Parser

A parser is a computer program or a component of a program that analyses the grammatical structure of an input, with respect to a given formal grammar, a process known as parsing. Parsers can be made both for natural languages and for programming languages. Programming language parsers tend to be based on context free grammars as fast and efficient parsers can be written for them. For example LALR parsers are capable of efficiently analysing a wide class of context free grammars. Such parsers are usually not written by hand but generated by parser generators.

The task of the parser can be summarized as to determine if and how the input can be derived from the start symbol with the rules of the formal grammar. A parser can do this in essentially two ways: it can start with the input and attempt to rewrite it to the start symbol, a so-called bottom-up parser, or it can start with the start symbol and try to rewrite it to the input, a so-called top-down parser. For example LL parsers are top-down parsers and LR parsers are bottom-up parsers.

Another important distinction is whether the parser generates a leftmost derivation or a rightmost derivation (see context-free grammar). LL parsers will generate a leftmost derivation and LR parsers will generate a rightmost derivation (although usually in reverse).

Overview of Parsers

Top-down parsers

Some of the parsers that use top-down parsing include:

• Recursive descent parser
• LL parser
• Packrat parser
• Unger parser

Bottom-up parsers

Some of the parsers that use bottom-up parsing include:

• LR parser
  • SLR parser
  • LALR parser
  • Canonical LR parser
• Earley parser
• CYK parser
• Tomita parser

See also

• Chart parser
• Compiler-compiler
• SableCC
• JavaCC
• Yacc
• Lex
• pass
• Spirit Parser Framework

References

External links

• Citations from CiteSeer (http://citeseer.org/cs?q=parser)
• UCalc Fast Math Parser (http://www.ucalc.com/mathparser) A commercial expression parser

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