A new algorithm for regularizing one-letter context-free grammars

Constructive methods for obtaining regular grammar counterparts for some sub-classes of context-free grammars (CFGs) have been investigated by many researchers. An important class of grammars for which this is always possible is the one-letter CFG. We show in this paper a new constructive method for transforming an arbitrary one-letter CFG to an equivalent regular expression of star-height 0 or 1. Our new result is considerably simpler than a previous construction by Leiss, and we also propose a new normal form for a regular expression with only a single-star occurrence. Through an alphabet factorization theorem, we show how to go beyond the one-letter CFG in a straight-forward way. c © 2003 Elsevier B.V. All rights reserved. MSC: 68Q42; 68Q45; 68N20