mix is not the English word mix, it’s options of Regexp.
See Regexp#to_s:
Returns a string containing the regular expression and its options (using the (
?opts:source) notation.
In your example, m is for multiline mode, i is for case insensitive, and x is for extended mode. Options before the dash are on, those after are off (default). The question’s example, ?-mix, has all options off.
You can turn them on like:
puts /^a$/mix
# =>(?mix:^a$)