isalnum

isalnum is a standard library function used to determine whether a character is alphanumeric. It is defined in the C standard library header ctype.h and has the prototype int isalnum(int ch). The function returns a non-zero value if ch is a letter or a digit, and zero otherwise. Its behavior is influenced by the current locale, so the set of characters considered alphanumeric can vary with locale settings.

In the default locale, often referred to as the "C" or "POSIX" locale, isalnum identifies ASCII letters A through Z, a through z, and digits 0 through 9 as alphanumeric. Other locales may treat additional characters as letters, affecting results accordingly. The argument to isalnum must be representable as an unsigned char or be the special value EOF; passing a plain signed char with a negative value that is not cast to unsigned char yields undefined behavior.

isalnum is frequently implemented as a macro on many platforms, sometimes backed by a lookup table for efficiency. The wide-character counterpart is iswalnum, declared in wctype.h, which operates on wchar_t values. Locale-aware variants can be used by setting the program locale with setlocale and by using related locale-aware functions.

Common uses include input validation and tokenization, such as checking whether a character in a string is allowed in an identifier or username. A simple check might evaluate isalnum(ch) and proceed if the result is non-zero; for example, it would be true for 'A' or '3' and false for '%'.