IsDigit

isdigit is a term used for functions in several programming languages that test whether a given character represents a decimal digit, typically the ASCII digits 0 through 9. The exact behavior depends on the language and, in some cases, the current locale.

In C and C++, isdigit is provided by the ctype.h or <cctype> header. The signature is int isdigit(int ch). It returns a nonzero value if ch represents a decimal digit and zero otherwise. The argument should be representable as unsigned char or EOF. Some implementations expose isdigit as a macro. In the default C locale, the function tests only the ASCII digits '0' to '9'; in other locales, classification may be locale-sensitive.

In Python, the string method str.isdigit checks whether all characters in the string are digits and that the string is non-empty. It recognizes digits defined by Unicode, including digits from scripts beyond ASCII. This differs from related checks such as str.isdecimal or str.isnumeric, which have different scope with respect to which characters are considered digits.

Java exposes similar functionality via Character.isDigit(char ch), which returns true if the character is a digit according to Unicode classification. This includes many digit characters beyond the ASCII '0'–'9'.

Unicode and broader text processing note: digit classification may include characters from multiple numeral systems. When parsing numeric input in software that handles international text, developers often rely on language-specific isdigit-like functions and consider locale and Unicode rules to ensure correct interpretation and validation.

Examples and usage vary by language, but the central concept remains: isdigit-type utilities identify decimal digit characters and are used in input validation, parsing, and numeric conversions.