Two Families of Languages Related to ALGOL
Journal of the ACM (JACM)
A New Normal-Form Theorem for Context-Free Phrase Structure Grammars
Journal of the ACM (JACM)
Formal languages and their relation to automata
Formal languages and their relation to automata
Regular-like expressions for some irregular languages
SWAT '68 Proceedings of the 9th Annual Symposium on Switching and Automata Theory (swat 1968)
Delayed-logic and finite-state machines
FOCS '61 Proceedings of the 2nd Annual Symposium on Switching Circuit Theory and Logical Design (SWCT 1961)
A characterization of context-free languages
Journal of Computer and System Sciences
Regular expressions of conditions for processing language modelling
MMACTEE'06 Proceedings of the 8th WSEAS international conference on Mathematical methods and computational techniques in electrical engineering
Model specifications of transition systems
MMACTEE'06 Proceedings of the 8th WSEAS international conference on Mathematical methods and computational techniques in electrical engineering
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This paper describes a characterization of context-free languages by means of expressions using union, substitution, and an iterative substitution called substitution star. The ''substitution expressions'' so defined have some properties analogous to those of regular expressions. The well-known cases of linear and regular subfamilies of context-free languages are obtainable from the general model by placing proper restrictions on the basis over which the substitution expressions are defined. The notion of substitution star height of a substitution expression is defined analogously to regular star height for regular expressions. It shown that for each n=0 there exists a context-free, in fact, linear language whose substitution star height is n.