Regular Expression - Validate only digits

Here is the simple program to validate the digits

private static void onlyDigits () {
   //Only Digits
    String expression = "^[0-9]+$";
    String str1 = "123"; //It will match.
    String str2 = "2dF1"; //It wont match.
    String str3 = "a33345345"; //It wont match.
    Pattern pattern = Pattern.compile(expression);

    Matcher matcher = pattern.matcher(str1);

    boolean found = false;
    if (matcher.find()) {
        System.out.println (str1 + "  - is a number");
        found = true;
    }
    if(!found){
        System.out.println(str1 + " - is not a number");
    }
}