metasyntaxとは 意味・読み方・使い方

A metasyntax describes the allowable structure and composition of phrases and sentences of a metalanguage, which is used to describe either a natural language or a computer programming language. Some of the widely used formal metalanguages for computer languages are Backus–Naur Form (BNF), Extended Backus–Naur Form (EBNF), Wirth syntax notation (WSN), and Augmented Backus–Naur Form (ABNF). These metalanguages have their own metasyntax each composed of terminals, nonterminals, and metasymbols. A terminal symbol, such as a word or a token, is a stand-alone structure in a language being defined. A nonterminal symbol represents a syntactic category, which defines one or more valid phrasal or sentence structure consisted of an n-element subset. Metasymbols provide syntactic information for denotational purposes in a given metasyntax. Terminals, nonterminals, and metasymbols do not apply across all metalanguages. Typically, the metalanguage for token-level languages (formally called “regular languages”) does not have nonterminals because nesting is not an issue in these regular languages. English, as a metalanguage for describing certain languages, does not contain metasymbols since all explanation could be done using English expression. There are only certain formal metalanguages used for describing recursive languages (formally called context-free languages) have terminals, nonterminals, and metasymbols in their metasyntax.