Substitution expressions

I.P. McWhirter
<span title="">1971</span> <i title="Elsevier BV"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/p6ovb2qpkfenhmb7mcksobrcxq" style="color: black;">Journal of computer and system sciences (Print)</a> </i> &nbsp;
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 eases 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
more &raquo; ... otion 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.
<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="https://doi.org/10.1016/s0022-0000(71)80020-x">doi:10.1016/s0022-0000(71)80020-x</a> <a target="_blank" rel="external noopener" href="https://fatcat.wiki/release/77dtptwrcfhzldnqjmbwa33ux4">fatcat:77dtptwrcfhzldnqjmbwa33ux4</a> </span>
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