How about:
String numbers = text.substring(text.length() - 7);
That assumes that there are 7 characters at the end, of course. It will throw an exception if you pass it “12345”. You could address that this way:
String numbers = text.substring(Math.max(0, text.length() - 7));
or
String numbers = text.length() <= 7 ? text : text.substring(text.length() - 7);
Note that this still isn’t doing any validation that the resulting string contains numbers – and it will still throw an exception if text
is null.