Formal language theory: refining the Chomsky hierarchy

G. Jager, J. Rogers
2012 Philosophical Transactions of the Royal Society of London. Biological Sciences  
The first part of this article gives a brief overview of the four levels of the Chomsky hierarchy, with a special emphasis on context-free and regular languages. It then recapitulates the arguments why neither regular nor context-free grammar is sufficiently expressive to capture all phenomena in the natural language syntax. In the second part, two refinements of the Chomsky hierarchy are reviewed, which are both relevant to the extant research in cognitive science: the mildly context-sensitive
more » ... languages (which are located between context-free and context-sensitive languages), and the sub-regular hierarchy (which distinguishes several levels of complexity within the class of regular languages).
doi:10.1098/rstb.2012.0077 pmid:22688632 pmcid:PMC3367686 fatcat:djhi3u3a6jf55glde77bj42ija