Sort the text in the region region lexicographically.
If called interactively, prompt for two regular expressions,
RECORD-REGEXP and KEY-REGEXP.
RECORD-REGEXP specifies the textual units to be sorted.
For example, to sort lines, RECORD-REGEXP would be "^.*$".
KEY-REGEXP specifies the part of each record (i.e. each match for
RECORD-REGEXP) to be used for sorting.
If it is "\\digit", use the digit'th "\\(...\\)"
match field specified by RECORD-REGEXP.
If it is "\\&", use the whole record.
Otherwise, KEY-REGEXP should be a regular expression with which
to search within the record. If a match for KEY-REGEXP is not
found within a record, that record is ignored.
With a negative prefix arg, sort in reverse order.
The variable `sort-fold-case' determines whether alphabetic case affects
the sort order.
For example: to sort lines in the region by the first word on each line
starting with the letter "f",
RECORD-REGEXP would be "^.*$" and KEY would be "\\