Regex based Char replacement.

RegexBasedTest.java

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class RegexBasedTest {
    private static final Pattern PASSWORD_PATTERN = Pattern.compile("[a-zA-z0-9]{3}");
    private static final String uuid = "b02fb649-797b-440b-9aa9-f3294ed0ce73";
    public static void main(String[] args) {
        for (int i = 0; i < Integer.MAX_VALUE; ++i) {
            String masked = replaceChar(uuid);
        }
    }

    private static String replaceChar(String uuid) {
        final Matcher pwdMatcher = PASSWORD_PATTERN.matcher(uuid);
        return pwdMatcher.replaceFirst("****");
    }
}