xdigit

Xdigit is a term used in computing to denote a hexadecimal digit. A hexadecimal digit is any character that represents a value in base-16: the decimal digits 0 through 9 and the letters A through F (in either upper or lower case). In practice, xdigit is used when parsing, validating, or formatting hexadecimal numbers, color codes, and other data that employ base-16 notation.

In the C standard library, the function isxdigit checks whether a given character is a hexadecimal digit. Declared in <ctype.h>, int isxdigit(int ch) returns a nonzero value if ch is a hexadecimal digit and zero otherwise. The input is treated as an unsigned char value, or as EOF; this ensures defined behavior for all possible input values. A wide-character variant, iswxdigit, serves a similar purpose for wide characters and is declared in <wctype.h>.

In POSIX and many regular-expression implementations, hexadecimal digits are represented by the character class [:xdigit:] or by equivalent patterns like [0-9A-Fa-f]. Some regex flavors also support properties such as \p{XDigit}. These classifications help implement validation rules, tokenization, and color-code parsing in text processing tools.

Examples: isxdigit('3') is true; isxdigit('A') is true; isxdigit('g') is false. For color codes such as #RRGGBB, xdigits define the allowable characters after the hash.

See also: isdigit, isalpha, isalnum, ctype.h, [:xdigit:], regular-expression digit classes.