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Concept
Comparison of parser generators
Résumé
This is a list of notable lexer generators and parser generators for various language classes.
Regular languages are a category of languages (sometimes termed Chomsky Type 3) which can be matched by a state machine (more specifically, by a deterministic finite automaton or a nondeterministic finite automaton) constructed from a regular expression. In particular, a regular language can match constructs like "A follows B", "Either A or B", "A, followed by zero or more instances of B", but cannot match constructs which require consistency between non-adjacent elements, such as "some instances of A followed by the same number of instances of B", and also cannot express the concept of recursive "nesting" ("every A is eventually followed by a matching B"). A classic example of a problem which a regular grammar cannot handle is the question of whether a given string contains correctly-nested parentheses. (This is typically handled by a Chomsky Type 2 grammar, also termed a context-free grammar.)
Context-free languages are a category of languages (sometimes termed Chomsky Type 2) which can be matched by a sequence of replacement rules, each of which essentially maps each non-terminal element to a sequence of terminal elements and/or other nonterminal elements. Grammars of this type can match anything that can be matched by a regular grammar, and furthermore, can handle the concept of recursive "nesting" ("every A is eventually followed by a matching B"), such as the question of whether a given string contains correctly-nested parentheses. The rules of Context-free grammars are purely local, however, and therefore cannot handle questions that require non-local analysis such as "Does a declaration exist for every variable that is used in a function?". To do so technically would require a more sophisticated grammar, like a Chomsky Type 1 grammar, also termed a context-sensitive grammar. However, parser generators for context-free grammars often support the ability for user-written code to introduce limited amounts of context-sensitivity.
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