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Terminal symbols are the elementary symbols of the language defined as part of a formal grammar. Nonterminal symbols (or syntactic variables) are replaced by groups of terminal symbols according to the production rules. The terminals and nonterminals of a particular grammar are in two completely separate sets.
<symbol> [2] is a nonterminal variable that is always enclosed between the pair <>. ::= means that the symbol on the left must be replaced with the expression on the right. __expression__ consists of one or more sequences of either terminal or nonterminal symbols where each sequence is separated by a vertical bar "|" indicating a choice , the ...
Each variable represents a different type of phrase or clause in the sentence. Variables are also sometimes called syntactic categories. Each variable defines a sub-language of the language defined by G. Σ is a finite set of terminals, disjoint from V, which make up the actual content of the sentence
A starting non-terminal produces loops. The rest of the grammar proceeds with parameter L {\displaystyle \mathbf {\mathit {L}} } that decide whether a loop is a start of a stem or a single stranded region s and parameter F {\displaystyle \mathbf {\mathit {F}} } that produces paired bases.
The vertical bar represents an alternative and the terminal symbols are enclosed with quotation marks followed by a semicolon as terminating character. Hence a digit is a 0 or a digit excluding zero that can be 1 or 2 or 3 and so forth until 9. A production rule can also include a sequence of terminals or nonterminals, each separated by a comma:
A → w, where A is a non-terminal in N and w is in a (possibly empty) string of terminals Σ * A → wB, where A and B are in N and w is in Σ *. Some authors call this type of grammar a right-regular grammar (or right-linear grammar) [1] and the type above a strictly right-regular grammar (or strictly right-linear grammar). [2]
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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 .