What is the metalanguage used to define the syntax of a programming language?

The metalanguage that is commonly used to define the syntax of a programming language is Backus-Naur Form (BNF). BNF provides a formal way to express the grammar of a language, specifying the rules for how symbols can be combined to form valid statements in that language. It uses a set of derivation rules to outline the structure of the language, making it easier to understand and implement compilers and interpreters that process the code written in that programming language.

BNF simplifies the representation of complex syntax into a series of production rules, which clearly define what constitutes a valid expression or statement. This characteristic distinguishes it as a powerful tool in language design, allowing programmers and language designers to convey the necessary grammatical rules systematically and precisely.

In contrast, syntax diagrams visually represent the structure of a language but do not formalize it as a metalanguage. A markup language is designed for annotating a document in a way that is syntactically distinguishable from the text, while a parse tree represents the syntactic structure of a string derived from a grammar but does not serve to define the grammar itself. These alternatives, while relevant to understanding programming languages and their syntactic structures, do not function as metalanguages for syntax definition in the same formalized way