Terminal and nonterminal symbols
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In computer science, terminal and nonterminal symbols are the lexical elements used in specifying the production rules constituting a formal grammar. Terminal symbols are the elementary symbols of the language defined by 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 two disjoint sets.
Contents
Terminal symbols
Terminal symbols are literal symbols which may appear in the outputs of the production rules of a formal grammar and which cannot be changed using the rules of the grammar. 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. Using pictoric marks interacting with each other:
 The symbol
ר
can becomeди
 The symbol
ר
can becomeд
Here д
is a terminal symbol because no rule exists which would change it into something else. On the other hand, ר
has two rules that can change it, thus it is nonterminal. A formal language defined or generated by a particular grammar is the set of strings that can be produced by the grammar and that consist only of terminal symbols.
Nonterminal symbols
Nonterminal symbols are those symbols which can be replaced. They may also be called simply syntactic variables. A formal grammar includes a start symbol, a designated member of the set of nonterminals from which all the strings in the language may be derived by successive applications of the production rules. In fact, the language defined by a grammar is precisely the set of terminal strings that can be so derived.
Contextfree grammars are those grammars in which the lefthand side of each production rule consists of only a single nonterminal symbol. This restriction is nontrivial; not all languages can be generated by contextfree grammars. Those that can are called contextfree languages. These are exactly the languages that can be recognized by a nondeterministic push down automaton. Contextfree languages are the theoretical basis for the syntax of most programming languages.
Production rules
A grammar is defined by production rules (or just 'productions') that specify which symbols may replace which other symbols; these rules may be used to generate strings, or to parse them. Each such rule has a head, or lefthand side, which consists of the string that may be replaced, and a body, or righthand side, which consists of a string that may replace it. Rules are often written in the form head → body; e.g., the rule a → b specifies that a can be replaced by b.
In the classic formalization of generative grammars first proposed by Noam Chomsky in the 1950s,^{[1]}^{[2]} a grammar G consists of the following components:
 A finite set of nonterminal symbols.
 A finite set of terminal symbols that is disjoint from .
 A finite set of production rules, each rule of the form
 where is the Kleene star operator and denotes set union, so represents zero or more symbols, and means one nonterminal symbol. That is, each production rule maps from one string of symbols to another, where the first string contains at least one nonterminal symbol. In the case that the body consists solely of the empty string—i.e., that it contains no symbols at all—it may be denoted with a special notation (often , or ) in order to avoid confusion.
 A distinguished symbol that is the start symbol.
A grammar is formally defined as the ordered quadruple . Such a formal grammar is often called a rewriting system or a phrase structure grammar in the literature.^{[3]}^{[4]}
Example
For instance, the following represents an integer (which may be signed) expressed in a variant of Backus–Naur form:
<digit> ::= '0'  '1'  '2'  '3'  '4'  '5'  '6'  '7'  '8'  '9'
<integer> ::= [''] <digit> {<digit>}
In this example, the symbols (,0,1,2,3,4,5,6,7,8,9) are terminal symbols and <digit> and <integer> are nonterminal symbols.
Note: This example supports strings with leading zeroes like "0056" or "0000", as well as negative zero strings like "0" and "00000".
Another example is:
S > cAd
A > a  ab
In this example, the symbols a,b,c,d are terminal symbols and S,A are nonterminal symbols.
See also
References
 ^ Chomsky, Noam (1956). "Three Models for the Description of Language". IRE Transactions on Information Theory. 2 (2): 113&ndash, 123. doi:10.1109/TIT.1956.1056813.
 ^ Chomsky, Noam (1957). Syntactic Structures. The Hague: Mouton.
 ^ Ginsburg, Seymour (1975). Algebraic and automata theoretic properties of formal languages. NorthHolland. pp. 8–9. ISBN 0720425069.
 ^ Harrison, Michael A. (1978). Introduction to Formal Language Theory. Reading, Mass.: AddisonWesley Publishing Company. p. 13. ISBN 0201029553.