4.9 Case Conversion in Lisp

The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (see Case Tables).

These functions do not modify the strings that are passed to them as arguments.

The examples below use the characters ‘X’ and ‘x’ which have ASCII codes 88 and 120 respectively.

function downcase string-or-char

This function converts string-or-char, which should be either a character or a string, to lower case.

When string-or-char is a string, this function returns a new string in which each letter in the argument that is upper case is converted to lower case. When string-or-char is a character, this function returns the corresponding lower case character (an integer); if the original character is lower case, or is not a letter, the return value is equal to the original character.

(downcase "The cat in the hat")
     ⇒ "the cat in the hat"

(downcase ?X)
     ⇒ 120

function upcase string-or-char

This function converts string-or-char, which should be either a character or a string, to upper case.

When string-or-char is a string, this function returns a new string in which each letter in the argument that is lower case is converted to upper case. When string-or-char is a character, this function returns the corresponding upper case character (an integer); if the original character is upper case, or is not a letter, the return value is equal to the original character.

(upcase "The cat in the hat")
     ⇒ "THE CAT IN THE HAT"

(upcase ?x)
     ⇒ 88

function capitalize string-or-char

This function capitalizes strings or characters. If string-or-char is a string, the function returns a new string whose contents are a copy of string-or-char in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case.

The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).

When string-or-char is a character, this function does the same thing as upcase.

(capitalize "The cat in the hat")
     ⇒ "The Cat In The Hat"

(capitalize "THE 77TH-HATTED CAT")
     ⇒ "The 77th-Hatted Cat"

(capitalize ?x)
     ⇒ 88

function upcase-initials string-or-char

If string-or-char is a string, this function capitalizes the initials of the words in string-or-char, without altering any letters other than the initials. It returns a new string whose contents are a copy of string-or-char, in which each word has had its initial letter converted to upper case.

The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).

When the argument to upcase-initials is a character, upcase-initials has the same result as upcase.

(upcase-initials "The CAT in the hAt")
     ⇒ "The CAT In The HAt"

Note that case conversion is not a one-to-one mapping of codepoints and length of the result may differ from length of the argument. Furthermore, because passing a character forces return type to be a character, functions are unable to perform proper substitution and result may differ compared to treating a one-character string. For example:

(upcase "fi")  ; note: single character, ligature "fi"
     ⇒ "FI"

(upcase ?fi)
     ⇒ 64257  ; i.e. ?fi

To avoid this, a character must first be converted into a string, using string function, before being passed to one of the casing functions. Of course, no assumptions on the length of the result may be made.

Mapping for such special cases are taken from special-uppercase, special-lowercase and special-titlecase See Character Properties.

See Text Comparison, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.