See Gruber, Lee & Shallit (2012) for a detailed exposition. Gruber, Hermann; Lee, Jonathan; Shallit, Jeffrey (2012). "Enumerating regular expressions and their languages". arXiv:1204.4982 [cs.FL].
See Berstel & Boasson (1990) for detailed account. Berstel, Jean; Boasson, Luc (1990). "Context-free languages"(PDF). In van Leeuwen, Jan (ed.). Handbook of Theoretical Computer Science, Volume B: Formal Models and Semantics. Elsevier and MIT press. pp. 59–102. ISBN0-444-88074-7.