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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 ...
An unrestricted grammar is a formal grammar = (,,,), where . is a finite set of nonterminal symbols,; is a finite set of terminal symbols with and disjoint, [note 1]; is a finite set of production rules of the form , where and are strings of symbols in and is not the empty string, and
Similar to a CFG, a probabilistic context-free grammar G can be defined by a quintuple: = (,,,,) where M is the set of non-terminal symbols; T is the set of terminal symbols; R is the set of production rules; S is the start symbol; P is the set of probabilities on production rules
T is a set ("alphabet") of terminal symbols, F is a set of so-called index symbols, or indices, S ∈ N is the start symbol, and; P is a finite set of productions. In productions as well as in derivations of indexed grammars, a string ("stack") σ ∈ F * of index symbols is attached to every nonterminal symbol A ∈ N, denoted by A[σ].
S is called the start symbol. In a left-regular grammar, (also called left-linear grammar), all rules obey the forms A → a; A → Ba; A → ε; The language described by a given grammar is the set of all strings that contain only terminal symbols and can be derived from the start symbol by repeated application of production rules.
where A, B, and C are nonterminal symbols, the letter a is a terminal symbol (a symbol that represents a constant value), S is the start symbol, and ε denotes the empty string. Also, neither B nor C may be the start symbol, and the third production rule can only appear if ε is in L(G), the language produced by the context-free grammar G.
where f is any index symbol, α is any string of terminals and/or non-terminal symbols, and x is a terminal is a terminal symbol. Because occasionally a rewrite rule might need to be conditioned on the stack being in some sense empty, the symbol # is used as the bottom-most stack symbol, meaning an "empty" stack contains exactly one symbol, #.
is the set of terminal symbols; is the set of productions; is the distinguished, or start, symbol; Then, given a string of nonterminal symbols and an attribute name , . is a synthesized attribute if all three of these conditions are met: