Regular Expression - Validate phone no

Here is a simple program to validate phone no with nnn-nnn-nnnn/nnnnnnnnnn format

private static void validatePhone () {
   //Validate phone no.
    String expression = "^\\d{3}-?\\d{3}-?\\d{4}$";
    String str1 = "1233456789"; //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 valid phone no.");
        found = true;
    }
    if(!found){
        System.out.println(str1 + " - is not a valid phone no.");
    }
}