string-downcase

(string-downcase string-designator &key (start 0) end)

Returns a new string with every uppercase letter converted to lowercase; the original string is unchanged. The argument is a string designator, so a symbol, a keyword or a character is also accepted -- a symbol's name is used and a keyword's leading colon is dropped, so (string-downcase :FOO) returns "foo" and (string-downcase #\A) returns "a". Anything that is not one of those three types is an error. Case conversion is full-Unicode and identical on every backend: each character is folded with char-downcase, so (string-downcase "ÉΛΩ") returns "éλω". Because the fold is per character, the result always has the same length as the argument and no context-sensitive rule applies (a Greek final sigma is not special-cased).

:start / :end bound the part that is converted; the characters outside it are kept as they are, and a nil :end means the end of the string. A range outside the string (:start below 0, :end past the length, or :start after :end) or a bound that is not an integer -- a nil :start included -- is the same type-error as subseq's, on every backend.