maxlength

Maxlength refers to a constraint on how many elements a string, sequence, or data field may contain. In software development, it is commonly used to limit input length, control storage requirements, and enforce data integrity.

In user interfaces, maxlength is most familiar as an attribute on input controls. For example, the HTML5 input element can specify a maximum number of characters that a user may enter. This client-side restriction helps guide the user and reduces invalid submissions, but it should not replace server-side validation.

In data storage and programming languages, maximum lengths are used to define the size of string types. Databases often implement this with types such as varchar(n) or char(n), where n sets an upper bound on stored characters. Fixed-length fields reserve a constant amount of space, while variable-length fields allocate space up to the defined maximum.

Considerations include handling of Unicode and multi-byte encodings. Length counting can differ between code points, bytes, and grapheme clusters, and truncating mid-character can produce invalid results. Many systems provide safe truncation methods or warn when an input would exceed the limit.

Validation and security are important: enforce maxlength on both client and server sides to prevent data corruption and security issues. Truncation for display is acceptable in some contexts, but apps should preserve the possibility of error messages and user feedback when the limit is reached.

See also: input validation, string length, database field sizes, and fixed vs. variable-length types.