string-capitalize

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

Returns a new string in which the first letter of each word is uppercased and the remaining letters of each word are lowercased, where words are runs of alphanumeric characters separated by other characters. 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-capitalize :foo-bar) returns "Foo-Bar" and (string-capitalize nil) returns "Nil". Anything that is not one of those three types is an error. As with the other case operators the fold is full-Unicode and identical on every backend, and a word constituent is any Unicode letter or digit, so (string-capitalize "élan vital") returns "Élan Vital".

: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 word starts at :start whatever precedes it. 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.