11 Feb 2004*
CS 3243 - Optional Lecture - NLP
6
Chomsky Hierarchy
Four classes of grammatical formalisms:
lRecursively
enumerable grammars
lUnrestricted rules: both sides of the rewrite rules can
have any number of terminal and
nonterminal symbols
AB ® C
lContext-sensitive
grammars
lThe
RHS must contain at least as many symbols as the LHS
ASB ® AXB
lContext-free
grammars (CFG)
lLHS
is a single nonterminal symbol
S ® XYa
lRegular
grammars
X ® a X ® aY