1) That’s not just Python. That’s essentially all regular expressions. It’s called “alternation,” if you’re curious. Look in the man page for grep(1).
2) The shorter and better way to say [0-9] is “\d”. So your regex could be shortened to
“\d{6}|\d{7}”.
3) The “character classes” (things like “[0-9]” or “[A-Za-z]“) are actually just more-concise ways of expressing alternation. So, for instance, “[0-9]” is identical to “0|1|2|3|…|8|9″.
4) You may already know that if you were looking for 6, 7, or 8 digits, you could use “\d{6,8}”.