Allowedletter

Allowedletter is a term used in discussions of character sets and input validation to denote a single letter that is permitted within a particular context or rule set. It functions as a placeholder for the permissible alphabetic elements in a system, language, or puzzle.

In linguistics and orthography, an allowed letter specifies which letters may be used to represent phonemes or morphemes in a language or transliteration scheme. Different writing systems impose different restrictions, ranging from a limited Latin alphabet to broader scripts that include diacritics and non-Latin characters.

In computing and programming, allowed letters are defined by lexical rules and identifier conventions. Many languages permit ASCII letters A–Z and a–z, often allowing an underscore and digits in subsequent positions. Unicode-aware systems may extend allowed letters to include letters from many scripts, with rules governing their position and use in identifiers, keywords, or file names.

In domain names, passwords, and form validation, allowed letters contribute to the overall character class eligible for input. For example, domain labels typically restrict to letters, digits, and hyphens, while passwords may specify required and optional letter categories as part of a broader policy.

In word games and puzzles, allowed letters define the usable alphabet for constructing words or solving challenges. Puzzles may restrict to a subset of letters or require letters to satisfy additional constraints such as positions or frequencies.

Regular expressions and programming libraries provide mechanisms to enforce allowed letters. For ASCII letters, a common pattern is ^[A-Za-z]+$, while Unicode-aware contexts may use patterns like ^\p{L}+$ with appropriate flags. See also character set, lexical analysis, and transliteration.