invaliddigit

Invalid digit refers to a character that is not valid as part of a numeric representation in a given numeral system or data format. In base-N notation, each position may contain one of N symbols representing digits. A character outside the allowed digit set is considered an invalid digit and may cause a parsing or conversion error when the value is read or processed.

In programming languages, invalid digits commonly occur when interpreting numeric literals or numeric strings. The allowed digits depend on the base: base 10 uses 0–9; base 2 uses 0 and 1; base 16 uses 0–9 and A–F (often case-insensitive). If a string contains a character that is not permitted for the chosen base, a parser typically signals an error. For example, some languages report messages such as invalid digit found in string or invalid literal for int with base 10 when encountering an illegal character like 'A' in a decimal literal, or '12a' in a decimal parse.

Common error handling approaches include validating input before parsing, or using parsing functions that specify the base and return clear error information. Regular expressions can be used to enforce allowed digit sets for a given base, e.g., patterns that permit only digits 0–9 for decimal, or 0–9 and A–F for hexadecimal. In other contexts, such as data entry or OCR, an invalid digit may indicate transcription or formatting issues that require correction or normalization.

See also: digit, numeral system, base (radix), parsing, input validation.