Comment by aexaey

0, which is a shorthand for 0.0.0.0 is likely the most code-golf-y way to write localhost, as many [EDIT: Linux] systems alias 0.0.0.0 to 127.0.0.1:

  $ ping 0
  PING 0 (127.0.0.1) 56(84) bytes of data.
  64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.032 ms

Of course, don't expect this to work universally. A lot of software will try to be clever with input validation, and fail.

Tangentially related: https://fosdem.org/2018/schedule/event/email_address_quiz/