How to replace all characters in a Java string with stars

Don't use regex at all, count the String length, and return the according number of stars.

Plain Java < 8 Version:

int len = str.length();
StringBuilder sb = new StringBuilder(len);
for(int i = =; i < len; i++){
    sb.append('*');
}
return sb.toString();

Plain Java >= 8 Version:

int len = str.length();
return IntStream.range(0, n).mapToObj(i -> "*").collect(Collectors.joining());

Using Guava:

return Strings.repeat("*", str.length());
// OR
return CharMatcher.ANY.replaceFrom(str, '*');

Using Commons / Lang:

return StringUtils.repeat("*", str.length());

Java 11 and later

str = "*".repeat(str.length());

Note: This replaces newlines \n with *. If you want to preserve \n, see solution below.

str = str.replaceAll(".", "*");

This preserves newlines.

To replace newlines with * as well in Java 10 and earlier, you can use:

str = str.replaceAll("(?s).", "*");

The (?s) doesn't match anything but activates DOTALL mode which makes . also match \n.