Regular expression to extract only numbers but only if sting doesn't contain any letters : Hello regular expression gurus! I need Your help! 

I want to extracts digits from the string that may contain some special character (let's say "+-() ") but not any other characters! I.e.:

    "+123 (456) 7-8" -> "1, 2, 3, 4, 5, 6, 7, 8" is extracted
    "123a45" -> pattern matching fails, nothing is extracted
    "1234 B" -> pattern matching fails, nothing is extracted

Thanks!