malformed

Malformed is an adjective describing something that is not correctly formed or structured according to a given system of rules. It signals deviation from the expected shape, organization, or syntax. The word combines the prefix mal-, meaning bad, with form, meaning shape.

In computing and data processing, malformed data, input, or documents fail to satisfy the syntax of a language or protocol. Examples include a malformed URL that does not conform to URI syntax, or malformed XML, JSON, or HTML that violates its grammar. Parsers and validators may reject such input or attempt error recovery, with potential security risks if malformed data is mishandled.

In linguistics, a sentence or utterance can be described as malformed or ungrammatical when it cannot be generated by the grammar of a language. The term is widely used in reference to syntax and sometimes contrasts with well-formed sentences produced by competent speakers under the grammar's rules.

In medicine and biology, malformation refers to abnormal development of a structure or organ. Congenital malformations are present at birth and can affect many systems, including limbs, the spine, or internal organs. The root malformation implies that development did not proceed normally rather than a deformation after birth.

Handling of malformed data or inputs includes validation, error reporting, and repair or rejection. In software, robust systems often distinguish between malformed input and incorrect semantics, applying appropriate parsing strategies and security measures to mitigate vulnerabilities.

See also well-formed, validation, congenital malformation, ill-formed sentence.