is a variable defined in `
Its value is
Regular expression matching delimiters controlling the partial-completion.
Typically, this regular expression simply matches a delimiter, meaning
that completion can add something at (match-beginning 0), but if it has
a submatch 1, then completion can add something at (match-end 1).
This is used when the delimiter needs to be of size zero (e.g. the transition
from lowercase to uppercase characters).