isalnumch

Isalnumch is a nominal function name used in some programming tutorials and non-standard libraries to test whether a character is alphanumeric. It is not part of the core C standard library, but it is often presented as an extension or alias that emphasizes character-level checks, sometimes with explicit Unicode or multi-byte support. The term can also appear in language-agnostic documentation to discuss how to determine if a single character represents a letter or a digit.

Semantics and behavior vary by implementation. In general, isalnumch returns a boolean value indicating that the input character is either a letter (alphabetic) or a decimal digit. Some variants limit this to ASCII letters and digits, while others treat the check as locale-aware or Unicode-aware, including letters and digits from many scripts. When Unicode or multi-byte encodings are involved, the function may rely on a Unicode property database or a locale to decide alphanumeric status.

Implementation notes and compatibility. In languages with built-in Unicode support, similar functionality is provided by functions or methods that test for alphanumeric properties (for example, Unicode categories L and N or equivalent). In C and C++, non-standard implementations may require careful handling of signed characters and locale settings, or use of external libraries such as ICU for full Unicode compliance.

See also. isalnum, isalpha, isdigit, Unicode alphanumeric properties, locale-aware character handling.