allowedletters

Allowedletters refers to the set of letters permitted in a given input or data field. It is used in validation, data sanitization, and string generation to restrict inputs to alphabetic characters. The exact composition depends on context and locale; it may include only ASCII letters (A–Z, a–z) or extend to accented characters and letters from other scripts to support internationalization.

In software development, allowedletters is typically implemented via character classes or regular expressions. For an ASCII-only field, a pattern may restrict input to the 26 lowercase and 26 uppercase Latin letters. For Unicode-aware applications, designers may select all characters classified as letters, using language features that recognize a broader alphabet. In some systems, allowedletters is a configurable parameter set by administrators or defined by locale settings.

Practical uses include ensuring usernames, display names, or identifiers contain only letters, or filtering input before storage or processing. The choice of allowed letters can affect user experience, accessibility, and security; overly restrictive sets may exclude legitimate names, while too-permissive sets can allow spoofing or injection.

Common patterns include explicit lists of letters, Unicode category checks, or built-in library utilities such as isalpha. Examples: an ASCII check may use a pattern like ^[A-Za-z]+$, while Unicode-aware approaches rely on language-specific APIs to test for alphabetic characters.

See also: character set, regular expression, input validation, Unicode, locale.