https://regex101.com/r/uI0zJ6/3 .