Since launch of a new site (Feb 2015), if your question is about Vi/Vim editor, you should consider posting questions at the dedicated Vim Stack Exchange site where it belongs.
However if your question is not about a compatible text-editor with the original Vi (such as vintage-mode in Sublime, evil, etc.), it should still belong to SuperUser, unless it's about Unix shell (such as vi-mode in shell). See: What is considered to be vi?
Questions about programming (such as regex), they still belong to StackOverflow.