The 'nstring-downcase' function takes a string argument and makes it lower case. This function modifies the string or string variable itself, it does not just make a copy. The lower case string is returned.
The keyword arguments allow for accessing substrings within 'string'. The keyword arguments require a keyword [':start' or ':end'] first and a single integer expression second. The ':start' keyword specifies the starting offset for the 'nstring-downcase' operation on 'string'. A value of 0 starts the string at the beginning [no offset]. The ':end' keyword specifies the end offset for the operation on 'string'.
(nstring-downcase "ABcd+-12&[")                ; returns "abcd+-&["
(nstring-downcase "ABCDEFGH" :start 2 :end 4)  ; returns "ABcdEFGH"
(setq mystr "ABcdEFgh")        ; set up variable
(nstring-downcase mystr)       ; returns "abcdefgh"
(print mystr)                  ; prints  "abcdefgh"
                               ; note that MYSTR is modified
See the
nstring-downcase
function in the