Case folding

Case folding is a term denoting the conversion of all characters in a string to lower or upper-case, typically to make case-insensitive comparisons.

Case Folding in some high-level languages

Most, if not all, BASIC dialects provide these basic functions:

UpperA$ = UCASE$("a") LowerA$ = LCASE$("A")

C and C++, as well as any C-like language that conforms to its standard library, provide these functions in the file ctype.h:

char upperA = toupper('a'); char lowerA = tolower('A');

Algorithms to fold case

Case folding is different with different character sets. In ASCII case can be folded in the following way, in C:

#define toupper(c) islower(c) ? (c) - 'a' + 'A' : (c) #define tolower(c) isupper(c) ? (c) - 'A' + 'a' : (c)

This only works because the alphanumeric letters are consecutive. This would not work, for instance, with EBCDIC.

Unicode case folding

Unicode defines case folding through the three case mapping properties of each character: uppercase, lowercase and titlecase. These properties relate all characters in scripts with differring cases to the other case variants of the character.