Separator between columns.
If character, sep is interpreted as a regular expression. The default
value is a regular expression that matches any sequence of
non-alphanumeric values.
If numeric, sep is interpreted as character positions to split at. Positive
values start at 1 at the far-left of the string; negative value start at -1 at
the far-right of the string. The length of sep should be one less than
into.