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Applying the rules recursively to a source string of symbols will usually terminate in a final output string consisting only of terminal symbols. Consider a grammar defined by two rules. In this grammar, the symbol Б is a terminal symbol and Ψ is both a non-terminal symbol and the start symbol. The production rules for creating strings are as ...
A derivation rule is composed by a nonterminal symbol and an expression . A special expression α s {\displaystyle \alpha _{s}} is the starting point of the grammar. [ 2 ] In case no α s {\displaystyle \alpha _{s}} is specified, the first expression of the first rule is used.
Metalanguages have their own metasyntax each composed of terminal symbols, nonterminal symbols, 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 ...
There is a procedure for each nonterminal in the grammar. Parsing descends in a top-down manner until the final nonterminal has been processed. The program fragment depends on a global variable, sym, which contains the current symbol from the input, and the function nextsym, which updates sym when called.
An EBNF consists of terminal symbols and non-terminal production rules which are the restrictions governing how terminal symbols can be combined into a valid sequence. Examples of terminal symbols include alphanumeric characters, punctuation marks, and whitespace characters.
The algorithm works by scanning a sequence of terminal symbols and building a list of all the symbol pairs which it has read. Whenever a second occurrence of a pair is discovered, the two occurrences are replaced in the sequence by an invented nonterminal symbol, the list of symbol pairs is adjusted to match the new sequence, and scanning ...
The grammar uses these terminal symbols but does not define them. They are always at the bottom bushy end of the parse tree. The capitalized terms like Sums are nonterminal symbols. These are names for concepts or patterns in the language. They are defined in the grammar and never occur themselves in the input stream.
BNFs describe how to combine different symbols to produce a syntactically correct sequence. BNFs consist of three components: a set of non-terminal symbols, a set of terminal symbols, and rules for replacing non-terminal symbols with a sequence of symbols. [1] These so-called "derivation rules" are written as <