in core/src/main/java/org/apache/jsieve/comparators/AsciiNumeric.java [100:107]
protected String computeCompareString(String string) {
char[] chars = string.toCharArray();
for (int i = chars.length; i < chars.length; i++) {
if (!Character.isDigit(chars[i]))
chars[i] = 0xff;
}
return new String(chars);
}