wellformedness

Wellformedness refers to the property of a string, structure, or data item that conforms to the structural rules of a given system. It is a binary notion: something is either well-formed or not, according to the rules in force.

In linguistics and formal language theory, a string is well-formed if it adheres to the syntax of a grammar. In formal languages, a well-formed string belongs to the language generated by a grammar. In natural language, well-formed sentences follow the syntactic rules of a language, though they may be odd or semantically nonsensical.

In computer science and data formats, well-formedness typically concerns syntactic legality. For programming languages, a program is well-formed if it adheres to the language’s syntax, allowing a compiler or interpreter to construct a parse or abstract syntax tree. In markup and data formats, well-formedness has specific technical criteria: for example, XML documents must have properly nested, closed tags, a single root element, and valid character data; JSON must have proper punctuation and structure. These criteria are distinct from validity, which adds semantic or contextual constraints such as type, value ranges, or schema compliance.

Wellformedness is a prerequisite step in parsing and validation. Parsers usually report well-formedness errors first; subsequent stages enforce deeper semantic constraints. Across disciplines, the concept provides a practical boundary between syntactic legality and broader notions of correctness or usefulness.