Nondigit

A nondigit is any character that is not a decimal digit. In many computing contexts, digits are the ten characters 0 through 9, and a nondigit is anything outside that set. In regular expressions, the nondigit metacharacter is typically written as \D and matches any character that is not a decimal digit. This is the complement of the digit class, usually represented as \d or [0-9].

Unicode usage: In Unicode-aware contexts, decimal digits are defined by the Nd (Decimal Digit) category. When a pattern engine supports Unicode, \d generally matches any character with the Nd property, and \D matches any character that does not have that property. Some older or non-Unicode engines restrict \d to ASCII digits, in which case many numerals from other scripts would be treated as nondigits.

Examples of nondigits include letters such as 'a', punctuation such as '+', whitespace such as a space, and symbols such as '•'. Digits such as '0' through '9' are digits and do not match \D. Depending on the engine, some characters that are numerals in non-Latin scripts may or may not be considered digits.

Applications: Nondigits are used in tokenization, validation, and pattern design to require, permit, or exclude numeric characters, and to separate alphabetic from numeric components in strings such as identifiers and filenames.

See also: digit, regular expression, Unicode general category Nd.